We have investigated the transcriptional and translational control mechanisms of the marRAB, multiple antibiotic resistance, operon of Escherichia coli. The operon is autorepressed by MarR. MarR binds to two palindromic sequences in vitro: Site I lies between and overlaps the -35 and -10 hexamers for RNA polymerase binding; Site II lies between the transcription start site and the GTG initiation codon of marR. To assess the importance of these sites in vivo, the effects of mutant sites on transcription were analysed using appropriate fusions to a lacZ reporter gene. When both sites were wild-type, transcription was repressed by MarR about 20-fold; when only Site I or Site II was wild-type, transcription was repressed 4.3- and 2.6-fold, respectively, indicating that repression at one site is largely independent of repression at the other. However, the possibility of an interaction between the MarR bound at each site has not been ruled out. Fusions of the wild-type promoter to lacZ demonstrated that marR translation proceeds at only 4.5% of the transcription rate. Analysis of translational fusions with mutant leader sequences demonstrated that the principal reason for inefficient translation is a weak Shine-Dalgarno sequence (SD), AGG(G). Although the SD is located within the potential stem-loop structure of Site II, no evidence for occlusion of the SD was found in the wild-type strain. However, a single bp mutation that would strengthen the stem-loop structure drastically reduced the translational efficiency. Substitution of ATG for GTG as the initial codon did not have a significant effect. Increasing the spacing between the SD and the GTG codon by one to four bases further reduced the translational efficiency by two- to four-fold. Interestingly, both downstream structural genes, marA and marB, appear to have strong SD sequences reasonably spaced from their AUG start codons. In fact, when marA is directly fused to the mar promoter, it is translated with an efficiency of ~34%. This raises the possibility that under some conditions, ribosome binding at the marA translation signals may surpass that of marR. If MarA were translated at a higher rate than MarR, it could provide a mechanism for rapid activation of the regulon in response to an external stimulus before the build-up of MarR again dampened expression.
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