The Regulatory Network Involving PcoR, RsaL, and MvaT Coordinates the Quorum-Sensing System in Pseudomonas fluorescens 2P24

Pseudomonas fluorescens 2P24 is a beneficial plant root-associated microorganism capable of suppressing several soilborne plant diseases. The capacity of P. fluorescens to aggressively colonize the rhizosphere is an important requirement for its biocontrol trait. We previously found that the PcoI/PcoR quorum-sensing system (QS) is involved in regulating the rhizosphere colonization of P. fluorescens. Here, we revealed a sophisticated regulatory network that connects PcoR, RsaL, and MvaT proteins to fine-tune the PcoI/PcoR QS system. Our data showed that PcoR could directly bind to the promoter region of pcoI thereby inducing the PcoI/PcoR QS system, whereas RsaL binds simultaneously with PcoR to the promoter region of pcoI and represses the PcoR-dependent activation of pcoI gene. In addition, RsaL indirectly downregulates the expression of pcoR. Furthermore, we showed that disruption of mvaT enhanced the expression of pcoI, pcoR, and rsaL, whereas MvaT controls the PcoI/PcoR QS in a RsaL-independent manner. Overall, this study elucidates that PcoR, RsaL, and MvaT regulate the PcoI/PcoR QS through a multi-tiered regulatory mechanism and that PcoR is necessary in the RsaL- and MvaT-mediated repression on the expression of pcoI. IMPORTANCE The PcoI/PcoR quorum-sensing system of Pseudomonas fluorescens 2P24 is important for its effective colonization in the plant rhizosphere. Many regulatory elements appear to directly or indirectly influence the QS system. Here, we found a complex regulatory network employing transcriptional factors PcoR, RsaL, and MvaT to influence the expression of the PcoI/PcoR QS in P. fluorescens 2P24. Our results indicate that PcoR and RsaL directly bind to the promoter region of pcoI and then positively and negatively regulate the expression of pcoI, respectively. Furthermore, the H-NS family protein MvaT negatively controls the PcoI/PcoR QS in a RsaL-independent manner. Taken together, our data provide new insights into the interplays between different regulatory elements that fine-tune the QS system of P. fluorescens.