title: Evidence that Ralstonia eutropha (Alcaligenes eutrophus) contains a functional homologue of the Ralstonia solanacearum Phc cell density sensing system.
abstract: 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 methyl ester (3-OH PAME)], an atypical two-component system (PhcS/PhcR), and a LysR-type activator (PhcA) to regulate a reversible switching between two different physiological states. While Phc is present in most R. solanacearum strains, it is apparently absent from other pseudomonad plant pathogens and prokaryotic genomes that have been sequenced. Here, we report discovery of a phcA orthologue in the non-pathogenic, facultative chemolithoautotroph Ralstonia eutropha (Alcaligenes eutrophus) that fully complements R. solanacearum phcA mutants. We also demonstrate that some R. eutropha produce an extracellular factor that complements R. solanacearum mutants deficient in production 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) and PhcS (a 3-OH PAME-responsive sensor kinase). Analysis of a phcA-null mutant of R. eutropha showed that phcA (and probably Phc) positively activates motility, in contrast to R. solanacearum where it represses motility. Similarly, the R. eutropha phcA mutant was unaffected in siderophore production, whereas inactivation of phcA in R. solanacearum increases siderophore production. Although our data strongly suggest that R. eutropha has a functional Phc-like system and support the phylogeny of Ralstonia, it implies that Phc may have a different physiological and ecological function in R. eutropha.
authors.affiliation: Department of Microbiology, University of Georgia, Athens, GA 30602, USA.
authors.forename: R P
authors.initials: RP
authors.lastname: Garg
authors.forename: W
authors.initials: W
authors.lastname: Yindeeyoungyeon
authors.forename: A
authors.initials: A
authors.lastname: Gilis
authors.forename: T P
authors.initials: TP
authors.lastname: Denny
authors.forename: D
authors.initials: D
authors.lastname: Van Der Lelie
authors.forename: M A
authors.initials: MA
authors.lastname: Schell
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism; D020118:Cupriavidus necator / Q000235:genetics*; D004268:DNA-Binding Proteins / Q000235:genetics* / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D008969:Molecular Sequence Data; D010169:Palmitic Acids / Q000378:metabolism*; D011401:Promoter Regions, Genetic; D012097:Repressor Proteins / Q000235:genetics; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; D010169:Palmitic Acids; C082933:PhcA protein, Ralstonia solanacearum; D012097:Repressor Proteins; D014157:Transcription Factors; C092197:XpsR protein, Ralstonia solanacearum; C068477:ferric uptake regulating proteins, bacterial; C108229:3-hydroxypalmitic acid