Title of Invention

HYBRID VECTOR HAVING A CYTOMEGALOVIRUS ENHANCER AND MYELOPROLIFERATIVE SARCOMA VIRUS PROMOTER

Abstract

ABSTRACT 2829/CHENP/20Q4 "HYBRID VECTOR HAVING A CYTOMEGALOVIRUS ENHANCER AND MYELOPROLIFERATIVE SARCOMA VIRUS PROMOTER " This invention relates to a non-retroviral expression vector comprising a cytomegalovirus (CMV) enhancer and a myeloproliferative sacrcoma virus (MPSV) promoter, wherein the CMV enhancer is located upstream from the 5' end of the MPSV promoter such that the CMV enhancer is located in a position closer to the 5' end of the MPSV promoter than the 3' end of the MPSV promoter.

Full Text

HYBRID VECTOR HAVING A CYTOMEGALOVIRUS ENHANCER AND MYELOPROLIFERATIVE SARCOMA VIRUS PROMOTER INTRODUCTION The present invention is related to the construction and utilization of a DNA plasmid vector, in particular, those hybrid n on-retroviral vectors that comprise the cytomegalovirus (CMV) enhancer and the myeloproliferative sarcoma virus (MPSV) promoter minus its negative control region. This hybrid sequence promotes the high expression of cloned genes under its transcriptional control when the vector is transfected into mammalian cell lines. Preferably, the vector also comprises other functional sequences to increase expression of the cloned sequence such as the Tg intron sequence, a viral internal ribosome entry site (IRES), a leader sequence to allow for secreted protein expression, and polyadcnylation signals. The vector can also comprise selectable markers and other features that facilitate the replication of the vector in mammalian, yeast, and ptakaryotic host cells, thus increasing the stability of the vector in whatever expression system is being used. BACKGROUND OF THE INVENTION The expression of foreign proteins by bacteria, yeast or mammalian cell lines has become routine. One type of commonly used means involves the construction of virion-plasmid hybrid vectors that possess the capacity to express cloned inserts in mammalian cells. The expression of the cloned gene with such hybrid vectors can occur in a transient, extrachromosomal manner, but higher production is usually obtained through random insertion of the vector into the host cell genome. The typical mammalian expression vector will contain regulatory elements, usually in the form of viral promoter or enhancer sequences and characterized by a broad host and tissue range, a polylinker sequence facilitating the insertion of a DNA fragment within the plasmid vector, and the sequences responsibie for nitron splicing and polyadcnylation of inRNA transcripts. This contiguous region of promoter-polylinker-polyadenylation site is commonly refciTed to as the transcription unit. Viral promoter and enhancer regions have long been utilized as regulatory elements for use in mammalian host cells. For example, the strength of the CMV enhancer caused it to be a suggested component in eukaryotic expression vectors upon its discovery (Boshart et al., Cell, 41 (2):521-30 (1985)) and it has been utilized as a universal cell control element in transgenic mice (Schmidt et a]. Mol. Cell. Biol. 10: 4406-44J J (1990)). The MPSV promoter coveys a wide host cell specificity to the virus including fibroblasts and hematopoietic stem cells (Stocking et al. Proc. Natl. Acad. Sci. USA, 82: 5746-5750 (1985)). Accordingly, this promoter has been used to express heterologous genes in a number of cell types, including skin fibroblasts (Pamcr et al., Blood, 73: 438-445 (1989), primary hepatocytes (Ponder et al., Hum. Gene Then 2:41-52 (1991), and rodent cells lines and human fibroblast cell lines (van den Wollenberg, Gene 144: 237-241 (1994)). Generally, there are two types of expression vectors suitable for use in eukaryotic cells, retrovirally-based systems and virion-plasmid hybrids described above, van den Wollenberg et al. describe a retroviral vector that comprises the CMV enhancer genetically engineered within the U3 region of the MPSV promoter. However, retroviral vectors have significant drawbacks for use in industrial level protein production. First, the level of protein production is severely hampered by the retroviral packaging sequence, a necessary component of such vectors, as it interferes with translational initiation. Second, protein production is reduced because the transport of retroviral messenger RNA is less efficient than a standard mRNA and there is competition between retroviral packaging and translation. Third, it is impossible to reach the gene copy numbers routinely achieved by standard vectors with an amplifying selection marker, due to the fact that a retroviral vector implants two promoters for each random integration, thus randomly activating downstream sequences with deleterious effects to the cell. Fourth, there are serious safety concerns with large-scale production of retroviral cultures due to random recombination to replication competency. Finally, relrovirally-established cell lines arc harder LO document and less efficient to develop since a viral production cell line must first be used to make a master cell bank, then the actual production cell line is produced, requiring a second round of analysis and banking. Accordingly, industrial production of protein is not routinely performed with retroviral vectors. Thus, the expression of foreign proteins in commercially acceptable quantities remains a challenge. This is especially true in mammalian cell lines. Very often expression of a mammalian protein in a mammalian cell line is required in order to mimic the native form of the protein in all respects: structure, catalytic activity, immunological reactivity, and biological function. Often glycosylation or other post-translational modifications are the key to the production of the desired form of the protein, and bacteria or yeast systems are unable to accomplish these modifications. Thus, there remains a need for improved plasmids that promote the production of mammalian proteins in commercially viable quantities within mammalian host systems. BRIEF SUMMARY OF TFfE INVENTION One aspect of the present invention is a non-retroviral expression vector comprising a cytomegalovirus (CMV) enhancer and a myeloproliferative sarcoma virus (MPSV) promoter. Preferably, the CVIV enhancer is located upstream from the 5'end of the MPSV promoter. Most preferably, the CMV enhancer and MPSV promoter construct comprises the polynucleotide sequence of SEQ ID NO:L The vector of the present invention can further comprise at least one additional clement selected from the group consisting of a consensus Ig intrcm, a tPA pre-proleadcr sequence, a polio IRES, a A CD8 selection marker, and a human growth hormone polyA signal sequence. Preferably, the vector comprises a consensus Ig intron, a tPA pre proleader sequence, and a polio IRES. The vector can also comprise a structural gene, such as prothrombin. A further aspect of the present invention is a mammalian cell transfected with the vector. The mammalian cell of the present invention is preferably a CHO cell, and most preferably a CHO of the strain DXB11. The present invention also encompasses a method of producing a recombinant protein comprising tranyfeclmg a mammalian host cell with the vector of the present invention, growing the cells under conditions that selectively propagates those cells that have integrated the vector into its genome, and growing the cells with the integrated vector under conditions that cause the recombinant protein to be secreted into the cell medium, and isolating the recombinant protein from the cell medium. BRIEF DESCRIPTION OF THE FIGURES Figure 1 shows a plasmid map containing the MPSV/CMV promoter/enhancer of the present invention. Clockwise the plasmid contains the CMV enhancer, the MPSV LTR promoter minus the negative signal sequence, a consensus Ig intron, a tPA pre-pro leader, polio IRES, a A CDS selection marker, a human growth hormone (hGH) poiyA sequence, a dihydrofolate reductase (DHFR) selection cassette with the SV40 promoter/enhancer and SV40 polyA, pUC ori, |3 lactamase selection, yeast CEN/ARS and URA3 selection- This vector has heen named pZMP21. Figure 2 compares the picograms per cell per day (pg/cell-day) of prethrombin production for Chinese Hamster Ovary (CHO) cells transfecled with pZMP20 (CMV promoter/enhancer) or pZMP21 (MPSV promoter/CMV enhancer). DESCRIPTION OF THE INVENTION The present invention fills this need by providing for a novel non-retroviral expression vector, which is able to transfect mammalian cell lines such as Chinese Hamster Ovary Cells (CHO cells) and promote the production of foreign proteins in unexpectedly high quantities. The plasmid of the present invention is comprised of a cytomegalovirus enhancer upstream from the 5' end of a myeloproliferative sarcoma virus (MPSV) promoter. Preferably the MPSV promoter is fused to a cytomegalovirus (CMV) enhancer. 1. Overview SEQ ID NO: 1 shows a CMV enhancer/MPSV LTR promoter construct of the present invention. The CMV enhancer extends from nucleotide 1 to and including nucleotide 374 of SEQ ED NO: 1. The MPSV LTR promoter extends from nucleotide 375 to and including nucleotide 851. 2. Definitions In the description that follows, a number of terms are used extensively. The following definiiions arc provided to facilitate understanding of the invention. As used herein, "nucleic acid" or "nucleic acid molecule" refers to polynucleotides, such as deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), oligonucleotides, fragments generated by the polymerase chain reaction (PCR), and fragments generated by any of ligation, scission, endonuclcase action, and exonuclease action. Nucleic acid molecules can be composed of monomers that are naturally occurring nucleotides (such as DNA and RNA), or analogs of naturally occurring nucleotides (e.g., a-enanliomeric forms of naturally-occurring nucleotides), or a combination of both. Modified nucleotides can have alterations in sugar moieties and/or in pyrimidine or purine base moieties. Sugar modifications include, for example, replacement of one or more hydroxy! groups with halogens, alkyl groups, amines, and azido groups, or sugars can be functionalized as ethers or esters. Moreover, the entire sugar moiety can be replaced with sterically and electronically similar structures, such as aza-sugars and carbocyclic sugar analogs. Examples of modifications in a base moiety include alkylated purines and pyrimidines, acylated purines or pyrimidines, or other well-known heterocyclic substitutes. Nucleic acid monomers can be linked by phosphodiester bonds or analogs of such linkages. Analogs of phosphodiester linkages include phosphorothioate, phosphorodithioate, phosphoroselenoatc. phosphorodiselenoale, phosphoroanilothioate, phosphoranilidale, phosphoramidate, and the like. The term 'nucleic acid molecule" also includes so-called "peptide nucleic acids," which comprise naturally occurring or modified nucleic acid bases attached to a polyamide backbone. Nucleic acids can be either single stranded or double stranded. The term "complement of a nucleic acid molecule" refers to a nucleic acid molecule having a complementary nucleotide sequence and reverse orientation as compared to a reference nucleotide sequence. The term "contig" denotes a nucleic acid molecule that has a contiguous stretch of identical or complementary sequence to another nucleic acid molecule. Contiguous sequences are said to "overlap" a given stretch of a nucleic acid molecule cither in their entirety or along a partial stretch of the nucleic acid molecule. The term "structural gene" refers to a nucleic acid molecule that is transcribed into messenger RNA (mRNA), which is then translated into a sequence of amino acids characteristic of a specific polypeptide. An "isolated nucleic acid molecule" is a nucleic acid molecule that is not integrated in the genomic DNA of an organism. For example, a DNA molecule that encodes a growth factor that has been separated from the genomic DNA of a cell is an isolated DNA molecule. Another example of an isolated nucleic acid molecule is a chemically-synthesized nucleic acid molecule that is not integrated in the genome of an organism. A nucleic acid molecule that has been isolated from a particular species is smaller than the complete DNA molecule of a chromosome from that species. A "nucleic acid molecule construct" is a nucleic acid molecule, either single- or double-stranded, that has been modified through human intervention to contain segments of nucleic acid combined and juxtaposed in an arrangement not existing in nature. "Linear DNA" denotes non-circular DNA molecules having free 5' and 3' ends. Linear DNA can be prepared from closed circular DNA molecules, such as plasmids, by enzymatic digestion or physical disruption. "Complementary DNA (cDNA)" is a single-stranded DNA molecule that is formed from an mRNA template by the enzyme reverse transcriptase. Typically, a primer complementary to portions of mRNA is employed for the initiation of reverse transcription. Those skilled in the art also use the term "cDNA" to refer to a double-stranded DNA molecule consisting of such a single-stranded DNA molecule and its complementary DNA strand. The term "cDNA" also refers to a clone of a cDNA molecule synthesized from an RNA template. A "promoter" is a nucleotide sequence that directs the transcription of a structural gene. Typically, a promoter is located in the 5' non-coding region of a gene, proximal to the transcriptional start site of a stnictural gene. Sequence elements within promoters that function in the initiation of transcription are often characterized by consensus nucleotide sequences. "These promoter elements include RNA polymerase binding sites, TATA sequences, CAAT sequences, differentiation-specific elements [DSEs; McGehee e.t al, Mol. Endocrinol. 7:551 (1993)], cyclic AMP response elements (CREs), serum response elements [SREs; Trcisman, Seminars in Cancer Bio!. 1:41 (1990)], glucocorticoid response elements (GREs), and binding sites for other transcription factors, sucli as CRE/ATF [O'Reilly el al, J. Biol. Che.m. 267:19938 (1992)], AP2 [Yeetal.,.1 Biol Chem. 269:25728 (1994)], SP1, cAMP response element binding protein [CREB; Loeken, Gene Expr. 3:253 (1993)] and octamer factors [see, in general, Watson et al, eds., Molecular Biology of the Gene, 4th ed. (The Bcnjamin/Cummings Publishing Company, Inc. 1987), and Lemaigre and Rousseau, lliochvm. J. 303:1 (1994)]. If a promoter is an inducible promoter, then the rate of transcription increases in response to an inducing agent. In contrast, the rate of transcription is not regulated by an inducing agent if the promoter is a constitutive promoter. Repressible promoters are also known. A "core promoter" contains essential nucleotide sequences for promoter function, including the TATA box and start of transcription. By (his definition, a core promoter may or may not have detectable activity in the absence of specific sequences that may enhance the activity or confer tissue specific activity. A "regulatory element" is a nucleotide sequence that modulates the activity of a core promoter or increases the translation of the mRNA product that results from transcription driven by the core promoter. For example, a regulatory element may contain a nucleotide sequence that binds with cellular factors that increases transcription over basal levels or imparts transcription exclusively or preferentially in particular cells, tissues, or organelles. Other regulatory elements increase translation of the mRNA message that results because of sequences that arc now included in the message, such as an IRES (due to increased ribosome entry) or a poly-A tail (due to increased mRNA stability). An "enhancer" is a type of regulatory element that can increase the efficiency of transcription, regardless of the distance or orientation of the enhancer relative to the start site of transcription. "Heterologous DNA" refers to a DNA molecule, or a population of DNA molecules, that does not exist naturally within a given host cell. DNA molecules heterologous to a particular host cell may contain DNA derived from the host cell species (i.e., endogenous DNA) so long as that host DNA is combined with non-host DNA (i.e., exogenous DNA). For example, a DNA molecule containing a non-host DNA segment encoding a polypeptide operably linked to a host DNA segment comprising a transcription promoter is considered to be a heterologous DNA molecule. Conversely, a heterologous DNA molecule can comprise an endogenous gene operably linked with an exogenous promoter, As another illustration, a DNA molecule comprising a gene derived from a wild-type cell is considered to be heterologous DNA if thai DNA molecule is introduced into a mutant cell that lacks the wild-type gene. A "polypeptide" is a polymer of amino acid residues joined by peptide -bonds, whether produced naturally or synthetically. Polypeptides of less than about 10 amino acid residues are commonly referred to as "peptides." A "protein" is a macromolecule comprising one or more polypeptide chains. A protein may also comprise non-peptidic components, .such as carbohydrate groups. Carbohydrates and other non-peptidic substituents may be added to a protein by the cell in which the protei.i is produced, and will vary with the type of cell. Proteins are defined herein in terms of their amino acid backbone structures; substituents such as carbohydrate groups are generally not specified, but may be present nonetheless. A peptide or polypeptide encoded by a non-host DNA molecule is a "heterologous" peptide or polypeptide. An "integrated genetic element" is a segment of DNA that has been incorporated into a chromosome of a host cell after that element is introduced into the cell through human manipulation. Within the present invention, integrated genetic elements are most commonly derived from linearized plasmids that are introduced into the cells by electroporation or other techniques. Integrated genetic elements are passed from the original host cell to its progeny. A "cloning vector" is a nucleic acid molecule, such as a plasmid, cosmid, or bacteriophage that has the capability of replicating autonomously in a host cell. Cloning vectors typically contain one or a small number of restriction endoniiclease recognition sites that allow insertion of a nucleic acid molecule in a determinable fashion without loss of an essential biological function of the vector, as well as nucleotide sequences encoding a marker gene that is suitable for use in the identification and selection of cells transformed with the cloning vector. Marker genes typically include genes that provide tetracycline resistance or ampieillin resistance. An "expression vector" is a nucleic acid molecule encoding a gene that is expressed in a host cell. I'ypically, an expression vector comprises a transcription promoter, a gene, and a transcription tcrminalor. Gene expression is usually placed under the control of a promoter, and such a gene is said to be "operably linked to" the promoter. Similarly, a regulatory element and a core promoter arc operably linked if the regulatory element modulates the activity of the core promoter. A "non-retroviral vector expression vector" is an expression vector that " does not contain a polynucleotide sequence encoding a retroviral packaging element. A "recombinant host" is a cell that contains a heterologous nucleic acid molecule, such as a cloning vector or expression vector. "Integrative transformants" are recombinant host cells, in which heterologous DNA has become integrated into the genomic DNA of the cells. The term "secretory signal sequence" denotes a DNA sequence that encodes a peptide (a "secretory peptide") that, as a component of a larger polypeptide, directs the larger polypeptide through a secretory pathway of a cell in which it is synthesized. The larger polypeptide is commonly cleaved to remove the secretory peptide during transit through the secretory pathway. An "isolated polypeptide" is a polypeptide that is essentially free from contaminating cellular components, such as carbohydrate, lipid, or other proteinaceous impurities associated with the polypeptide in nature. Typically, a preparation of isolated polypeptide contains the polypeptide in a highly purified form, i.e., at least about 80% pure, at least about 90% pure, at least about 95% pure, greater than 95% pure, or greater than 99% pure. One way to show that a particular protein preparation contains an iso:atcd polypeptide is by the appearance of a single band following sodium dodecyl sulfate (SDSJ-polyacrylamide gel electrophoresis of the protein preparation and Coomassie Brilliant Blue staining of the gel. However, the term "isolated" does not exclude the presence of the same polypeptide m alternative physical forms, such as dimers or alternatively glycosylated or derivatized forms. The terms "amino-terminal or N-terminal" and "carboxyl-terminal or C-terminal" are used herein to denote positions within polypeptides. Where the context allows, these terms are used with reference to a particular sequence or portion of a polypeptide to denote proximity or relative position. For example, a certain sequence positioned carboxyl-terminal to a reference sequence wilhin a polypeptide is located proximal to the carboxyl terminus of the reference sequence, but is not necessarily at the carboxyl terminus of the complete polypeptide. The term "expression" refers to the biosynthesis of a gene product. For example, in the case of a structural gene, expression involves transcription of the structural gene into mRNA and the translation of mRNA into one or more polypeptides. The term "complement/anti-complement pair" denotes non-identical moieties that form a non-covalently associated, stable pair under appropriate conditions. For instance, biotin and avidin (or streplavidin) are prototypical members of a complement/anti-complement pair. Other exemplary complement/anti-complement pairs include receptor/Iigand pairs, antibody/antigen (or hapten or epitope) pairs, sense/antisense polynucleotide pairs, and the like. Where subsequent dissociation of the complement/anli-complement pair is desirable, the complement/anti-complement pair preferably has a binding affinity of less than lO5 M"1. "Upstream" and "downstream" arc terms used to describe the relative orientation between two elements present in a nucleotide sequence. An element that is "upstream" of another is located in a position closer to the 5' end of the sequence (i.e., closer to the end of the molecule that has a phosphate group attached to the 5' carbon of the ribosc or dcoxyribose backbone if the molecule is linear) than the other element. An elemenl is said to be "downstream" when it is located in a position closer to the 3' end of the sequence (i.e., the end of the molecule that has an hydroxyl group attached to the 3' carbon of the ribose or deoxyribose backbone in the linear molecule) when compared to the other element. hi eukaryotes, RNA polymerase II catalyzes the transcription of a structural gene lo produce mRNA. A nucleic acid molecule can be designed to contain an RNA polymerase II template in which the RNA transcript has a sequence that is complementary to that of a specific mRNA. The RNA transcript is termed an "anti-sense RNA" and a nucleic acid molecule that encodes the anti-sense RNA is termed an "anti-sense gene." Anti-sense RNA molecules are capable of binding to mRNA molecules, resulting in an inhibition of mRNA translation. Due to the imprecision of standard analytical methods, molecular weights and lengths of polymers are understood to be approximate values. When such a value is expressed as "about" X or "approximately" X, the stated value of X will be understood to be accurate to +KKL Polynucleotides, generally a cDNA sequence, of the present invention encode the described polypeptides herein. A cDNA sequence which encodes a polypeptide of the present invention is comprised of a series of codons, each amino acid residue of the polypeptide being encoded by a codon and each codon being comprised of three nucleotides.. The amino acid residues are encoded by their respective codons as follows. Alanine (Ala) is encoded by GCA, GCC, GCG or GCT; Cysteine (Cys) is encoded by TGC or TGT; Aspartic acid (Asp) is encoded by GAC or GAT; Glutamic acid (Glu) is encoded by GAA or GAG; Phenylalanine (Phe) is encoded by TTC or TTT; Glycine (Gly) is encoded by GGA, GGC, GGG or GGT; Histidinc (His) is encoded by CAC or CAT; holeucme (lie) is encoded by ATA, ATC or ATT; Lysine (Lys) is encoded by AAA, or AAG; Leucine (Leu) is encoded by TTA, TTG, CTA, CTC, CTG or CTT; Methionine (Met) is encoded by ATG; Aspiiraginc (Asn) is encoded by AAC or AAT; Proline (Pro) is encoded by CCA, CCC, CCG or CCT; Glutamine (Gin) is encoded by CAA or CAG; Arginmc (Arg) is encoded by AGA, AGG, CGA, CGC, CGG or CGT; Serine (Ser) is encoded by AGC, AGT, TCA, TCC, TCG or TCT; Threonine (Thr) is encoded by ACA, ACC, ACG or ACT; Valine (Val) is encoded by GTA, GTC, GTG or GTT; Tryptophan (Tip) is encoded by TGG; and Tyrosine (Tyr) is encoded by TAC or TAT. It is to be recognized that according to the present invention, when a polynucleotide is claimed as described herein, it is understood that what is claimed are both the sense strand, the anti-sense strand, and the DNA as double-stranded having both the sense and anti-sense strand annealed together by their respective hydrogen bonds. -Also claimed is the messenger RNA (mRNA) that encodes the polypeptides of the president invention, and which mRNA is encoded by the cDNA described herein. Messenger RNA (mRNA) will encode a polypeptide using the same codons as those defined herein, with the exception that each thymine nucleotide (T) is replaced by a uracil nucleotide (Uj. 3. Detailed Description The vector of the present invention can be used to produce polypeptides having value in industry, therapeutics, diagnostics, or research. Illustrative proteins include antibodies and antibody fragments, receptors, immunomodulators, hormones, and the like. For example, the expression vector can include a nucleic acid molecule that encodes a pharmaceutically active molecule, such as prethrombin, Factor Vila, proinsulin, insulin, follicle stimulating hormone, tissue type plasminogen activator, tumor necrosis factor, intcrlcukins (e.g., interleuldn-1 (IL-1), 1L-2, FL-3, FL-4, FL-5, EL-6, IL-7, IL-8, FL-9, IL-10, IL-U, IL-12, 1L-13, IL-14, IL-15, IL-16, IL-17, 1L-1S, and IL-19), colony stimulating factors (e.g., granulocyte-colony stimulating factor, and granulocyte macrophage-colony stimulating factor), interferons (e.g., interferons-a, -P, -y, -to, -8, -T, and -e), a stem cell growth factor, erythropoietin, and tbrombopoietin. Additional examples of a protein of interest include an antibody, an antibody fragment, an anti-idiotype antibody (or, fragment thereof), a chimeric antibody, a humanized antibody, an antibody fusion protein, and the like. An example of such an antibody fusion protein would be a fusion of the extracellular portion of the transmembrane activator and CAML-intcractor (TACI) protein, such as amino acids 30-110, fused to the Fc portion of human IgGl. The Fc ponion can be the native sequence, or one that has been mulated to remove the, immunoglobulin effector functions. Examples of these mutations include changes at amino acids 234, 235, 237, 330 and 331 of (he IgGl Fc sequence. The vectors of (he present invention have been found to produce these proteins of interest at higher than expected levels. Without being bound by theory, it is anticipated that the greater than average protein expression displayed by the vectors of the present invention is due, at leas! in part, to the greater than average stability of expression exhibited by this vector when integrated into the genome of a mammalian host cell. The gene of interest can be isolated from genomic or cDNA sequences using methods well known to one of ordinary skill or chemically synthesized. If chemically synthesized and double stranded DNA is required, then each complementary strand is made separately. The production of short genes (60 to 80 base pairs) is technically straightforward and can be accomplished by synthesizing the complementary strands and then annealing them. For the production of longer genes (>300 base pairs), however, special strategies may be required, because the coupling efficiency of each cycle during chemical DNA synthesis is seldom 100%. To overcome this problem, synthetic genes (double-stranded) are assembled in modular form from single-stranded fragments that arc from 20 to 100 nucleotides in length. One method for building a synthetic gene requires the initial production of a set of overlapping, complementary oligonucleotides, each of which is between 20 to 60 nucleotides long. The sequences of the strands arc planned so that, after annealing, the two end segments of the gene are aligned to give blunt ends. Each interna! section of the gene has complementary 3' and 5' terminal extensions that are designed to base pair precisely with an adjacent section. Thus, after the gene is assembled, the only remaining requirement to complete the process is to seal the nicks along the backbones of the two strands with T4 DNA Hgase. In addition to the protein coding sequence, synthetic genes can be designed with terminal sequences that facilitate insertion into a restriction endonudcase sites of a cloning vector and other sequences should also be added that contain signals for the proper initiation and termination of transcription and translation. An alternative way to prepare a full-size gene is to synthesize a specified set of overlapping oligonucleotides (40 to 100 nucleotides). After the 3' and 5' extensions (6 to 10 nucleotides) arc annealed, large gaps still remain, but the base-paired regions are both long enough and stable enough to hold the structure together. The duplex is completed and tire gaps filled by enzymatic DNA synthesis with E. coli DNA polymerase I. This enzyme uses the 3'-hydroxyl groups as replication initiation points and the single-stranded regions as templates. After the enzymatic synthesis is completed, the nicks are sealed with T4 DNA ligase. For larger genes, the complete gene sequence is usualiy assembled from double-stranded fragments that are each put together by joining four to six overlapping oligonucleotides (20 to 60 base pairs each). If there is a sufficient amount of the double-stranded fragments after each synthesis and annealing step, they arc simply joined to one another. Otherwise, each fragment is cloned into a vector to amplify the amount of DNA available. In both cases, the double-stranded constructs are sequentially linked to one another to form the entire gene sequence. Each double-stranded fragment and the complete sequence should be characterized by DNA sequence analysis to verify that the chemically synthesized gene has the correct nucleotide sequence. For reviews on polynucleotide synthesis, see, for example, Glick and Pasternak, Molecular Biotechnology, Principles and Applications of Recombinant DNA {ASM Press 1994), Itakura et at., Annu. Rev. Biochem. 53:323 (1984), and Climie etal., Proc. Nat'lAcacL Sci. USA 87:633 (1990). Expression vectors that arc suitable for production of an amino acid sequence of interest in eukaryotic cells typically contain (1) eukaryotic or viral DNA elements that control initiation and level of transcription, such as a promoter and an enhancer; (2) DNA elements that control the processing of transcripts, such as a transcription tcrmination/polyadenylation sequence; and (3) one or more selectable marker gene(s) and other sequences useful for stable gene expression for all anticipated host cells. Expression vectors can also include nucleotide sequences encoding a secretory sequence that directs the heterologous polypeptide into the secretory pathway of a host cell. To express a gene of interest or a selectable marker gene, a nucleic acid molecule encoding the amino acid sequence must be operably linked to regulatory sequences that control transcriptional expression and then, introduced into a host cell. The vector of the present invention comprises the MPSV promoter with the CMV enhancer in a 5' position to the promoter. MPSV is a member of the Moloney murine sarcoma virus family (Mo-MuSV) and can transform fibroblasts in vitro and cause sarcoma in vivo. Additionally, MPSV causes an acute myeloprolcralivc disease in adult mice. The mos oncogene, which is a component of the virus genome, is necessary for the virus' transforming function, but it is sequences specific to its long tcrmina) repeat (LTR) thai account for expanded cell target specificity when compared to Mo-MuSV. These additional cell targets makes the MPSV LTR an attractive promoter for mammalian cell line expression. The MPSV LTR is generally defined as nucleotides between —4-1 The second regulatory element of the present invention is the CMV enhancer and can be generally defined as the nucleotides between -118 and -524 5' of the transcription initiation site of the major immediate-early gene of CMV. Preferably, the CMV enhancer has the sequence of nudctodes 1 to 374 of SEQ ID NO; J. The enhancer function of this fragment of the viral genome was discovered based on its ability to produce recombinant viruses when cotransfected with enhancerless SV40 viral genome (Boshart et al, Cell, 41(2):521-30 (1985)). For the vectors of the present invention, this sequence, or functionally fragments thereof, is placed within the vector such that an increase in transcription results when compared to the transcription without the presence of the CMV enhancer. Preferably, this location is 5' of the MPSV promoter sequence. The vector of the present invention can comprise other regulatory elements that can increase the expression of the recombinant protein of interest within mammalian host cells. Among the other regulatory elements that can be included is the transcription enhancer located within the intron of an immunoglobulin gene. Particularly preferred is a consensus Ig intron sequence that comprises sequences that have been optimized for use in mammalian host ceils such as CHO DXB11. A second additional regulatory element is an internal ribosome entry site (ERES), a sequence derived from viral genomes that allows for the translation of a dicistronic message. Particularly preferred is the IRES derived from the polio virus. A third regulatory element is a poly-A signal sequence that results in the addition of adenosine residues on the end of the mRNA message, which increases the message stability. Particularly preferred is the poly-A signal sequence derived from the human growth hormone (hGll) gene sequence. Recombinant host cells can be produced that secrete the ammo acid sequence of interest into surrounding medium. Accordingly, the present invention contemplates .expression vectors comprising a nucleotide sequence that encodes a secretory signal sequence, which is also known as a "signal peptide," a "leader sequence," a "prepro sequence," or a "pre sequence." The secretory signal sequence is operably linked to a gene of interest such that the two sequences are joined in the correct reading frame and positioned to direct the newly synthesized polypeptide of interest into the secretory pathway of the host cell. Secretory signal sequences are commonly positioned 5' to the nucleotide sequence encoding the amino acid sequence of interest, although certain secretory signal sequences may be positioned elsewhere in the nucleotide sequence of interest (see, e.g., Welch et al, U.S. Patent No. 5,037,743; Holland el al, U.S. Patent No. 5,143,830), The present invention can utilize a tissue plasminogen activator (tPA) pre-prolcader derived from the sequence described in U.S. Patent No. 5,641,655. Mutations have been introduced into the pre-proleader so that it is optimized for use within mammalian expression systems. Expression vectors can also comprise nucleotide sequences that encode a peptide tag to aid the purification of the polypeptide of interest. Peptide tags that are useful for isolating recombinant polypeptides include polyHistidine tags (which have an affinity for nickel-chekiting resin), c-rnyc tags, calmodulin binding protein (isolated with calmodulin affinity chromatography), substance P, the RYIRS tag (which binds with anti-RYIRS antibodies), the Gtu-Glu tag, and the FLAG tag (which binds with anti-FLAG antibodies). See, for example, Luo et al, Arch. Biochem. Biophys. 329:215 (1996), Morganti et al, Biotechnol Appl. Biochem. 23:61 (1996), and Zheng et al. Gene 186:55 (1997). Nucleic acid molecules encoding such peptide tags are available, for example, from Sigma-Aldrich Corporation (St. Louis, VIO). A wide variety of selectable marker genes for use in mammalian expression vectors are available (see, for example, Kaufman, Meth. Enzymol iS5:4S7 (1990); Kaufman, Meth. Enzymol. 185:531 (1990)). Selectable marker genes generally confer growth resistance to a chemical or drug, that allow selection of nviiial positive transform an Is in bacterial, yeast, or mammalian host cells. Selectable markers fall into two functional categories: recessive and dominant. The recessive markers are usually genes that encode products that are not produced in the host cells, i.e., host cells that lack the "marker" product or function. Marker genes for thymidine kinase (TK)S dihydrofolale .reductase (DHFR), adenine phosphoribosyl transferase (APRT), and hypoxanthine-guanine phosphoribosyl transferase (HGPRT) are in this category, (see, for example, Srivastava and Schlessinger, Gene 103:53 (1991); Romanos et al-, "Expression of Cloned Genes in Yeast," in DNA Cloning 2: Expression Systems, 2"d Edition, pages 123-167 (1RL Press 1995); Markie, Methods MoL Biol. 54:359 (1996); Pfeifer et al.. Gene i 58:183 (1997); Tucker and Burke, Gene 199:25 (1997); Hashida-Ok-ddoetaL, FEDS Letters 425:111 (1998)). Dominant markers include genes that encode products that confer resistance to growth-suppressing compounds (such as antibiotics or other drugs) and/or permit growth of the host cells in metabolically restrictive environments. Commonly used markers within this category include a mutant DHFR gene that confers resistance to methotrexate; the gpt gene for xanthine-guanine phosphoribosyl transferase, which permits host cell growth in mycophenolic acid/xanthine containing media; and the neo gene for aminoglycoside 3'-phosphotransferase, which can confer resistance to G418, gentamycin, kanamycin, and neomycin. More newly developed markers include resistance to zeoc'm, bleomycin, .blastocidin, and hy gromycm (see, e.g., Gatignol et al, Mo!. Gen. Genet. 207:342 (1987); Drocourt et al, Nucl. Acids Res. i 5:4009 (1990)). The use of selectable markers has been extended beyond isolation of cells that have incoi-porated the vector sequences to selection for cells that are expressing the recombinant protein at a high level. An example of this selection process is co-expression of green fluorescent protein with the recombinant protein. The use of autofluorescent proteins provides a visual mechanism to assess if host cells are overexpressing recombinant protein. Similar selection can be performed with a cell surface protein that can be detected with an antibody (e.g. CD4, CD8, Class I major histocompatibility complex (MHC) protein, etc.). Preferably, the cytoplasmic domain of ) the cell surface protein has been deleted, m order to reduce the cytological effect on the host cell of over-expression of the protein. The expression products of such selectable marker genes can be used to sort trans fee ted cells from untransfected cells by such standard means as FACS sorting or magnetic bead separation technology. Selectable marker genes can be cloned or synthesized using published nucleotide sequences, or marker genes can be obtained commercially. .The present vector preferably utilizes as selectable makers a DHFR cassette with the SV40 promoter/enhancer for use in mammalian host cells, a CD8 A construct (A indicating that the sequence encoding the cytoplasmic domain of the protein has been deleted) to determine recombinant gene expression at the cell surface of mammalian cells, [5 lactamase for use in bacterial host cells, and UR.A3 for use in yeast host cells. A final common component of expression vectors arc sequences that facilitate the replication of the vector in mammalian, yeast, and bacterial hosts such as centromeres, origins of replication, chromatin stability sequences, and the like, that increase the stability of the vector in the host system. For example, the vector of present invention can comprise the pUC origin of replication for use in bacterial host cells and the S. cerevisiae CEN/ARS origin of replication for use in yeast host cells. Chromatin elements that may modulate protein expression levels and/or stability are; locus control regions (LCR), matrix or scaffold attachment regions(MAR or SAR) or insulators. Both during and after construction of the expression vector comprising the amino acid-encoding sequences of interest, the vector is typically propagated in a host cell. Vector propagation can be carried out in a prokaryotic host cell, such as E. colt. Suitable strains of E. coli include BL21(DE3), BL21(DE3)pLysS, BL21(DE3)pLysE, DH1, DH41, DH5, DHM, DH5IF, DH51MCR, DH10B, DH10B/p3, DH11S, C600, HB101, JM101, JM105, JM109, JM110, K38, RR1, Y1088, Y1089, CSH18, ER1451, and ER1647 (see, for example, Brown (cch), Molecular Biology Labfax (Academic Press 1991)). Standard techniques for propagating vectors in prokaryotic hosts are well-known to those of skill in the arl (see, for example, Ausnbel et al. (eds.), Short Protocols in Molecular Biology, 3rd Edition (John Wiley & Sons 1995) ["Ausubel 1995"]; Wu et al., Methods in Gene Biotechnology (CRC Press, Inc. 1997)). Alternatively, vector propagation both during or after vector construction can be carried out in CLikaryotic cells, such as yeast. Yeast species of particular interest in this regard include Saccharoinyces cercvisiae, Pichia pastoris, and Pirhia methanol ica. Methods for transforming .S'. cerevisiae cells with exogenous 1DNA and producing recombinant polypeptides therefrom are disclosed by, for example, Kawasaki, U.S. Patent No. 4,599,311, Kawasaki et al, U.S. Patent No. 4,931,373, Brake, U.S. Patent No. 4,870,008, Welch et al, U.S. Patent No. 5.037,743, and Murray et al, U.S. Patent No. 4,845,075. Transformed cells are selected by phenotype determined by the selectable marker, commonly drug resistance or the ability to grow in the absence of a particular nutrient {e.g., leucine). Transformation systems for other yeasts, including Hansenula polymorpha, Schizosaccharomyces pombe, Kluyveromyces lactis, Kluyveromyces fragilis, Ustilago mayclis, Pichia pastoris. Pic hi a methanolica, Pichia guillermondii and Candida maltosa are known in the art. See, for example, Gleeson et al, J. Gen. Microbiol. 732:3459 (1986), and Gregg, U.S. Patent No. 4,882,279. Ultimately, the amino acid sequence of interest may be expressed in any prokaryotic or eukaryotic host cell as described above. Preferably, using the vector of the present invention, the amino acid sequence of interest is produced by a eukaryotic cell, such as a mammalian cell. Examples of suitable mammalian host cells include African green monkey kidney cells (Veto; ATCC CRL 1587), human embryonic kidney cells (293-HEK; ATCC CRL 1573), baby hamster kidney cells (BHK-21, BHK-570; ATCC CRL 8544, ATCC CRL 10314), canine kidney cells (MDCK; ATCC CCL 34), Chinese hamster ovary cells (CHO-K1; ATCC CCL61; CHO DG44; CHO DXB11 (Hyclone, Logan, UT); see also, e.g., Chasin et al., Som. Cell. Molec. Genet, 72:555, 1986)), rat pituitary cells (GH1; ATCC CCL82), HeLa S3 cells (ATCC CCL2.2), rat hepatoma cells (H-4-II-E; ATCC CRL 1548) SV40-transformcd monkey kidney cells (COS-1; ATCC CRL 1650) and murine embryonic cells (NIH-3T3; ATCC CRL 1658). The CHO strain DXBU is the preferred host cell for protein production utilizing the vector of the present invention. An expression veclor can be introduced into host cells using a variety of standard techniques including calcium phosphate transfection, liposome-mediated transfectinn, microprojectile-mediated delivery, electroporation, and the like. Transfected cells can be selected and propagated to provide recombinant host ceils that comprise the gene of interest slably integrated in the host cell genome. Standard methods for introducing nucleic acid molecules into bacterial, yeast, insect, mammalian, and plant cells aiv; provided, for example, by Ausubel (1995). General methods for expressing and recovering foreign protein produced by a mammalian cell system are provided by, for example, Etcheverry, "Expression of Engineered Proteins in Mammalian Cell Culture," in Protein Engineering; Principles and Practice, Cleland et al. (eds.), pages 163 (Wiley-Liss, Inc. 1996). The present invention, thus generally described, will be understood more readily by reference to the following examples, which are provided by way of illustration and are not intended to be limiting of the present invention. Example 1 Construction of MPSV promoter and pZMP21 The MPSV LTR promoter was constructed synthetically by assembling oligonucleotides in sets of four using PCR. First the oligos were assembled in pairs by PCR:SEQ ED NOs: 4 + 5,6 + 7,8 +9, 10+ 11, 12+ 13, 14+ 15. Then the pairs were assembled into three sets of four oligos SEQ ED NOS: 4 + 5 and 6 + 7, with oligos SEQ ID NOs: 4 and 7 as primers, 8 + 9 and 10 + 11 with oligos 8 and 11 as primers, and 12 + 13 and 14 + 15 with oligos 12 and 15 as primers in PCR reactions. When the three PCR fragments were assembled a smaller than expected product was observed. A new primer, 16, was made to get around the internal repeat that lead to this deletion. The product of 4 + 7 was extended with primers 4 and 16 to make a better overlap with the product of 8 + 15. 4 + 16 and 8 + 15 were assembled with primers 4 and 15 by PCR to make a full length product. The PCR reactions were run as follows: to a 100 p\ final volume was added, 10 \i\ 10X Taq polymerase Reaction Buffer (Perkin Elmer), 8 itl of 2.5 mM dNTPs, 78 ;J,I dH/jO, 2 /ri each of a 20 mM stock solution of the two primers described above, and taq polymerase (2.5 units, Life Technology). An equal volume of mineral oil was added and the reaction was heated to 94°C for 2 minutes, followed by 25 cycles at 94°C for 30 seconds, 45°C for 30 seconds, 72°C for 30 seconds followed by a 5 minute extension at 72°C. Tn the case of the first stage of assembly the pnmers were also the templates of the reaction. For the later steps, 10 /tl of PCR product was used as template for the each level of assembly. Ten rtl of the 100 /xl PCR reaction is run on a 1.0% agarose gel with 1 x TBE buffer for analysis. The remaining 90 (il of PCR reaction is precipitated with the addition of 5 f±\ ] M NaCl and 250 til of absolute cthanol. The plasmid pZMP20 which has been cut with Nhel is used for recombination with the PCR fragment. Plasmid pZMP20 was constructed from pZP9 (deposited at the American Type Culture Collection, 10801 University Boulevard, Manassas, VA 20110-2209, and is designated No. 98668) with the yeast genetic elements taken from pRS316 (deposited at the American Type Culture Collection, 10801 University Boulevard, Manassas, VA 20110-2209, and designated No. 77145), an IRES clement from poliovirus, and the extracellular domain of CD8, truncated at the carboxyl terminal end of the transmembrane domain. pZMP20 is a mammalian expression vector containing an expression cassette having the cytomegalovirus immediate early promoter, immunoglobulin signal peptide intron, multiple restriction sites for insertion of coding sequences, a stop codon and a human growth hormone terminator. The plasmid also has an E. coli origin of replication, a mammalian selectable marker expression unit having an SV40 promoter, enhancer and origin of replication, a DHFR gene, the SV40 terminator, as well as the URA3 and CEN-ARS sequences required for selection and replication in S. cerevisiae. One hundred microliters of competent yeast cells (5. cerevisiae) are independently combined with 10 /xl of the various DNA mixtures from above and transferred to a 0.2 cm electroporation cuvette. The ycast/DNA mixtures arc elcctropulsed at 0.75 kV (5 kV/cm), co ohms, 25 tiF. TO each cuvette is added 600 \i\ of 1.2 M sorbitol and the yeast is plated in two 300 /J.I aliquots onto two URA-D plates and incubated at 30°C. After about 48 hours, the Ura-t- yeast transformants from a single plate are resuspended in 1 ml H20 and spun briefly to pellet the yeast cells. The cell pellet is resuspended m 1 ml of lysis buffer (2% Triton X-100, 1% SDS, 100 mM NaC], 10 mM Tris, pH 8.0, I mM EDTA). Five hundred microliters of the lysis mixture is added to an Eppendorf tube containing 300 /rl acid washed glass beads and 200 iil phenol-chloroform, vortexed for 1 minute intervals two or three times, followed by a 5 minute spin in a Eppendorf centrifuge at maximum speed. Three hundred microliters of the aqueous phase is transferred to a fresh tube, and the DNA precipitated with 600 fil ethanol (EtOH), followed by centrifugal] on for 10 minutes at 4JC. The DNA pellet is resuspended in 10 /il LLC). Transformation of electrocompetent E. coli cells (DH10B, GibcoBRL) is done with 0-5-2. ml yeast DNA prep and 40 ul of DHI0B cells. The cells are elcctropulsed at 1.7 kV, 25 /iF and 400 ohms. Following electroporation, 1 ml SOC (2% Bacto' Tryptone (Difco, Detroit, MI), 0.5% yeast extract (Difco), 10 mM NaCl, 2.5 mM KC1, 10 mM MgC12, 10 mM MgSOt, 20 mM glucose) is plated in 250 til aliquots on four LB AMP plates (LB broth (Lennox), 1.8% Bacto Agar (Difco), 100 mg/L Ampicillin). Individual clones harboring the correct construct are identified by restriction digest to verify the presence of the MPSV promoter and to confirm that the various DNA sequences have been joined correctly to one another. The insert of positive clones are subjected to sequence analysis. Larger scale plasmid DNA is isolated using the Qiagen Maxi kit (Qiagen) according to manufacturer's instruction. pZMP21 was deposited on June 17, 2003 at the American Type Culture Collection (ATCC) 10801 University Boulevard, Manassas, VA 20110-2209, designated as ATCC # . Example 2 Construction of Prethrombin Expression Vectors An expression plasmid containing all or part of a polynucleotide encoding prethrombin is constructed via homologous recombination. A fragment of prethrombin cDNA is isolated using PCR that includes the polynucleotide sequence from nucleotide 1 to nucleotide 1380 of SEQ ID NO: 15 with flanking regions at the 5' and 3' ends corresponding to the vectors sequences Hanking the prothrombin insertion point. The primers for PCR each include from 5' to 3' end: 40 bp of flanking sequence from the vector and 17 bp corresponding to the amino and carboxyl termini from the open reading frame of prethrombin. Ten /AI of the 100 til PCR reaction is run on a 0.8% LMP agarose gel (Seaplaque GTG) with 1 x TBE buffer for analysis. The remaining 90 til of PCR reaction is precipitated with the addition of 5 fil J. M NaCl and 250 pA of absolute ethanol. The plasmids pZMP20 and pZMP21, described in the previous example, which were cut with " Bglll were used for recombination with the PCR fragment. One hundred microliters of competent yeast cells (5. cc.revisiac) are independently combined with 10 /A! of the various DNA mixtures from above and transferred to. a 0.2 cm clcctroporalion cuvette. The yeast/DNA mixtures are electropulsed at 0.75 kV (5 kV/cm), »> ohms, 25 /iF. To each cuvette is added 600 \i\ of 1.2 M sorbitol and the yeast is plated in two 300 til aliquots onfo two URA-D plates and incubated at 30°C. After about 48 hours, the Ura+ yeast transformants from a single plate are resuspended in 1 nil FLO and spun briefly to pellet the yeast cells. The cell pellet is resuspended in 1 ml of lysis buffer (2% Triton X-100, 1% SDS, 100 mM NaCl, 10 mM Tns, pH 8.0, 1 mM EDTA). Five hundred microliters of the lysis mixture is added to an Eppendorf tube containing 300 fil acid washed glass beads and 200 ji\ phenol-chloroform, vortexed for 1 minute intervals two or three times, followed by a 5 minute spin in a Eppendorf centrifuge at maximum speed. Three hundred microliters of the aqueous phase is transferred to a fresh tube, and the DNA precipitated with 600 fil ethanol (EtOH), followed by centrifugation for 10 minutes at 4°C. The DNA pellet is resuspended in 10 ji\ FLO. Transformation of eleclrocompetent E. coli cells (DLI10B, Invitrogen) is done with 0.5-2 ml yeast DNA prep and 40 ul of DH10B cells. The cells arc electropulsed at 1.7 kV, 25 fiF and 400 ohms. Following electroporation, 1 ml SOC (2% Bacto' Tryptone (Difco, Detroit, Ml), 0.5% yeast extract (Difco), 10 mM NaCl, 2.5 mM KC1, 10 mM MgCI2, 10 mM MgS04, 20 mM glucose) is plated in 250 \i\ aliquots on four LB AMP plates (LB broth (Lennox), 1.8% Bacto Agar (Difco), 100 mg/L Ampicillin). Individual clones harboring the correct expression construct for prethrombin arc identified by restriction digest lo verify the presence of the prothrombin insert and to confirm that the various DNA sequences have been joined correctly to one another. The insert of positive clones are subjected to sequence analysis. Larger scale piasmid DNA is isolated using the Qiagen Maxi kit (Qiagen) according to manufacturer's instruction. Example 3 Expression of Prethrombin in protein-free, suspension-adapted CHO cells Serum-free, suspension-adapted CHO DG-44 cells were electroporated mth two of .the plasmids described above: pZMP21-prelhrombin and the control Dlasmid, pZMP20-prethrombin, by the following method. The plasmids were linearized ?y digestion with Pvul, precipitated with sodium acetate and ethanol then rinsed with 70% ethanol and dried. The pellets were resuspended at a concentration of 200 /Kg/100 fi\ 3er electroporation in PFCHO medium supplemented with 4 mM L-Glut, 1% Hypoxanthine/ Thymidine, 1% vitamins, and 1% Na pyruvate (Invitrogen). Cells, growing at log phase, were pelleted and resuspended at 5E6/800 fil per electroporation reaction. The electroporation was performed in a BioRad GencPulser II with Capacitance extender (BioRad, Hercules, CA), at 300 v and 950 jiVd in 4mm cuvettes. The cells were suspended in 25 ml of the medium described above in 125mL shake flasks and put on shakers in cell culture incubators at 37"C, at 80 rpm for 24h to recover. The cells were then pelleted and resuspended at 2.5E5 in selective medium, consisting of PFCHO supplemented with 4 mM L-Glut, 1% vitamins, 1% Na Pyruvate. Cell lines were further cultured in increasing concentrations of methotrexate up to 1 fiM once the cultures were capable of growing in the absence of hypoxanthine/ thymidine supplementation. Once the cultures were growing actively in selection media and the viability had increased to over 95%, cultures were established for harvest and analysis of protein. Cultures were seeded at 5E5/mL at 25 mL in shake flasks, and allowed to grow for 48h then harvested. The supernatants were filtered through 0.22 fim fillers and analyzed by EUSA assay. The ELISA assay was performed using two polyclonal antibodies: capture antibody, sheep anti-humaui prethrombin fragment 2 (Accurate Chemical #20Ii2AP) and detection antibody, sheep anti-human prethrombin-HRP conjugate (Accurate Chemical #20! 10HP). The coating antibody was diluted in 0.1 M Na carbonate pH9.6 at 1 /ig/mL, dispensed into 96 wells and incubated at 4°C overnight. The plates were rinsed five times in wash buffer (PBS plus 0.05% Twccn) and blocked by incubating twices with SuperBlock (Pierce, Rockford, 1L, #37515) 200 /il/wcll 5 minutes at room temperature. The samples and standards were applied to the plate in binding buffer (PBS, 0.05% Tween, 1 mg/mL BSAJ and incubated 1 hour at 37CC. The plates were washed five times in wash buffer and detection antibody diluted to 2 ng/rnL in binding buffer. The detection antibody was applied lo the wells and incubated I h at 37°C. The plates were rinsed five times with wash buffer and the detection reagent, OPD, was applied. OPD was prepared by adding hydrogen peroxide immediately before use according to the manufacturer's instructions (Pierce, Rockford, IL, #34006), 100 (il added to each well, allowed to develop 10 minutes at RT and stopped with 100 fxl per well of 1 N H2S04. Plates were read at 492 nm. The results were calculated via SoftMaxPro. Production rales of prcthrombin by CTIO cell pools was calculated by dividing the preLhrombin titer by the average number of cells and the number of days in culture. These comparative results are shown in a bar graph in Figure 2 and indicate that pZMP21-prethrombin produces approximately 3.6 times the amount of recombinant protein as the pZMP20-prethrombin control. Example 4 Construction of zsig37 Expression Vectors An expression plasmid containing all or part of a polynucleotide encoding zsig37 is constructed via homologous recombination. A fragment of zsig37 cDNA is isolated using PCR that includes the polynucleotide sequence from nucleotide 1 to nucleotide 873 of SEQ ID NO: 16 with flanking regions at the 5' and 3' ends corresponding to the vectors sequences flanking the zsig37 insertion point. The primers for PCR each include from 5' lo 3' end: 40 bp of flanking sequence from the vector and 17 bp corresponding to the amino and carboxyl lermini from the open reading frame of zsig37. Ten jj.\ of the 100 (x\ PCR reaction is run on a 0.8% LMP agarose gel (Seaplaque GTG) with I x TBE buffer for analysis. The remaining 90 fd of PCR reaction is precipitated with the addition of 5 \i\ I M NaCl and 250 \i\ of absolute cthanol. The plasmids pZMP20 and pZMP21, described in the previous example, which were cut with BglP were used for recombination with the PCR fragment. One hundred microliters of competent yeast cells (S. cercvisiae) are independent!; combined with 10 pA of the various DNA mixtures from above and transferred to a 0,2 cm electroporation cuvette. The yeast/DNA mixtures are -electropulsed at 0.75 kV (5 kV/cm), oo ohms, 25 fiV. To each cuvelle is added 600 til of 1.2 M sorbitol and the ye us t is plated in two 300 fi\ aliquots onto two URA-D platen and incubated at 30°C. After about 4S hours, the Ura+ yeast transformants from a single plate are resuspended in 1 ml H20 and spun briefly to pellet the yeast cells. The cell pellet is resuspended in 1 ml of lysis buffer (2% Triton X-100, 1%"SDS, 100 mM NaCl, 10 mM Tris, pll 8.0, 1 mM EDTA). Five hundred microliters of the lysis mixture is added to an Eppendorf tube containing 300 fil acid washed glass beads and 200 ji\ phenol-chloroform, vortexed for 1 minute intervals two or three times, followed by a 5 minute spin in a Eppendorf centrifuge at maximum speed. Three hundred microliters of the aqueous phase is transferred to a fresh tube, and the DNA precipitated with 600 ji\ ethanol (EtOH), followed by centrifugation for 10 minutes at 4°C. The DNA pellet is resuspended in 10 ji\ H2O. Transformation of electrocompctent E. colt cells (D1I10B, fnvitrogen) is done with 0.5-2 ml yeast DNA prep and 40 ul of DH10B cells. The cells are electro pulsed at 1.7 kV, 25/*F and 400 ohms. Following elcctroporafion, 1 ml SOC (2% Bacto' Tryptone (Difco, Detroit, MI), 0.5% yeast extract (Difco), 10 mM NaCl, 2.5 mM KC1, 10 mM MgC12, 10 mM MgS04, 20 mM glucose) is plated in 250 pA aliquots on four LB AMP plates (LB broth (Lennox), 1.8% Bacto Agar (Difco), 100 mg/L Ampicillin). Individual clones harboring the correct expression construct for zsig37 are identified by restriction digest to verify the presence of the zsig37 insert and to confirm that the various DNA sequences have been joined correctly to one another. The insert of positive clones are subjected to sequence analysis. Larger scale plasmid DNA is isolated using the Qiagcn Maxi kit (Quigen) according to manufacturer's instruction. Example 5 Analysis of the stability of production of zsi^37 by cells Iransfected with MPSV vs. CMV expression vectors Serum-free, suspension-adapted CHO DG44 cells are eicctroporated with the plasmids described above, by the following method. The plasmids are linearized by digestion with Pvul, precipitated with sodium acetate and ethanol then rinsed with 70% ethanol and dried. The pellets are resuspended at a concentration of 200 /ig/100 [i\ per eleclroporalion in PFCHO medium supplemented with 4 mM L-Glut, 1% Hypoxantliine/ Thymidine, 1% vitamins, and 1% N;i pyruvate (Invitrogen). Cells, growing at log phase, are pelleted and resuspended at 5E6/S00 p.\ per eleclroporalion reaction. The eleclroporalion is performed in , at 300 v and 950 /iFd in 4mm cuvettes. The cells are suspended in 25 ml of the medium described above in 125mL shake flasks and put on shakers in cell culture incubators al 37°C, at 80 rpm for 24h to recover. The cells are then pelleted and resuspended at 2.5E5 in selective medium, consisting of PFCHO supplemented with 4 mM L-Glut, 1% vitamins, 1% Na Pyruvate. Cell lines are further cultured in increasing concentrations of methotrexate up to I jxM once the cultures are capable of growing in the absence of hypoxanthine/ thymidine supplementation. Once the cultures are growing actively in selection media and the viability has increased to over 95%, cultures are established for harvest and analysis of protein. Cultures are passaged over a period of three months and samples are removed weekly for analysis by ELISA. The supernatants were filtered through 0.22 fim filters and analyzed by ELISA assay. The ELISA assay is performed using two polyclonal antibodies: capture antibody, sheep anti-human zsig37 and detection antibody, sheep anti-human zsig37-HRP conjugate. The coating antibody is diluted in 0.1 M Na carbonate pH9.6 al 1 ftg/niL, dispensed into 96 wells and incubated at 4°C overnight. The plates are rinsed five times in wash buffer (PBS plus 0.05% Tween) and blocked by incubating twiccs with SuperBlock (Pierce, Rockford, IL, #37515) 200 /j.]/well 5 minutes at room temperature. The samples and standards are applied to the plate in binding buffer (PBS, 0.05% Tween. 1 mg/mL BSA) and incubated 1 hour at 37°C. The plates are washed five times in wash buffer and detection antibody diluted to 2 ng/mL in binding buffer. The detection antibody is applied to the wells and incubated 1 h at 37°C. The plates are rinsed five times with wash buffer and the detection reagent, OPD, was applied. OPD is prepared by adding hydrogen peroxide immediately before use according to the manufacturer's instructions (Pierce, Rockford, IL, #3-1006), 100 fil added to each well, allowed to develop 10 minutes al RT and stopped with 100 jil per well of 1 N H2S04. Plates are read al 492 nrn. The results arc calculated via SoftMaxPro. Produclion rates of zsig37 by CHO ceil pools is calculated by dividing the zsig37 titer by the average number of cells and the number of days in culture. The levels of productivity as a function of time arc calculated for the two cultures for comparison. From the foregoing, H will be appreciated that, although specific embodiments .of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims. WE CLAIM: 1. A non-retroviral expression vector comprising a cytomegalovirus (CMV) enhancer and a myeloproliferative sacrcoma virus (MPSV) promoter, wherein the CMV enhancer is located upstream from the 5' end of the MPSV promoter such that the CMV enhancer is located in a position closer to the 5' end of the MPSV promoter than the 3' end of the MPSV promoter. 2. The vector as claimed in claim 1, wherein the CMV enhancer and MPSV promoter comprises the polynucleotide sequence of SEQ ID NO: 1. 3. The vector as claimed in claim 1, wherein it comprises at least one additional element selected from the group consisting of a consensus Ig intron, a tPA pre-proleader sequence, a polio IRES, a A CD8 selection marker, and a human growth hormone polyA signal sequence. 4. The vector as claimed in claim 1, wherein it comprises a consensus Ig intron, a tPA pre-porleader sequence, and a polio IRES. 5. The vector as claimed in claim 2, wherein it comprises a consensus Ig intron, a tPA pre-proleader sequence, and a polio IRES. 6. The vector as claimed in claim 5, wherein it comprises a structural gene such that the gene is operably linked to the CMV enhancer and MPSV promoter. 7. The vector pZMP21 as deposited with the ATCC, having the reference number ATCC PTA - 5266. 8. A method of producing a recombinant protein comprising a. transfecting a mammalian host cell with the vector of claim 1; b. growing the cells to selectively propagates those cells that have integrated the vector of claim 1 into its genome; c. growing the cells of step b) to cause the recombinant protein to be secreted into the cell medium; d. isolating the recombinant protein from the cell medium. 9. The method as claimed in claim 8, wherein the transfection occurs by electroporation. 10. The method as claimed in claim 8, wherein the conditions that selectively propagates cells that have integrated the vector of claim 1 into its genome comprises growing the cells in the presence of methotrexate. 11. A method of producing a recombinant protein comprising a. randomly integrating the vector of claim 6 into the genome of CHO cells; b. growing the cells in the presence of increasing concentrations of methotrexate; c. isolating cells from step b) and growing the CHO cells to produce the recombinant protein into the culture medium; d. isolating the recombinant protein from the culture medium. 12. The method as claimed in claim 11, the CHO cells are of the strain DXB11. 13. A non-retroviral expression vector as claimed in claim 1, wherein the MSPV promoter is fused to the CMV enhancer.

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