title: The regulatory cascade that activates the Hrp regulon in Erwinia herbicola pv. gypsophilae.
abstract: The pathogenicity of Erwinia herbicola pv. gypsophilae (Ehg) is dependent on a plasmid (pPATH(Ehg)) that harbors the hrp gene cluster and additional virulence genes. The hrp regulatory cascade of Ehg comprises an hrpXY operon encoding a two-component system; hrpS encoding a transcriptional factor of the NtrC family and hrpL encoding an alternative sigma factor. Results obtained suggest the following signal transduction model for activating the Hrp regulon: phosphorylated HrpY activates hrpS, HrpS activates hrpL, and HrpL activates genes containing "hrp box" promoter. This model was supported by studies on the effects of mutations in the regulatory genes on pathogenicity and complementation analysis. Nonpolar mutations in hrpX did not affect virulence or transcription of downstream genes. Site-directed mutagenesis of the conserved aspartate 57 in HrpY suggested that its phosphorylation is crucial for activating the hrp regulatory cascade. Studies on the effects of mutations in the hrp regulatory genes on transcriptional activity of downstream genes or of their isolated promoters in planta showed dependency of hrpS expression on active HrpY, of hrpL expression on active HrpS, and of hrpN or hrpJ expression on active HrpL. These results were also partially supported by overexpression of regulatory genes under in vitro conditions. The hrpXY is constitutively expressed with high basal levels under repressive conditions, in contrast to hrpS and hrpL, which exhibit low basal expression levels and are environmentally regulated.
authors.affiliation: Department of Plant Sciences, Faculty of Life Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel.
authors.forename: R
authors.initials: R
authors.lastname: Nizan-Koren
authors.forename: S
authors.initials: S
authors.lastname: Manulis
authors.forename: H
authors.initials: H
authors.lastname: Mor
authors.forename: N M
authors.initials: NM
authors.lastname: Iraki
authors.forename: I
authors.initials: I
authors.lastname: Barash
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D004885:Erwinia / Q000235:genetics* / Q000254:growth & development; D015964:Gene Expression Regulation, Bacterial; D017930:Genes, Reporter / Q000235:genetics; D005816:Genetic Complementation Test; D008969:Molecular Sequence Data; D005810:Multigene Family / Q000235:genetics; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D010766:Phosphorylation; D020161:Physical Chromosome Mapping; D010944:Plants / Q000235:genetics / Q000382:microbiology; D010957:Plasmids / Q000235:genetics; D011401:Promoter Regions, Genetic / Q000235:genetics; D018085:Regulon / Q000235:genetics*; D017422:Sequence Analysis, DNA; D012808:Sigma Factor / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000235:genetics; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D015533:Transcriptional Activation
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D004268:DNA-Binding Proteins; C085855:HrpL protein, Pseudomonas syringae; C421032:HrpY protein, Erwinia amylovora; D012808:Sigma Factor; D014157:Transcription Factors; C083700:hrpJ protein, Pseudomonas syringae; C060789:hrpS protein, Pseudomonas syringae