Joint transcriptional control of xpsR, the unusual signal integrator of the Ralstonia solanacearum virulence gene regulatory network, by a response regulator and a LysR-type transcriptional activator.

Ralstonia (Pseudomonas) solanacearum is a soil-borne phytopathogen that causes a wilting disease of many important crops. It makes large amounts of the exopolysaccharide EPS I, which it requires for efficient colonization, wilting, and killing of plants. Transcription of the eps operon, encoding biosynthetic enzymes for EPS I, is controlled by a unique and complex sensory network that responds to multiple environmental signals. This network is comprised of the novel transcriptional activator XpsR, three distinct two-component regulatory systems (VsrAD, VsrBC, and PhcSR), and the LysR-type regulator PhcA, which is under the control of PhcSR. Here we show that the xpsR promoter (PxpsR) is simultaneously controlled by PhcA and VsrD, permitting XpsR to act like a signal integrator, simultaneously coordinating signal input into the eps promoter from both VsrAD and PhcSR. Additionally, we used in vivo expression analysis and in vitro DNA binding assays with substitution and deletion mutants of PxpsR to show the following. (i) PhcA primarily interacts with a typical 14-bp LysR-type consensus sequence around position -77, causing a sixfold activation of PxpsR; a weaker, less-defined binding site between -183 and -239 likely enhances PhcA binding and activation via the -77 site another twofold. (ii) Full 70-fold activation of PxpsR requires the additional interaction of the VsrD response regulator (or its surrogate) with a 14-bp dyadic sequence centered around -315 where it enhances activation (and possibly binding) by PhcA; however, VsrD alone cannot activate PxpsR. (iii) Increasing the distance between the putative VsrD binding site from that of PhcA by up to 232 bp did not dramatically affect PxpsR activation or regulation.