A 2-COMPONENT SYSTEM IN RALSTONIA (PSEUDOMONAS) SOLANACEARUM MODULATES PRODUCTION OF PHCA-REGULATED VIRULENCE FACTORS IN RESPONSE TO 3-HYDROXYPALMITIC ACID METHYL-ESTER

Expression of virulence factors in Ralstonia solanacearum is controlle d by a complex regulatory network, at the center of which is PhcA, a L ysR family transcriptional regulator. We report here that expression o f phcA and production of PhcA-regulated virulence factors are affected by products of the putative operon phcBSR(Q). phcB is required for pr oduction of an extracellular factor (EF), tentatively identified as th e fatty acid derivative 3-hydroxypalmitic acid methyl ester (3-OH PAME ), but a biochemical function for PhcB could not be deduced from DNA s equence analysis. The other genes in the putative operon are predicted to encode proteins homologous to members of two-component signal tran sduction systems: PhcS has amino acid similarity to histidine kinase s ensors, whereas PhcR and OrfQ are similar to response regulators. PhcR is quite unusual because its putative output domain strongly resemble s the histidine kinase domain of a sensor protein, Production of the P hcA-regulated factors exopolysaccharide I, endoglucanase, and pectin m ethyl esterase was reduced 10- to 100-fold only in mutants with a nonp olar insertion in phcB [which express phcSR(Q) in the absence of the E F]; simultaneously, expression of phcA was reduced fivefold. Both a wi ld-type phenotype and phcA expression were restored by addition of 3-O H PAME to growing cultures. Mutants with polar insertions in phcB or l acking the entire phcBSR(Q) region produced wild-type levels of PhcA-r egulated virulence factors. The genetic data suggest that PhcS and Phc R function together to regulate expression of phcA, but the biochemica l mechanism for this is unclear. At low levels of the EF, it is likely that PhcS phosphorylates PhcR, and then PhcR interacts either with Ph cA (which is required for full expression of phcA) or an unknown compo nent of the signal cascade to inhibit expression of phcA. When the EF reaches a threshold concentration, we suggest that it reduces the abil ity of PhcS to phosphorylate PhcR, resulting in increased expression o f phcA and production of PhcA-regulated factors.