ERWINIA-AMYLOVORA SECRETES HARPIN VIA A TYPE-III PATHWAY AND CONTAINSA HOMOLOG OF YOPN OF YERSINIA SPP

Type III secretion functions in flagellar biosynthesis and in export o f virulence factors from several animal pathogens, and for plant patho gens, it has been shown to be involved in the export of elicitors of t he hypersensitive reaction. Typified by the Yop delivery system of Yer sinia spp., type III secretion is sec independent and requires multipl e components, Sequence analysis of an 11.5-kb region of the hrp gene c luster of Erwinia amylovora containing hrpI, a previously characterize d type III gene, revealed a group of eight more type III genes corresp onding to the virB or lcrB (yscN-to-yscU) locus of Yersinia spp. A hom olog of another Yop secretion gene, yscD, was found between hrpI and t his group downstream. Immediately upstream of hrpI, a homolog of yopN was discovered. yopN is a putative sensor involved in host-cell-contac t-triggered expression and transfer of protein, e.g., YopE, to the hos t cytoplasm. In-frame deletion mutagenesis of one of the type III gene s, designated hrcT, was nonpolar and resulted in a Hrp(-) strain that produced but did not secrete harpin, an elicitor of the hypersensitive reaction that is also required for pathogenesis, Cladistic analysis o f the HrpI (herein renamed HrcV) or LcrD protein family revealed two d istinct groups for plant pathogens. The Yersinia protein grouped more closely with the plant pathogen homologs than with homologs from other animal pathogens; flagellar biosynthesis proteins grouped distinctly, A possible evolutionary history of type III secretion is presented, a nd the potential significance of the similarity between the harpin and Yop export systems is discussed, particularly with respect to a poten tial role for the YopN homolog in pathogenesis of plants.