Contribution of Stenotrophomonas maltophilia MfsC transporter to protection against diamide and the regulation of its expression by the diamide responsive repressor DitR.

Stenotrophomonas maltophilia contains an operon comprising mfsB and mfsC, which encode membrane transporters in the major facilitator superfamily (MFS). The results of the topological analysis predicted that both MfsB and MfsC possess 12 transmembrane helices with the N- and C-termini located inside the cells. The deletion of mfsC increased the susceptibility to diamide, a chemical oxidizing agent, but not to antibiotics and oxidative stress-generating substances relative to wild-type K279a. Moreover, no altered phenotype was observed against all tested substances for the ΔmfsB mutant. The results of the expression analysis revealed that the mfsBC expression was significantly induced by exposure to diamide. The diamide-induced gene expression was mediated by DitR, a TetR-type transcriptional regulator encoded by smlt0547. A constitutively high expression of mfsC in the ditR mutant indicated that DitR acts as a transcriptional repressor of mfsBC under physiological conditions. Purified DitR was bound to three sites spanning from position + 21 to -57, corresponding to the putative mfsBC promoter sequence, thereby interfering with the binding of RNA polymerase. The results of electrophoretic mobility shift assays illustrated that the treatment of purified DitR with diamide caused the release of DitR from the mfsBC promoter region, and the diamide sensing mechanism of DitR required two conserved cysteine residues, Cys92 and Cys127. This suggests that exposure to diamide can oxidize DitR through the oxidation of cysteine residues, leading to its release from the promoter, thus allowing mfsBC transcription. Overall, MfsC and DitR play a role in adaptive resistance against the diamide of S. maltophilia.