Evidence that Ralstonia eutropha (Alcaligenes eutrophus) contains a functional homologue of the Ralstonia solanacearum Phc cell density sensing system

In the phytopathogen Ralstonia (Pseudomonas) solanacearum, control of many virulence genes is partly mediated by the Phc cell density sensing system. Phc uses a novel self-produced signal molecule [3-hydroxypalmitic acid meth yl ester (3-OH PAME)], an atypical two-component system (PhcS/PhcR), and a LysR-type activator (PhcA) to regulate a reversible switching between two d ifferent physiological states. While Phc is present in most R. solanacearum strains, it is apparently absent from other pseudomonad plant pathogens an d prokaryotic genomes that have been sequenced. Here, we report discovery o f a phcA orthologue in the non-pathogenic, facultative chemolithoautotroph Ralstonia eutropha (Alcaligenes eutrophus) that fully complements R. solana cearum phcA mutants. We also demonstrate that some R. eutropha produce an e xtracellular factor that complements R. solanacearum mutants deficient in p roduction of the 3-OH PAME signal molecule that controls phcA. Additionally , Southern blot hybridization analysis suggested that R. eutropha harbours other Phc components, such as PhcB (a biosynthetic enzyme for 3-OH PAME) an d PhcS (a 3-OH PAME-responsive sensor kinase). Analysis of a phcA-null muta nt of R. eutropha showed that phcA (and probably Phc) positively activates motility, in contrast to R. solanacearum where it represses motility. Simil arly, the R. eutropha phcA mutant was unaffected in siderophore production, whereas inactivation of phcA in R. solanacearum increases siderophore prod uction. Although our data strongly suggest that R. eutropha has a functiona l Phc-like system and support the phylogeny of Ralstonia, it implies that P hc may have a different physiological and ecological function in R. eutroph a.