Pseudomonas syringae HRP gene pathogenicity islands
The type III protein secretion system is required by most plant pathogenic bacteria for pathogenicity in host plants and elicitation of the hypersensitive response (HR) in non-host plants. Thus, this secretion system is called the Hrp system (hypersensitive response and pathogenicity). At least three types of proteins are believed to be secreted through the Pseudomonas syringae Hrp system: harpins, a Hrp pilus protein, and avirulence (Avr) proteins. Avrs are likely translocated directly into plant cells where they either promote bacterial parasitism or interact with a plant resistance gene product and elicit the HR. The number of proteins secreted through the Hrp system by plant pathogenic bacteria and the functions of most of the secreted proteins are unknown. P. syringae appears to secrete Avrs only when in contact with plant cells, complicating attempts to determine a complete inventory of these proteins. To better understand the secretion process, I investigated the localization of the HrpZ harpin in P. syringae pv. syringae mutants with mutations in genes controlling the production of components of the Hrp secretion system. A mutant defective in the outer membrane HrcC protein failed to secrete HrpZ but accumulated some HrpZ in the periplasm, suggesting that secretion through the two bacterial membranes can be partially uncoupled. To search for additional genes encoding harpins and Avrs in P. syringae pv. tomato DC3000 and P. syringae pv. syringae B728a, a model pathogens of regions flanking the hrp gene clusters in these bacteria. One flank was highly variable between strains and contained mobile elements and diverse avrs, while the other was conserved and lacked mobile elements. Several avr homologs and a pectate-binding harpin were identified. The contribution to bacterial virulence of several of these proteins was determined. The hrp cluster from P. syringae was demonstrated to be a pathogenicity island inserted next to a tRNA gene. My results support a model for bacterial plant pathogenicity based on secretion through a conserved Hrp system of at least two conserved harpin proteins and many diverse Avr proteins, with the latter controlling host range and many other pathogenic properties
Thesis, Dissertation, English, 1998