MexZ is a transcriptional regulator of the mexXY multidrug transporter operon, which confers aminoglycoside resistance on Pseudomonas aeruginosa. Highly purified MexZ showed direct binding with a specific site of the mexZ-mexX intergenic DNA when probed by a gel retardation assay. Both in vitro chemical cross-linking experiments and an in vivo two-hybrid expression system showed that the active form of MexZ, which is capable of binding the intergenic DNA, appeared to be a dimer. These results explain the mechanism by which MexZ represses transcription of the mexXY operon, but do not explain the substrate-induced hyperproduction of MexXY. The presence of inducer antibiotic in the gel-retardation assay mixture failed to detect altered MexZ-probe DNA interaction suggesting the possible involvement of an additional regulator.

MexZ is a transcriptional regulator of the mexXY multidrug transporter operon, which confers aminoglycoside resistance on Pseudomonas aeruginosa. Highly purified MexZ showed direct binding with a specific site of the mexZ-mexX intergenic DNA when probed by a gel retardation assay. Both in vitro chemical cross-linking experiments and an in vivo two-hybrid expression system showed that the active form of MexZ, which is capable of binding the intergenic DNA, appeared to be a dimer. These results explain the mechanism by which MexZ represses transcription of the mexXY operon, but do not explain the substrate-induced hyperproduction of MexXY. The presence of inducer antibiotic in the gel-retardation assay mixture failed to detect altered MexZ-probe DNA interaction suggesting the possible involvement of an additional regulator.