mexEF-oprN multidrug efflux operon of Pseudomonas aeruginosa: regulation by the MexT activator in response to nitrosative stress and chloramphenicol.

A null mutation in the mexS gene of Pseudomonas aeruginosa yielded an increased level of expression of a 3-gene operon containing a gene, xenB, whose product is highly homologous to a xenobiotic reductase in Pseudomonas fluorescens shown previously to remove nitro groups from trinitrotoluene and nitroglycerin (D. S. Blehert, B. G. Fox, and G. H. Chambliss, J. Bacteriol. 181:6254, 1999). This expression, which paralleled an increase in mexEF-oprN expression in the same mutant, was, like mexEF-oprN, dependent on the MexT LysR family positive regulator previously implicated in mexEF-oprN expression. As nitration is a well-known result of nitrosative stress, a role for xenB (and the coregulated mexEF-oprN) in a nitrosative stress response was hypothesized and tested. Using s-nitrosoglutathione (GSNO) as a source of nitrosative stress, the expression of xenB and mexEF-oprN was shown to be GSNO inducible, although in the case of xenB, this was seen only for a mutant lacking MexEF-OprN. In both instances, this GSNO-inducible expression was dependent upon MexT. Chloramphenicol, a nitroaromatic antimicrobial that is a substrate for MexEF-OprN, was shown to induce mexEF-oprN but not xenB, again dependent upon the MexT regulator, possibly because it resembles a nitrosated nitrosative stress product accommodated by MexEF-OprN.