Characterization of a major facilitator superfamily (MFS) tripartite efflux pump EmrCABsm from Stenotrophomonas maltophilia

Objectives: To characterize the emrRCABsm operon of Stenotrophomonas maltophilia. Methods: The presence of the emrRCABsm operon was verified by RT-PCR. The regulatory role of EmrRsm was investigated by Delta emrRsm mutant construction and promoter transcriptional fusion assay. A susceptibility test was employed to assess the substrate spectrum of the EmrCABsm efflux pump. The requirement for each component of the EmrCABsm pump was assessed by individual mutant construction and susceptibility testing. The expression of the emrRCABsm operon was evaluated by an induction assay, using different compounds as inducers. Results: emrRsm, emrCsm, emrAsm and emrBsm formed a four-member operon that was negatively regulated by the MarR-type transcriptional regulator EmrRsm. The emrRCABsm operon was intrinsically poorly expressed and the EmrCAB pump favoured extrusion of the uncoupling agents carbonyl cyanide 3-chlorophenylhydrazone (CCCP) and tetrachlorosalicylanilide (TCS), and the hydrophobic antibiotics nalidixic acid and erythromycin. However, the emrRCABsm operon could not be derepressed by CCCP, nalidixic acid, TCS, 2-chlorophenylhydrazine hydrochloride or salicylate, which are known to be possible inducers for MarR-type regulons. Each component of the EmrCABsm pump was apparently essential for pump function. Conclusions: The EmrRsm-regulated EmrCABsm efflux pump is involved in the extrusion of hydrophobic compounds.