title: A signal transfer system through three compartments transduces the plant cell contact-dependent signal controlling Ralstonia solanacearum hrp genes.
abstract: Ralstonia solanacearum hrp genes encode a type III secretion system required for disease development in host plants and for hypersensitive response elicitation on non-hosts. hrp genes are expressed in the presence of plant cells through the HrpB regulator. This activation, which requires physical interaction between the bacteria and the plant cell, is sensed by the outer membrane receptor PrhA. PrhA transduces the plant cell contact-dependent signal through a complex regulatory cascade integrated by the PrhJ, HrpG, and HrpB regulators. In this study, we have identified two genes, named prhI and prhR, that belong to the hrp gene cluster and whose predicted products show homology with extracytoplasmic function sigma factors and transmembrane proteins, respectively. Strains carrying a mutation in prhIR show a delayed pathogenic phenotype toward host plants. PrhIR control the plant cell contact-dependent activation of hrp genes. prhIR gene expression is induced by a signal present in the plant cell coculture that is not PrhA-dependent. Genetic evidence shows that PrhIR act upstream of PrhJ in the regulatory cascade, likely transducing the signal sensed by PrhA through the periplasm as described for signal transfer systems through three compartments. This is the first report of such a surface signaling mechanism activating pathogenicity determinants in response to a nondiffusible plant cell wall signal.
authors.affiliation: Laboratoire de Biologie Moléculaire des Relations Plantes-Microorganismes, INRA-CNRS, Castanet-Tolosan, France.
authors.forename: Belen
authors.initials: B
authors.lastname: Brito
authors.forename: Didier
authors.initials: D
authors.lastname: Aldon
authors.forename: Patrick
authors.initials: P
authors.lastname: Barberis
authors.forename: Christian
authors.initials: C
authors.lastname: Boucher
authors.forename: Stéphane
authors.initials: S
authors.lastname: Genin
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000382:microbiology* / Q000502:physiology; D001426:Bacterial Proteins / Q000235:genetics*; D005798:Genes, Bacterial; D016946:Gram-Negative Aerobic Rods and Cocci / Q000235:genetics* / Q000472:pathogenicity; D018551:Solanum lycopersicum / Q000382:microbiology* / Q000502:physiology; D008969:Molecular Sequence Data; D010935:Plant Diseases / Q000382:microbiology; D015183:Restriction Mapping; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction / Q000502:physiology*; D014774:Virulence
chemical_list: D001426:Bacterial Proteins