MepR, a Repressor of the Staphylococcus aureus MATE Family Multidrug Efflux Pump MepA, Is a Substrate-Responsive Regulatory Protein

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Abstract
The mepRAB gene cluster of Staphylococcus aureus encodes a MarR family repressor (MepR; known to repress mepA expression), a MATE family multidrug efflux pump (MepA), and a protein of unknown function (MepB). In this report, we show that MepR also is autoregulatory, repressing the expression of its own gene. Exposure of strains containing a mepR :: lacZ fusion with mepR provided in trans under the control of an inducible promoter, or a mepA :: lacZ fusion alone, to subinhibitory concentrations of MepA substrates resulted in variably increased expression mainly of mepA . Mobility shift assays revealed that MepR binds upstream of mepR and mepA , with an apparently higher affinity for the mepA binding site. MepA substrates abrogated MepR binding to each site in a differential manner, with the greatest effect observed on the MepR- mepA operator interaction. DNase I footprinting identified precise binding sites which included promoter motifs, inverted repeats, and transcription start sites for mepR and mepA , as well as a conserved GTTAG motif, which may be a signature recognition sequence for MepR. Analogous to other multidrug efflux pump regulatory proteins such as QacR, the substrate-MepR interaction likely results in its dissociation from its mepA , and in a more limited fashion its mepR , operator sites and relief of its repressive effect. The enhanced effect of substrates on mepA compared to mepR expression, and on the MepR- mepA operator interaction, results in significant relief of mepA and relative maintenance of mepR repression, leading to increased MepA protein unimpeded by MepR when the need for detoxification exists.