CmeR functions as a transcriptional repressor for the multidrug efflux pump CmeABC in Campylobacter jejuni

CmeABC, a resistance-nodulation-division (RND) type of efflux pump, contributes to Campylobacter resistance to a broad spectrum of antimicrobial agents and is also essential for Campylobacter colonization of the animal intestinal tract by mediation of bile resistance. As one of the main systems for Campylobacter adaptation to different environments, CmeABC is likely subject to control by regulatory elements. We describe the identification of a transcriptional repressor for CmeABC. Insertional mutagenesis of cmeR, an open reading frame immediately upstream of the cmeABC operon, resulted in overexpression of cmeABC, as determined by transcriptional fusion and immunoblotting with CmeABC-specific antibodies. Overexpression of the efflux pump was correlated with a moderate increase in the level of resistance of the cmeR mutant to several antimicrobials. In vitro, recombinant CmeR bound specifically to the promoter region of cmeABC, precisely, to the inverted repeat sequences in the cmeABC promoter. A single nucleotide deletion between the two half sites of the inverted repeat reduced the level of CmeR binding to the promoter sequence and resulted in overexpression of cmeABC. Together, these findings indicate that cmeR encodes a transcriptional repressor that directly interacts with the cmeABC promoter and modulates the expression of cmeABC. Mutation either in CmeR or in the inverted repeat impedes the repression and leads to enhanced production of the MDR efflux pump.