EMRR IS A NEGATIVE REGULATOR OF THE ESCHERICHIA-COLI MULTIDRUG-RESISTANCE PUMP EMRAB

The emrAB locus of Escherichia coli encodes a multidrug resistance pum p that protects the cell from several chemically unrelated antimicrobi al agents, e.g., the protonophores carbonyl cyanide m-chlorophenylhydr azone (CCCP) and tetrachlorosalicyl anilide and the antibiotics nalidi xic acid and thiolactomycin. The mprA gene is located immediately upst ream of this locus and was shown to be a repressor of microcin biosynt hesis (I. del Castillo, J. M. Gomez, and F. Moreno, J. Bacteriol. 173: 3924-3999, 1991). There is a putative transcriptional terminator seque nce between the mprA and emrA genes, To locate the emr promoter, singl e-copy lacZ operon fusions containing different regions of the emr loc us were made. Only fusions containing the mprA promoter region were ex pressed, mprA is thus the first gene of the operon, and we propose tha t it be renamed emrR, Overproduction of the EmrR protein (with a multi copy vector containing the cloned emrR gene) suppressed transcription of the emr locus, A mutation in the emrR gene led to overexpression of the EmrAB pump and increased resistance to antimicrobial agents. CCCP , nalidixic acid, and a number of other structurally unrelated chemica ls induced expression of the emr genes, and the induction required Emr R. We conclude that emrRAB genes constitute an operon and that EmrR se rves as a negative regulator of this operon. Some of the chemicals tha t induce the pump serve as its substrates, suggesting that their extru sion is the natural function of the pump.