Pseudomonas syringae Hrp type III secretion system and effector proteins

Pseudomonas syringae is a member of an important group of Cram-negative bac terial pathogens of plants and animals that depend on a type III secretion system to inject virulence effector proteins into host cells, In P. syringa e, hrp/hrc genes encode the Hrp (type III secretion) system, and avirulence (avr) and Hrp-dependent outer protein (hop) genes encode effector proteins , The hrp/hrc genes of P. syringae pv syringae 61, P, syringae pv syringae B728a, and P, syringae pv tomato DC3000 are flanked by an exchangeable effe ctor locus and a conserved effector locus in a tripartite mosaic Hrp pathog enicity island (Pai) that is linked to a tRNA(Leu) gene found also in Pseud omonas aeruginosa but without linkage to Hrp system genes. Cosmid pHIR11 ca rries a portion of the strain 61 Hrp pathogenicity island that is sufficien t to direct Escherichia coli and Pseudomonas fluorescens to inject HopPsyA into tobacco cells, thereby eliciting a hypersensitive response normally tr iggered only by plant pathogens. Large deletions in strain DC3000 revealed that the conserved effector locus is essential for pathogenicity but the ex changeable effector locus has only a minor role in growth in tomato. P. syr ingae secretes HopPsyA and AvrPto in culture in a Hrp-dependent manner at p H and temperature conditions associated with pathogenesis. AvrPto is also s ecreted by Yersinia enterocolitica. The secretion of AvrPto depends on the first 15 codons, which are also sufficient to direct the secretion of an Np t reporter from Y, enterocolitica, indicating that a universal targeting si gnal is recognized by the type III secretion systems of both plant and anim al pathogens.