THE OUTER-MEMBRANE PROTEIN OPRM OF PSEUDOMONAS-AERUGINOSA IS ENCODED BY OPRK OF THE MEXA-MEXB-OPRK MULTIDRUG-RESISTANCE OPERON

An outer membrane protein (OprK) overproduced in a multiply antibiotic -resistant strain of Pseudomonas aeruginosa was previously identified as the product of the third gene of a multidrug resistance operon, mex A-mexB-oprK (K. Poole, K. Krebes, C. McNally, and S. Neshat, J. Bacter iol. 175:7363-7372, 1993). To determine whether this protein was ident ical to another outer membrane protein (OpriM) also overproduced in so me multiply resistant strains, attempts were made to map the transposo n insertion site of several OprM-deficient mutants to the mex operon. Amplification of chromosomal DNA of several Tn5 insertion OprM-deficie nt mutants with primers specific to each gene of the mex operon reveal ed that the transposon had inserted into mexB in one instance and into oprK in two others. Furthermore, introduction of the cloned mexA-mexB -oprK operon into these mutants restored expression of multidrug resis tance, concomitant with OprM production. These data demonstrated that OprM is encoded by the mex operon. OprM and OprK were not, however, im munologically cross-reactive, indicating that they are distinct protei ns and that OprK is, in fact, not encoded by the mex operon. This oper on is thus renamed mexA-mexB-oprM.