1Unité de Microbiologie et Génétique UMR CNRS-INSA-UCBL 5122 Domaine Scientifique de la Doua, Université Claude Bernard Lyon I Bâtiment A. Lwoff, 10 rue Raphaël Dubois 69622 Villeurbanne Cedex, France; 2Laboratoire de Pathologie Végétale, UMR 217 INRA-INAP/G-Paris 6 16 rue Claude Bernard 75005 Paris, France
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Accepted 14 July 2005.
Erwinia chrysanthemi strain 3937 is a necrotrophic bacterial plant pathogen. Pectinolytic enzymes and, in particular, pectate lyases play a key role in soft rot symptoms; however, the efficient colonization of plants by E. chrysanthemi requires additional factors. These factors include HrpN (harpin), a heat-stable, glycine-rich hydrophilic protein, which is secreted by the type III secretion system. We investigated the expression of hrpN in E. chrysanthemi 3937 in various environmental conditions and different regulatory backgrounds. Using lacZ fusions, hrpN expression was markedly influenced by the carbon source, osmolarity, growth phase, and growth substrate. hrpN was repressed when pectinolysis started and negatively regulated by the repressors of ectate lyase synthesis, PecS and PecT. Primer extension data and in vitro DNA-protein interaction experiments support a model whereby PecS represses hrpN expression by binding to the hrpN regulatory region and inhibiting transcript elongation. The results suggest coordinated regulation of HrpN and pectate lyases by PecS and PecT. A putative model of the synthesis of these two virulence factors in E. chrysanthemi during pathogenesis is presented.
The American Phytopathological Society, 2005