Antibiotic inducibility of the MexXY multidrug efflux system of Pseudomonas aeruginosa: involvement of the antibiotic-inducible PA5471 gene product.

The MexXY components of the MexXY-OprM multidrug efflux system of Pseudomonas aeruginosa are encoded by a MexZ repressor-regulated operon that is inducible by antibiotics that target the ribosome. Mutant strains disrupted in a gene, PA5471, were shown to be compromised for drug-inducible mexXY expression and, therefore, MexXY-OprM-mediated antimicrobial resistance. The PA5471 gene was inducible by the same ribosome-targeting agents that induce mexXY expression. Moreover, vector-driven expression of cloned PA5471 was sufficient to promote mexXY expression and MexXY-mediated resistance in the absence of antibiotic exposure, consistent with PA5471 directly or indirectly activating mexXY expression following its own upregulation in response to antibiotics. The requirement for PA5471 for mexXY expression and antimicrobial resistance was, however, obviated in mutants lacking the MexZ repressor of mexXY expression, suggesting that PA5471 directly or indirectly modulates MexZ activity in effecting mexXY expression. While the recruitment of PA5471 and MexXY in response to ribosome disruption by antimicrobials is consistent with their genes playing a role in protecting cells from the adverse consequences of disrupting the translation process, reminiscent of trans-translation, these genes appear to operate independently in their contribution to resistance: mutants defective in trans-translation showed a much more modest (twofold) decrease in resistance to ribosome-targeting agents than those lacking PA5471 or MexXY, and this decrease was observed whether functional PA5471/MexXY was present or not.