Characterization of MexT, the regulator of the MexE-MexF-OprN multidrug efflux system of Pseudomonas aeruginosa

We investigated the regulation of the MexEF-OprN multidrug efflux system of Pseudomonas aeruginosa, which is overexpressed in nfxC-type mutants and co nfers resistance to quinolones, chloramphenicol and trimethoprim, Sequencin g of the DNA region upstream of the mexEF-oprN operon revealed the presence of an open reading frame (ORF) of 304 amino acids encoding a LysR-type tra nscriptional activator, termed MexT, By using T7-polymerase, a 34-kDa prote in was expressed in Escherichia coli from a plasmid carrying the mexT gene. Expression of a mexE::lacZ fusion was 10-fold higher in nfxC-type mutants than in the wild-type strain; however, transcription of mexT as well as the mexT DNA region was unchanged. Located adjacent to mexT but transcribed in opposite direction, the beginning of an ORF termed qrh (quinone oxidoreduc tase homologue) was identified. Expression of a qrh::lacZ fusion was also f ound to he activated by MexT. Further, we present evidence for coregulation at the transcriptional and the posttranscriptional level between the MexEF -OprN efflux system and the OprD porin responsible for cross-resistance of nfxC-type mutants to carbapenem antibiotics.