Assignment of the substrate-selective subunits of the MexEF-OprN multidrug efflux pump of XXXX .

@entity2892 @entity796 @entity3405 @entity2887 @entity803 @entity2755 @entity206
@entity796 expresses a low level of the MexAB-OprM efflux pump and shows natural resistance to many structurally and functionally diverse antibiotics. The mutation that has been referred to previously as nfxC expresses an additional efflux pump, MexEF-OprN, exhibiting resistance to @entity2887 , @entity2892 , and @entity2755 and hypersusceptibility to @entity3405 antibiotics. To address the antibiotic specificity of the MexEF-OprN efflux pump, we introduced a plasmid carrying the mexEF-oprN operon into @entity796 lacking the mexAB-oprM operon. The transformants exhibited resistance to @entity2887 , @entity206 , and @entity2755 but, unlike most nfxC-type mutants, did not show @entity3405 hypersusceptibility. The transformants exhibited additional resistance to @entity803 . In the next experiment, we analyzed the MexEF-OprN pump subunit(s) responsible for substrate selectivity by expressing MexE, MexF, OprN, and MexEF in strains lacking MexA, MexB, OprM, and MexAB, respectively. The MexEF-OprM/DeltaMexAB transformants exhibited MexEF-OprN-type pump function that rendered the strains resistant to @entity2887 and @entity2755 but did not change susceptibility to @entity3405 antibiotics compared with the host strain. The MexAB-OprN/DeltaOprM, MexAF-OprM/DeltaMexB, and MexEB-OprM/DeltaMexA mutants exhibited antibiotic susceptibility indistinguishable from that in the mutant lacking both types of efflux pumps. The results imply that the MexEF-OprM pump selects substrates by a MexEF functional unit. Interestingly, OprN did not link functionally with the MexAB complex, despite the fact that OprM interacted functionally with MexEF.
