Bile-mediated activation of the acrAB and tolC multidrug efflux genes occurs mainly through transcriptional derepression of ramA in Salmonella enterica serovar Typhimurium.

In Salmonella Typhimurium, the genes encoding the AcrAB-TolC multidrug efflux system are mainly regulated by the ramRA locus, composed of the divergently transcribed ramA and ramR genes. The acrAB and tolC genes are transcriptionally activated by RamA, the gene for which is itself transcriptionally repressed by RamR. Previous studies have reported that bile induces acrAB in a ramA-dependent manner, but none provided evidence for an induction of ramA expression by bile. Therefore, the objective of this study was to clarify the regulatory mechanism by which bile activates acrAB and tolC. qRT-PCR was used to address the effects of bile (using choleate, an ox-bile extract) on the expression of ramA, ramR, acrB and tolC. Electrophoretic mobility shift assays and surface plasmon resonance experiments were used to measure the effect of bile on RamR binding to the ramA promoter (PramA) region. We show that ramA is transcriptionally activated by bile and is strictly required for the bile-mediated activation of acrB and tolC. Additionally, bile is shown to specifically inhibit the binding of RamR to the PramA region, which overlaps the putative divergent ramR promoter, thereby explaining our observation that bile also activates ramR transcription. We propose a regulation model whereby the bile-mediated activation of the acrAB and tolC multidrug efflux genes occurs mainly through the transcriptional derepression of the ramA activator gene.