ANALYSIS OF THE ROLE OF THE PSEUDOMONAS-SYRINGAE PV SYRINGAE HRPZ HARPIN IN ELICITATION OF THE HYPERSENSITIVE RESPONSE IN TOBACCO USING FUNCTIONALLY NONPOLAR HRPZ DELETION MUTATIONS, TRUNCATED HRPZ FRAGMENTS, AND HRMA MUTATIONS

Pseodomonas syringae pv. syringae, like many plant pathogenic bacteria , secretes a 'harpin' protein that can elicit the hypersensitive respo nse (HR), a defensive cellular suicide, in non-host plants, The harpin -encoding hrpZ gene is located in an operon that also encodes Hrp secr etion pathway components and is part of the functional cluster of hrp genes carried on cosmid pHIR11 that enables saprophytic bacteria like Escherichia coli and Pseudomonas fluorescens to elicit the HR in tobac co leaves, We have constructed functionally non-polar hrpZ deletion mu tations, revealing that HrpZ is necessary for saprophytic bacteria car rying pHIR11 to elicit a typical HR, whereas it only enhances the elic itation activity of P. s. syringae. Partial deletion mutations reveale d that the N-terminal 153 amino acids of HrpZ can enable E. coli MC410 0-(pHIR11) to elicit a strong HR. hrpZ subclone products comprising th e N-terminal 109 amino acids and C-terminal 216 amino acids, respectiv ely, of the 341 amino acid protein were isolated and found to elicit t he HR, P. fluorescens (pHIR11 hrmA::TnphoA) mutants do not elicit the HR, but cell fractionation and immunoblot analysis revealed that they produce and secrete wild-type levels of HrpZ. Therefore, elicitor acti vity resides in multiple regions of HrpZ, P. syringae produces elicito r(s) in addition to HrpZ, and HrpZ is essential but not sufficient for HR elicitation by saprophytic bacteria carrying pHIR11.