Unique regulator SrpR mediates crosstalk between efflux pumps TtgABC and SrpABC in Pseudomonas putida B6-2 (DSM 28064).

Coexistence of multiple homologous resistance-nodulation-division (RND) efflux pumps in bacteria is frequently described with overlapping substrate profiles. However, it is unclear how bacteria balance their transcription in response to changing environment. Here, we characterized a repressor, SrpR, in Pseudomonas putida B6-2 (DSM 28064), whose coding gene is adjacent to srpS that encodes the local repressor of the RND-type efflux pump SrpABC gene cluster. SrpR was demonstrated as a specific repressor of another RND efflux pump gene cluster ttgABC that is locally repressed by TtgR. SrpR was found to be capable of binding to the ttgABC operator with a higher affinity (KD , 138.0 nM) compared to TtgR (KD , 15.4 μM). EMSA and β-galactosidase assays were performed to survey possible effectors of SrpR with 35 available chemicals being tested. Only 2,3,4-trichlorophenol was identified as an effector of SrpR. A regulation model was then proposed, representing a novel strategy for balancing the efflux systems with partially overlapping substrate profiles. This study highlights sophisticated interactions among the RND efflux pumps in a Pseudomonas strain, which may endow bacteria with certain advantages in fluctuant environment.