Multiple antibiotic resistance in bacteria has become a clinical albatross. One of the mechanisms by which the enteric bacteria can become resistant to a wide spectrum of antibiotics is through induction of the mar (multiple antibiotic resistance) locus. This locus specifies an autorepressor, MarR, and a transcriptional activator, MarA. Of particular interest is the mechanism by which MarA induces the various genes of the mar regulon, the dozen or so genes dispersed throughout the chromosome whose expression is elevated by MarA, thereby conferring the antibiotic resistance phenotype as well as resistance to superoxide generating agents. The MarA protein of E. coli has been purified and its ability to stimulate RNA polymerase activity has been demonstrated in vitro with a number of promoters of the regulon. Curiously, among these promoters is the mar promoter itself. In analyzing this promoter, we have discovered that optimal induction also requires a small DNA binding protein, Fis. We have characterized the binding of both MarA and Fis to the mar promoter and defined their binding sites on the mar promoter. Recently, we have constructed trascriptional fusions for eight of the genes of the mar regulon and assayed their expression in vivo under a variety of conditions. We are particularly interested in determining not only how these genes are activated by MarA, but how they are induced by two other related transcriptional activators, SoxS and Rob. The former of these is expressed on exposure of E. coli to superoxide generating compounds, the latter is normally present at an estimated level of 5000 molecules per cell. Our studies demonstrate that only three of these genes are influenced by the ambient levels of Rob. We are currently examining mutants of MarA that differentially affect the various genes of the regulon and attempting a structural analysis of MarA.
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