prhKLM genes of Ralstonia solanacearum encode novel activators of hrp regulon and are required for pathogenesis in tomato

The genes in the hrp regulon encode the proteins composing type III secretion system in Ralstonia solanacearum. The hrp regulon is positively controlled by HrpB, and hrpB expression is activated by both HrpG and PrhG. We have identified three genes, prhK, prhL, and prhM, which positively control the hrp regulon in strain OE1-1. These genes are likely to form an operon, and this operon is well conserved in the genera Ralstonia and Burkholderia. This indicates that the operon is not specific to the plant pathogens. Mutations in each of these three genes abolished hrpB and prhG expression. prhK, prhL, and prhM mutant strains lost pathogenicity toward tomato completely, and they were less virulent toward tobacco. PrhK and PrhL share sequence similarity with allophanate hydrolase and PrhM with LamB. This suggests that the three gene products are not transcriptional regulators in the strict sense, but regulate hrp regulon indirectly. This novel class of virulence-related genes will mark the beginning of new findings regarding the overall infection mode of R. solanacearum.