The baeSR two-component regulatory system activates transcription of the yegMNOB (mdtABCD) transporter gene cluster in Escherichia coli and increases its resistance to novobiocin and deoxycholate.

Screening of random fragments of Escherichia coli genomic DNA for their ability to increase the novobiocin resistance of a hypersusceptible (Delta)acrAB mutant resulted in the isolation of a plasmid containing baeR, which codes for the response regulator of the two-component regulatory system BaeSR. When induced for expression, baeR cloned in multicopy plasmid pTrc99A significantly increased the resistance of the (Delta)acrAB host strain to novobiocin (16-fold) and to deoxycholate (8-fold). Incubation of cells with novobiocin followed by a chromatographic assay for intracellular drug showed that overproduced BaeR decreased drastically the drug accumulation, presumably via increased active efflux. The genes baeSR are part of a putative operon, yegMNOB baeSR. Direct binding of BaeR to the yegM promoter was demonstrated in vitro by gel retardation assay. The gene yegB, which codes for a major facilitator superfamily transporter, was not necessary for increased resistance, but deletion of yegO or an in-frame deletion of yegN, both of which code for resistance-nodulation-cell division-type multidrug transporters, abolished the BaeR-induced increase in resistance. It is likely that both YegN and YegO produce a complex(es) with the membrane fusion protein family member YegM and pump out novobiocin and deoxycholate. We accordingly propose to rename yegMNOB as mdtABCD (mdt for multidrug transporter). Finally, the expression of two other genes, yicO and ygcL, was shown to be regulated by BaeR, but it is not known if they play any roles in resistance.

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