Abstract
A genetic approach was undertaken to identify normal bacterial genes whose products function to limit the effective concentration of antibiotics. In this approach, a multicopy plasmid library containing cloned Escherichia coli chromosomal sequences was screened for transformants that showed increased resistance to a number of unrelated antibiotics. Three such plasmids were identified, and all contained sequences originating from the mar locus. DNA sequence analysis of the minimal complementation unit revealed that the resistance phenotype was associated with the presence of the marA gene on the plasmids. The putative marA gene product is predicted to contain a helix-turn-helix DNA binding domain that is very similar to analogous domains found in three other E. coli proteins. One such similarity was to the SoxS gene product, the elevated expression of which has previously been associated with the multiple antibiotic resistance (Mar) phenotype. Constitutive expression of marA conferred antibiotic resistance even in cells carrying a deletion of the chromosomal mar locus. We have also found that transformants bearing marA plasmids show a significant reduction in ompF translation but not transcription, similar to previously described mar mutants. However, this reduction in ompF expression plays only a minor role in the resistance mechanism, suggesting that functions encoded by genes unlinked to mar must be affected by marA. These results suggest that activation of marA is the ultimate event that occurs at the mar locus during the process that results in multiple antibiotic resistance.