Regulation and Assembly of a Type III Protein Secretion Apparatus in Pseudomonas syringae

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Hutcheson 9729524 Type III protein secretion systems are required for the pathogenicity of many mammalian and plant pathogenic bacteria. This type of secretion system has been associated with the trans-location of proteins from bacteria into target cells of the host. We have previously shown that the hrp/hrmA gene cluster isolated from Pseudomonas syringae Pss61 encodes for the conserved components of a putative type III protein export apparatus (PEA) as well as an apparently dedicated regulatory system involving HrpL. Pathogenesis by P. syringae is thus likely to involve adhesion of the bacteria to plant cells, plant cell contact-dependent induction of hrp gene expression, assembly of a hrp-encoded type III PEA and translocation of virulence and host range determinants into target plant cells. The mechanisms for coordinating assembly of the PEA have only been partially characterized thus far. The overall goal of this project will be to elucidate the mechanisms controlling assembly of the hrp-encoded PEA and define the role of the PEA in the pathogenicity of P. syringae strains. In the proposed experiments we will: 1) complete the characterization of the HrpL-dependent regulatory system; 2) use a novel reporter gene system to dissect the secretion pathway for Avr products; and 3) employ a genetic screen to identify the genetic determinants regulated by HrpL-linked regulatory system. Plants have the capability to recognize and respond to invading pathogens to protect themselves and thereby minimize disease. This involves recognition of pathogen signals by the cells of the plant. The hrp genes of Pseudomonas syringae have been shown to function in the recognition process and the products of these bacterial genes are predicted to form an apparatus that injects proteins into plants cells. These injected proteins are the stimulus to which the plant cells respond. These experiments will use a genetic approach to investigate early steps in the recognition process. Elucidation of the regulation a nd function of the hrp-encoded secretion apparatus will influence current research in molecular plant-microbe interactions by providing insights into: 1) molecules and regulatory pathways controlling disease resistance in plants; 2 innate mechanisms controlling pathogenicity of bacteria in plants; 3) new strategies for breeding stable crop resistance against a broad array of pathogens by targeting specific attributes of the pathogen; and 4) evolution of protein secretion systems functioning in the pathogenicity of mammalian and plant pathogens.