Expression of virulence factors in Ralstonia solanacearum is controlled by a complex regulatory network, at the center of which is PhcA, a LysR family transcriptional regulator. We report here that expression of phcA and production of PhcA-regulated virulence factors are affected by products of the putative operon phcBSR(Q). phcB is required for production of an extracellular factor (EF), tentatively identified as the fatty acid derivative 3-hydroxypalmitic acid methyl ester (3-OH PAME), but a biochemical function for PhcB could not be deduced from DNA sequence analysis. The other genes in the putative operon are predicted to encode proteins homologous to members of two-component signal transduction systems: PhcS has amino acid similarity to histidine kinase sensors, whereas PhcR and OrfQ are similar to response regulators. PhcR is quite unusual because its putative output domain strongly resembles the histidine kinase domain of a sensor protein. Production of the PhcA-regulated factors exopolysaccharide I, endoglucanase, and pectin methyl esterase was reduced 10- to 100-fold only in mutants with a nonpolar insertion in phcB [which express phcSR(Q) in the absence of the EF]; simultaneously, expression of phcA was reduced fivefold. Both a wild-type phenotype and phcA expression were restored by addition of 3-OH PAME to growing cultures. Mutants with polar insertions in phcB or lacking the entire phcBSR(Q) region produced wild-type levels of PhcA-regulated virulence factors. The genetic data suggest that PhcS and PhcR function together to regulate expression of phcA, but the biochemical mechanism for this is unclear. At low levels of the EF, it is likely that PhcS phosphorylates PhcR, and then PhcR interacts either with PhcA (which is required for full expression of phcA) or an unknown component of the signal cascade to inhibit expression of phcA. When the EF reaches a threshold concentration, we suggest that it reduces the ability of PhcS to phosphorylate PhcR, resulting in increased expression of phcA and production of PhcA-regulated factors.
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