HRPW OF ERWINIA-AMYLOVORA, A NEW HARPIN THAT CONTAINS A DOMAIN HOMOLOGOUS TO PECTATE LYASES OF A DISTINCT CLASS

Harpins, such as HrpN of Erwinia amylovora, are extracellular glycine- rich proteins that elicit the hypersensitive reaction (HR). We identif ied hrpW of E, amylovora, which encodes a protein similar to known har pins in that it is acidic, rich in glycine and serine, and lacks cyste ine, A putative HrpL-dependent promoter was identified upstream of hrp W, and Western blot analysis of hrpL mutants indicated that the produc tion of HrpW is regulated by hrpL. HrpW is secreted via the Hrp (type III) pathway based on analysis of wild-type strains and hip secretion mutants. When infiltrated into plants, HrpW induced rapid tissue colla pse, which required active plant metabolism. The HR-eliciting activity was heat stable and protease sensitive. Thus, we concluded that HrpW is a new harpin, HrpW of E. amylovora consists of two domains connecte d by a Pro and Ser rich sequence, A fragment containing the N-terminal domain was sufficient to elicit the HR. Although no pectate lyase act ivity was detected, the C-terminal region of HrpW is homologous to pec tate lyases of a unique class, suggesting that HrpW may be targeted to the plant cell wall. Southern analysis indicated that hrpW is conserv ed among several Erwinia species, and hrpW, provided in trans, enhance d the HR-inducing ability of a hrpN mutant. However, HrpW did not incr ease the virulence of a hrpN mutant in host tissue, and hrpW mutants r etained the wild-type ability to elicit the HR in nonhosts and to caus e disease in hosts.