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

The 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 the 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 a fluctuant environment.