PhcA and PhcR Regulate Ralsolamycin Biosynthesis Oppositely in Ralstonia solanacearum

Abstract

Ralsolamycin, one of secondary metabolites in <i>Ralstonia solanacearum</i>, is known to be involved in crosstalk between <i>R. solanacearum</i> and fungi. Ralsolamycin formation is catalyzed by two-hybrid synthetases of RmyA (non-ribosomal peptide synthetase) and RmyB (polyketide synthase). A methyltransferase PhcB catalyzes formation of 3-OH MAME or 3-OH PAME, signals for the quorum sensing (QS) in <i>R. solanacearum</i>, while PhcB positively modulates ralsolamycin biosynthesis. A two-component system of PhcS and PhcR can response these QS signals and activate <i>phcA</i> expression. Here, we experimentally demonstrated that deletion of <i>phcA</i> (Δ<i>phcA</i>) substantially impaired the ralsolamycin production and expression of <i>rmyA</i> and <i>rmyB</i> in <i>R. solanacearum</i> strain EP1, and failed to induce chlamydospore formation of plant fungal pathogen <i>Fusarium oxysporum</i> f. <i>cubense</i> (stran FOC4). However, deletion of <i>phcR</i> significantly increased ralsolamycin production and expression of <i>rmyA</i> and <i>rmyB</i>, and <i>phcR</i> mutants exhibited enhanced ability to induce chlamydospore formation of FOC4. Results of the electrophoretic mobility shift assay suggested that both PhcA and PhcR bind to promoter of <i>rmy</i> operon. Taken together, these results demonstrated that both PhcA and PhcR bind to promoter of <i>rmy</i> operon, but regulate ralsolamycin biosynthesis in an opposite way. It could extend our knowledge on the sophisticated regulatory networks of ralsolamycin biosynthesis in <i>R. solanacearum</i>.