Title of Invention	A METHOD OF IDENTIFYING A LIGAND OF A BACTERIAL SIGMA70 SUBUNIT
Abstract	ABSTRACT The invention provides a method of identifying a ligand , in particular an inhibitor, of a bacterial RNA polymerase sigma subunit.

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TECHNICAL FIELD The present invention relates to a method of identifying a ligand, in particular an inhibitor, of a bacterial RNA polymerase sigma subunit. BACKGROUND ART Sigma subunits of RNA polymerase Transcription of genes to the corresponding RNA molecules is a complex process which is catalyzed by DNA dependent RNA polymerase, and iavolves many different protein factors. In eubacteria, the core RNA polymers is composed of a, P, P' subunits in the ratio 2:1:1. To direct RNA polymerase to promoters of specific genes to be transcribed, bacteria produce a variety of proteins, known as sigma (o) factors, which interact with RNA polymerase to form an active holoenzyme. The resulting complexes are able to recognize and attach to selected nucleotide sequences in promoters. Physical measurements have shown that the sigma subunit induces conformational transition upon binding to core RNA polymerase. Binding of the sigma subunit to the core enzyme increases the binding constant of the core enzyme for DNA by several orders of magnitude (Chamberlin, M.J. (1974) Ann. Rev. Biochem. 43,721-). Bacterial sigma factors do not have any homology with eukaryotic transcription factors, and are consequently a potential target for antibacterial compounds. Mutations in the sigma subunit, effecting its association and ability to confer DNA sequence specificity to the enzyme, are known to be lethal to the cell. Characterization of sigma subunits, identified and sequenced from various organisms, allows them to be classified into three groups. The Group I sigma has also been referred to the sigma ^^ class, or the "house keeping" sigma group (for a review see Lonetto et.al (1992) J. Bacterial. 174, 3843-3849). Sigma subunits belonging to this group recognize similar promoter sequences in the cell. These properties are reflected in certain regions of the proteins which are highly conserved between species. Another important feature of sigma-dependent transcription is that the sigma subunit dissociates from the core enzyme during elongation of mRNA. Consequently, molecules which stabilize the interaction of the sigma subunit with the core enzyme would also be capable of inactivating transcription. Mycobacterium tuberculosis. Mycobacterium tuberculosis is a major pulmonary pathogen which is characterized by its very slow growth rate. As a pathogen it gains access to the alveolar macrophages where it multiplies within the phagosome, finally lysing the cells and being disseminated through the blood stream, not only to other areas of the lung, but also to extrapulmonary tissues. The pathogen thus multiplies in at least two entirely different environments, involving the utilization of different nutrients and a variety of possible host factors. A successful infection would thus involve the coordinated expression of new sets of genes, transcribed by RNA polymerase associating with different sigma factors. This opens the possibility of targeting not only the sigma ™ subunits of M. tuberculosis, but also other sigma subunits specific for the different stages of infection and dissemination. The cloning and expression of Mycobacterium tuberculosis sigA and sigB genes are disclosed in the International Patent AppUcation WO 96/38478 (Astra AB). Anti-sigma factors Anti-sigma (Asi) proteins are known in the art. Lysates of bacteriophage T2 (Khesin et. Al (1972) Mol. Gen. Genet 119, 299) or phase T4 (Bogdanova et.al (1970) Mol. Biol, 4,435; Stevens, A (1972) Proc. Natl. Acad. Sci. U.S.A. 69, 603) have been reported to inhibit transcription of bacterial genes. It has been established that the T4-dependent anti-sigma™ activity is borne by a 10 KDa protein (Stevens, A, In: RNA Polymerase p. 617-627 (Eds. R. Losick & M. Chamberlin) Cold Sping Harbor Laboratory, Cold Spring Harbor, N.Y. 1976). A 10 kDa protein was shown to co-purify with RNA polymerase from T4 infected E.coli cells and is detached together with sigma™ from the core enzyme on phosphocellulose columns. A gene called asiA coding for the lOkDa anti-sigma™ factor of bacteriophage T4, has been identified by Orsini et.al (1993) J. Bacteriol. 175, 85-93. The open reading frame encoded a 90 amino acid protein with the deduced sequence MNKNIDTVRE UTVASILIK FSREDIVENR ANFIAFLNEI GVTHEGRKLN QNSFRKIVSE LTQEDKKTLI DEFNEGFEGV YRYLEMYTNK (SEQ JD NO. 5). The asiA-encoded protein was overproduced in a phase T7 expression system and partially purified. It showed a strong inhibitory activity towards sigma™-directed transcription by E.coli RNA polymerase holoenzyme. The nucleotide sequence of gene asiA has been deposited in the GenBank data base imder accession no. M99441. Examples of proteins regulating the sigma subunit of RNA polymerase are known also fi-om other systems. An example is the S. typhimurium flagellar regulation system, which is a complex system controlled by a set of over 50 genes grouped into 13 flagellar operons. Late operon expression is positively regulated by ihefliA gene coding for the sigma factor FliA (Suzuki et.al (1978) J. Bacterial. 133, 904; Suzuki et.al (1981) J. Bacteriol. 145, 1036). On the other hand, the late operons are negatively regulated by the flgM gene. A 7.8 kDa protein has been identified as the yZgM gene product and purified (Ohnishi et.al. (1992) Mol. Microbiol. 6, 3149-3157). This FlgM protein was identified as an anti-sigma factor since it was capable to bind the FliA protein and disturbed its ability to form a complex with the RNA polymerase core enzyme. Sinodlarly, in B. subtilis gene expression, the sigma' factor has been shown to be regulated by a 14 kDa anti-sigma factor encoded by the spoIIAB gene (Duncan & Losick (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 2325-2329), while the sigma^ factor is regulated by a 16 kDa anti-sigma factor encoded by the rsbW gene (Benson & Haldenwang (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 2330-2334). The nucleotide sequences of the geaesflgM, spoIIAB and rsbW are available in the GenBank data base (Accession Nos. FLGMST.PRO for flgM, SPOIIAB.PRO for spoIIAB, M34995. PRO for rsbW). The sequences do not show any gross similarity with the asiA sequence disclosed by Orsini et.al. WO 96/25170 discloses methods for treating diseases caused by bacterial pathogens by administering the T4 AsiA protein, which protein is shown to inhibit the RNA polymerase activity of Bacillus subtilis and Mycobacterium smegmatis. DISLCOSURE OF THE INVENTION In accordance with the present invention, there is provided a method of identifying a ligand of a bacterial sigma™ subxinit which comprises contacting the sigma™ subunit or a portion thereof comprising the anti- sigma binding region, with a test compound and a fusion protein of an anti-sigma™ factor of bacteriophage T4, and determining whether the test compoimd binds competitively with the anti- sigma™ factor to the sigma™ subunit or portion thereof The test compound can be any test compoimd to identify whether the same is a ligand of a bacterial Sigma™ subunit. Preferably, the method comprises: (i) immobilizing the sigma™ subunit or portion thereof on a matrix or solid support; (ii) adding the test compound and the fusion protein; (iii) adding a first antibody against the fusion protein; (iv) adding a labeled second antibody against the fu-st antibody; and (v) determining the amount of second antibody bound to the (first antibody-fusion protein - sigma™ subimit or portion thereof) complex formed on the matrix or solid support. The invention provides a method of screening peptide libraries or chemical libraries for compounds (ligands) which mimic the anti- sigma™ factor and which thereby have the capability of inhibiting the interaction of core RNA polymerase and sigma™ subunits. Such inhibitor compounds are potentially useful in the treatment of bacterial infections. The capability of the identified ligands to inhibit the interaction between a bacterial core RNA polymerase and a sigma™ subunit can be tested by known methods, in particular by a RNA polymerase assay, such as that disclosed in Orsini et.al (supra), wherein transcription of a DNA template, such as calf thymus DNA, T4 DNA or poly (dA-dT), is measured by the incorporation of a labeled RNA precursor such as [5-^H]UTP. In the method according to the invention, the sigma™ subunit or portion thereof is preferably obtained fcom Escherichia coli (particularly the C-terminal 99 amino acids of the sigma™ subunit containing the anti-sigma binding region) or Salmonella typhimurium. In the fusion protein, the anti- sigma™ factor of bacteriophage T4 will preferably have an amino acid sequence as shown in SEQ ID NO. 1 or SEQ ID N0.2. The fusion protein preferably also comprises glutathione-S-transferase (GST-Asi) or any other suitable marker gene protein product. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 Map of plasmid vector pARC 8112. Fig.2 Map of plasmid vector pARC 8100. Fig.3 Map of plasmid vector pARC 8115. Fig.4 Map of plasmid vector pARC 8101. Fig.5 Map of plasmid vector pARC 8114. Fig.6 Map of plasmid vector pARC 8105. Fig.7 Map of plasmid vector pARC 8180. Fig. 8 Activity of GST-Asi protein on sigma™-dependent transcription. Assay: O.Sjig E.coli core polymerase, 2.5 jig sigma™ protein, 1.0 ^g T4 DNA and NTPs. Varying concentrations of GST-Asi was preincubated with sigma protein before addition to the reaction mixture. ^H-UTP specific activity was 2800-3000 cpm/nmole. EXAMPLES Throughout this description the terms "standard protocols" and "standard procedures", when used in the context of molecular cloning techniques, are to be understood as protocols and procedures found in an ordinary laboratory manual such as : Current Protocols in Molecular Biology, editors F. Ausubel et.al., John Wiley and Sons, Inc. 1994, or Sambrook, J. Fritsch, E.F. and Maniatis T. Molecular Cloning: A laboratory manual, 2"^ Ed..Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY 1989. EXAMPLE 1 : Cloning, expression and purification of E.coli sigma™ PCR primers corresponding to the 5' - and 3'-ends of the coding sequences of E.coli sigma were designed with the 5'-sequence including a site for the restriction enzyme EcoRI and the 3'-sequence including a site for the restriction enzyme Sail. The PCR amplified DNA fragment was restricted with the above mentioned restriction enzymes and cloned into the EcoRI- Sail sites of the expression vector pTrc99a (Amann et.al. (1988) Gene 69, 301), whereafter the ligation mix was transformed into E.coli DH5a. Transformants harboring the recombinant plasmid with the expected restriction profile were identified. E.coli DH5a cells harboring the recombinant plasmid, labeled pARC 8112 (Fig.l) were grown at +37°C in LB till an OD (600 nm) of 0.4 and induced with 1 mM IPTG. More than 50% of the 90 kDa sigma™ protein was found as inclusion bodies as had been reported earlier (Borukhov & Goldfarb (1993) Protein expression and purification 4,503). The overexpressed sigma™ was purified and renatured following standard protocols. The activity of the purified sigma'" protein was confirmed by its ability to support transcription mediated by E.coli RNA polymerase core enzyme. E.coli RNA polymerase core and holoenzyme forms were purified following the protocol of Burgess and Jendriask (1975) Biochemistry 14, 4634-4638. Polymerase activity using T4 DNA as template was assayed as described by Orsini et.al. (1993) J.Bacteriol. 175, 85-93. EXAMPLE 2: Cloning, expression and purification of Salmonella typhimurium sigma™ The coding sequence of the sigma^" of S. typhimurium was amplified using the forward primer shown as SEQ ID NO: 3 and the reverse primer shown as SEQ.ED NO: 4. The forward primer includes the site for the restriction enzyme EcoRI, while the reverse primer includes the sequence encoding the stop codon and a site for the restriction enzyme Sail . S. typhimurium DNA was used as template and the amplified fi^gment digested with appropriate enzymes. This Augment was then ligated to EcoRI - Sail digested pTrc 99A (Amann et.al (1988) Gene 69, 301) and transformed into E.coli DH5a to obtain the recombinant plasmid pARC 8118. Plasmid DNA of pARC 8118 was digested with EcoRI-Hindm and the released 2.3 kb DNA Augment encompassing the entire coding sequence of 5. typhimurium sigma™ was cloned into the EcoRI-Hindm digested pRSET B (KroU et.al. (1993) DNA and Cell Biol. 12, 441) vector which has not only the T7 promoter but also sequences encoding a his tag which is now fused at the C-terminus of S. typhimurium sigma™. The recombinant plasmid encoding the S. typhimurium sigma™ fused to a His tag was labeled pARC 8133. Plasmid DNA of pARC 8133 was then used to transform the expression host E.coli BL21 (DE3) and transformants obtained at +37°C. Cells harboring pARC 8133 were grown in LB at +37°C till an OD of 0.4 at 600 nm and induced with ImM IPTG for 4 hours. The cells were pelleted, suspended in buffer and sonicated, according to standard methods. The lysed cells were clarified by centrifugation at 45,000 rpm for 60 min at +4°C and the clarified supernatant was loaded directly on Ni-agarose (Pharmacia) and washed with 50mM imidazole. The boxmd protein was eluted with 200 mM imidazole and dialyzed overnight at +4°C against 50 mM Tris-Cl pH 7.5. The eluate was concentrated using Amicon cone filters to 1/10 volume. The concentrated protein was cleaved with enterokinase, used at a ratio of 1:50 of the enzyme at +25°C for 14 hours. The cleaved mixture was passed through a Ni^"^ agarose colimm and the unboimd material collected. The cleaved protein was analysed for homogeneity by SDS-PAGE. EXAMPLE 3 : Toxicity of the anti-sigma protein 3.1 Cloning of the osiA gene The coding sequence for the asiA gene (Orsini et.al., supra) was amplified from the genomic DNA of bacteriophage T4 by PCR using the forward primer shown as SEQ ID NO: 5 and the^ reverse primer designated shown as SEQ ID NO.: 6. The 5'-primer included the sequence of the restriction enzyme Ncol while the 3'-primer included the sequence for the restriction enzyme BaniHl. The coding sequence was amplified by PCR following standard protocols and the amplified firagment ligated to 'Ncol-BamlO. restricted pBR329 and transformed into competent cells E.coli DH5a. Recombinants were selected at +37°C as chloramphenicol sensitive, ampicillin resistant transformants. One of the transformants having the desired restriction pattern was labeled pARC 8100 (Fig.2). The inclusion of the sequence encoding Ncol resulted in the change of the second amino acid fi-om asparagine to glycine (SEQ ID N0.2). The nucleotide sequence of the asiA gene cloned in pARC 8100 was verified by double stranded sequencing and found to be identical to the asiA sequence as disclosed by Orsini et.al., except for the expected change of codon for the second amino acid as a result of the PCR cloning protocol used. 3.2 In vivo toxicity of asiAproduct in E.coli (A) The Ncol - BamYD. DNA fragment from pARC 8100 was ligated to pET 8c Km (Umender K. Sharma et.al., J. Bact., 177. 6745 (1996) and kanamycin resistant transformants with E.coli DH5a were selected at +37°C. One of the transformants harboring a plasmid with the expected restriction enzyme profile was labeled pARC 8115 (Fig.3). pARC 8115 plasmid DNA was then used to transform the expression host E.coli BL21 (DE3) (Studier et.al., supra) and transformants selected both at +37°C and at +30°C. However, no viable transformants could be obtained at either of the temperatures. The leaky expression from the T7 promoter in pET 8c being much higher than from pET lld(Km) (Studier et.al., supra), the toxicity of the asiA product could explain the non-transformability. Transformants could however be obtained using E.coli BL21(DE3)/pLysS as host, in which the leaky expression is additionally repressed by the T7 lysozyme expressed from pLysS. (B) The Ncol - BamHI 284bp DNA fragment was obtained from pARC 8100 and ligated to the Ncol - BamHI sites of pET lid Km (Umender K. Sharma et.al., J. Bact. 177, 6745 (1996)) and transformed into E.coli DH5a. Transformants were selected for kanamycin resistance. Plasmid preparations from individual transformants were digested with restriction enzymes and the correct transformant that released the 284 bp fragment after Ncol-Bamffl digest was labeled pARC 8101 (Fig.4). The transformants were selected at +37°C and appeared normal. pARC 8101 DNA was then used to transform the expression host E.coli BL26(DE3). BL26 is a lac Iq isogenic strain of BL21 (Studier et.al., supra). Transformants selected for kanamycin both at +37'C and at +30°C. The transformants obtained at +37°C were morphologically sick and non¬viable indicating the acute toxicity of the leaky expression of the asiA gene. In contrast, healthy colonies could be obtained when transformants were selected at +30°C where the leaky expression from the T7 promoter can be expected to be negligible. BL26(DE3)/pLysS colonies were healthy both at +37°C and +30°C, indicating that the tight regulation of expression in this strain made the asLA. gene non-toxic to the host. Transformants obtained from DH5a were also healthy both at +37°C and +30°C, indicating that the gene when transformed into a non-expression host was non-toxic. (C) The asiA. gene was excised from pARC 8101 as a Xbal - BamHl DNA fragment to include the sequence for the ribosome binding site and ligated to the low copy vector, Xbal — BamHI cleaved pWKS129 (Wang & Kushner (1991) Gene 100, 195) and the ligation mix transformed to E.coli DH5a. Recombinant plasmid harboring the asiA gene was identified by restriction profile and labeled pARC 8114 (Fig.5). This plasmid DNA when transformed into E.coli BL21 (DE3) gave viable transformants both at +37°C and +30°C, since the low copy mmiber of the plasmid reduced the level of the AsiA protein expressed through leaky expression. EXAMPLE 4: Glutathione-S-transferase - Anti-sigma fiision protein 4.1 Cloning affusion protein in E. coli The coding sequence for the asiA gene was excised as an Ncol - BaniHl fi°agment from pARC 8100 and ligated to Ncol - Bamm cleaved pARC 0499. The plasmid pARC 0499 has a Ncol site in frame with the glutathione-S-transferase encoding sequence, enabling fusion of the N-terminus of asiA sequences. The ligation mix was transformed into E.coli DH5a and transformants selected at +37°C and +30°C. Viable colonies were obtained at both temperatures indicating the N-terminal fusion reduces the toxicity of the protein. The recombinant plasmid obtained with the sequences encoding GST-AsiA was labeled pARC 8105 (Fig.6). The GST-AsiA fusion protein was purified as follows: E. coli DH5a harboring pARC 8105 was grown in LB till an OD at 600 nm of 0.6 at +37°C followed by the addition of 1 mM IPTG and allowing growth for a further period of 2 hours. The cells were then centrifuged at 5000 rpm for 10 minutes. The pelleted cells were suspended in Buffer A (phosphate buffered saline (PBS) pH 7.5, 5 Jig / ml aprotinin, 5 jig / ml leupeptin) and sonicated. The sonicate was clarified at 45,000 rpm for 10 minutes at +4°C and the supernatant passed through a Glutathione-Sepharose 4B column previously equilibrated with PBS. The column was washed with 5 bed volumes of 1 M urea once followed by washing with PBS (10 bed volumes). The bound protein was eluted with 10 mM glutathione and the eluate dialysed overnight against buffer containing 50 mM Tris (hydroxymethyl) aminomethane hydrochloride (Tris HCl), pH 7.5. The dialysed protein was then concentrated to 1/10 vol using Amicon cone filters. The concentrated protein was then treated with 1:50 ratio of Factor Xa protease in a buffer containing 50mM Tris HCl, pH 7.5, 100 mM NaCl, ImM CaCl2 at +25°C for 4 hours. The digested elute was then passed through a Glutathione-Sepharose column and the imbound fi:action collected and analyzed for the purity of the AsiA protein by SDS-PAGE. 4.2 Cloning affusion protein in yeast The expression and purification of the GST-AsiA fusion protein in E.coli did not allow large scale production because of the inherent toxicity of the fusion protein. To obtain large quantities of the AsiA protein, the possibility of expressing the GST-AsiA fusion protein in a yeast host was investigated. As the Saccharomyces species do not have a sigma^" homologue, the AsiA protein was expected not to have toxicity effects on the transcription apparatus of the host and thus for initial experiments a Saccharomyces cerevisiae expression system was chosen. The gene encoding the GST-AsiA fusion protein on the plasmid pARC 8105 was amplified using the forward primer shown as SEQ ID NO: 7 and the reverse primer shown as SEQ ID NO. 8. The amplified 1.0 kb fi^gment was restricted with BglU. and HindQI which are the sites introduced by the primer sequence at the 5' - and 3'- ends of the amplified PCR Augment. The BglU. and Hindin restricted fragment was ligated into the EgM and HindTTT restricted Saccharomyces cerevisiae cloning vector pSW6 (Pascall et.al. (1991) J. Mol. Endocrinol 6, 63-70). The ligation mixture was transformed into the Saccharomyces cerevisiae host CGY1585 and transfonnants selected on leucine (3 |ig / ml). Individual transfonnants from both reaction mixtures were screened for the presence of plasmid with the expected restriction profile. The recombinant plasmid obtained with the vector PSW6 was labeled pARC 8180 (Fig.7). S. cerevisiae CGY 1585 strain harbouring pARC 8180 were grown using standard procedures and lysed by passing the concentrated cell suspension through a French Press. The lysate was then centrifuged at 10,000 rpm and the supernatant applied to a Glutathione-Sepharose column (Pharmacia) and then fusion protein purified following standard protocols. The purified GST-AsiA fusion protein was cleaved with Factor Xa as recommended and the AsiA protein separated. The yield of the purified AsiA protein was 500 \ig /I. The activity of the GST-AsiA protein, expressed and purified fi-om Saccharomyces, was compared to that obtained fi-om E.coli and found to be identical in its ability to inhibit sigma™ mediated transcription of T4 template (Fig.8). AsiA protein purified firom GST-AsiA after Factor Xa cleavage also inhibited E.coli sigma^° dependent transcription. The ability to overexpress and purify large quantities of the AsiA protein in Saccharomyces without toxicity problems for the host also indicates that the AsiA protein does not have a corresponding sigma™ homologue in this species. As the Saccharomyces RNA polymerase is similar to that of higher eukaryotes it also substantiates the fact the AsiA protein is toxic only to the prokaryotic transcription apparatus. In order to test the feasibility of expressing AsiA in alternative yeast expression systems, Pichia Pastoris expression system was chosen. The gene encoding GST-AsiA was amplified by PCR from pARC 8105 and cloned into pPIC9k (Invitrogen). 5' primer (asi34) - TA TAG GTA TCC CCT ATA CTA GGT TAT TGG 3' primer (asi35) - T TGC GGC CGC TTA TTT GTT CGT ATA CAT PCR conditions used were melting temperature 94°C annealing temperature 50°C extension temperature 72°C and 30 cycles were performed. Ikb PCR product was cloned as SnaBI-NotI fragment into pPIC9K (pARC 8274). Pichia Pastoris transformants of pARC 8274 were selected based on G 418 resistance. Expression studies indicated that GST-AsiA was secreted into the culture supernatant. GST-AsiA purified from the culture supernatant was found to inhibit in vitro transcription as efficiently as that of either E.coli or the Saccharomyces GST-AsiA. This indicates that it is feasible to produce recombinant GST-AsiA in a yeast expression system. EXAMPLE 5 : In vitro Asia-sigma™ interaction assays 5.1 RNA polymerase assay The E.coli RNA polymerase assay was standardized following the protocol of Orsini et al. (J. Bacteriol 175, 85-93, 1993) using T4 phage DNA as template to quantify sigma™ dependent transcription. E.coli RNA polymerase core enzyme was purified following the protocol of Burgess and Jendriask (1975) Biochemistry 14,4634-4638. 5.2 Inhibition of E.coli transcription by AsiA protein Increasing concentrations of AsLA protein expressed in S. cerevisiae was added to the reaction mixture and the transcription mediated by the E.coli RNA polymerase quantified. The results showed that increasing concentrations of AsiA protein completely inhibited transcription mediated by E.coli sigma™ 5.3 Inhibition of E.coli transcription by GST-AsiA fusion protein The GST-AsiA fusion protein was purified from S. cerevisiae as described in Section 5.2 and used at different concentrations in the E.coli sigma^" mediated RNA polymerase transcription assay, using phage T4 DNA as template. The GST-AsiA fusion protein inhibited >80% the sigma™ mediated transcription when the core enzyme was reconstituted with 2.5 jig of sigma™ protein. 5.4 Reactivation of E.coli sigma^" mediated transcription Increasing concentrations of E.coli sigma™ protein were added to the reaction mixture containing 0.1 ng of AsiA. As shown in Table 1, addition of sigma'" protein could reactivate transcription mediated by E.coli RNA polymerase. This reversal of inhibition demonstrates the specific interaction of the AsiA protein with sigma™. As described in Section 5.1, the E.coli polymerase activity assay was standardized with purified enzyme and phage T4 DNA as template. The RNA polymerase could be >80% inhibited by 0.07 ^g of purified AsiA protein. To reaction mixture containing the E.coli RNA polymerase, T4 DNA and 0.1 ng of AsiA, increasing concentrations of purified Salmonella typhimurium sigma™ was added and the RNA polymerase activity quantified. As shown in Table 1, addition of 4 jig of S. typhimurium sigma™ could restore the activity of the RNA polymerase. 5.6 Preincubation o/sigma™ with AsiA To a reaction mixture containing E.coli RNA polymerase and T4 DNA template there was added 0.1 (ig of AsiA and 4 ^g E.coli or S. typhimurium sigma™ preincubated at +37°C for 10 min with 0.1 |xg of AsiA. As shown in Table 1, preincubation of the sigma™ with AsiA abolished the ability of the sigma™ from both E.coli and S. typhimurium, to activate E.coli RNA polymerase. EXAMPLE 6: Competitive ELISA for qixantification of AsiA-sigma™ interaction Screening assays The above principles provide a basis for a method of identification of oligonucleotide sequences encoding peptides, which may be either related or unrelated to the AsiA peptide, and which could efficiently bind to sigma™ subunits fi-om E.coli, S. typhimurium and/or other housekeeping or virulence associated sigma subunits of bacterial pathogens. The identification of such peptide structures would enable the fiuther identification, by known methods, of peptoids, peptidomimetics and organic molecules which can be tested in transcription assays for inhibition of sigma™ dependent transcription. Truncated sigma'" (C-terminal 99 amino acids) which has been shown to be the anti-sigma binding region of sigma™ of E.coli (Severinova et.al., J.Mol. Biol (1996) 263 (5), pp 637-647) was cloned and expressed in a His tag expression vector resulting in a (His)5-tagged protein product. A solution of the protein product (200 pmoles/ml) in a pH 7.5 buffer comprising Tris HCl (10 mM) and sodium chloride (50 mM) was prepared and 200 )i\ of solution were added per well of a nickel-coated microtitre plate, followed by incubation at room temperature for a period of two hours. The microtitre plate was then washed 5 times with the buffer solution and then incubated at 37°C for one hour with a solution of bovine serum albumin (3% w/v) in the above buffer. After washing 5 times with phosphate buffered saline (PBS) with "Tween" 20 (polyoxyethylene (20) sorbitan monolaurate, 0.5% v/v) the GST-AsiA fusion protein prepared as described in Example 4.1 above (100 ng) was added together with a defined amount of a putative inhibitor of truncated sigma'" protein (corallopyronin, myxopyronin or ripostatin) and incubated at 37°C for one hour. Further washing (5 times) with PBS-"Tween" 20 was carried out and then anti-GST antibodies (1:2000) raised in rabbits were added to the wells, with incubation at 37°C for one hour. The previous washing step was repeated and thereafter antirabbit IgG-horse radish peroxidase (HRP) conjugate (1:2000) was added and incubated at 37°C for one hom-. A final wash with PBS-"Tween" 20 was carried out, before adding the HRP enzyme substrate, tetramethyl benzidene / H2O2 The enzymic reaction (resulting in the development of colour) was stopped after a suitable period of time by the addition of 6N H2S04 and the microtitre plate was "read" in a spectrophotometer using light of wavelength (A,) 450 nm. The results obtained are shown in Table 2 following. WE CLAIM 1. A method of identifying a ligand of a bacterial sigma70 subunit which comprises contacting the sigma70 subunit or a portion thereof comprising the anti-sigma binding region, with a test compound and a GST-AsiA fusion protein produced in a yeast expression system and determining whether the test compound binds competitively with the anti-sigma™ factor to the sigma70 subunit or portion thereof. 2. A method according to claim 1, which comprises: (i) immobilizing the sigma70 subunit or portion thereof on a matrix or solid support; (ii) adding the test compound and the fusion protein; (iii) adding a first antibody against the fusion protein; (iv) adding a labeled second antibody against the first antibody; and (v) determining the amount of second antibody bound to the (first antibody-fusion protein - sigma70 subunit or portion thereof) complex formed on the matrix or solid support. 3. A method according to claim 1 or claim 2, wherein the sigma70 subunit or portion thereof is obtained from Escherichia coli or Salmonella typhimurium. 4. A method according to any of the preceding claims, wherein the AsiA protein has an amino acid sequence as shown in SEQ ID NO: 1 or SEQ ID NO: 2. 5. A method according to any one of the preceding claims wherein the ligand is an inhibitor of a bacterial sigma70 subunit. 6. A method according to any one of the previous claims, wherein the fusion protein is produced in a Saccharomyces cerevisiae or Pichia pastoris expression system.

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