THE HRPZ PROTEINS OF PSEUDOMONAS-SYRINGAE PVS SYRINGAE, GLYCINEA, ANDTOMATO ARE ENCODED BY AN OPERON CONTAINING YERSINIA YSC HOMOLOGS AND ELICIT THE HYPERSENSITIVE RESPONSE IN TOMATO BUT NOT SOYBEAN

The Pseudomonas syringae pathovars are composed of host-specific plant pathogens that characteristically elicit the defense-associated hyper sensitive response (HR) in nonhost plants. P. s. pv. syringae 61 secre tes an HR elicitor, harpin(Pss), (HrpZ(Pss)), in a hrp-dependent manne r. An internal fragment of the It s. pv. syringae 61 hrpZ gene was use d to clone the hrpZ locus from P. s. pv. glycinea race 4 (bacterial bl ight of soybean) and P. s. pv. tomato DC3000 (bacterial speck of tomat o). DNA sequence analysis revealed that hrpZ is the second ORF in a po lycistronic operon. The amino acid sequence identities of HrpZ(Pss)/Hr pZ(Psg), and HrpZ(Pss)/Hrp(Pst) were 79 and 63%, respectively, Althoug h none of the HrpZ proteins shelved significant overall sequence simil arity with other known proteins, HrpZ(Pst) contained a 24-amino acid s equence that is homologous with a region of the PopA1 elicitor protein of the tomato pathogen, Pseudomonas solanacearum GMI1000. hrpA, the u pstream ORF, was highly divergent: The amino acid sequence identities of HrpA(Pss)/HrpA(Psg) and HrpA(Pss)/HrpA(Pst) were 91 and 28%, respec tively, and no HrpA sequence showed similarity to known proteins. In c ontrast, the predicted products of the downstream ORFs in P. s. pv. sy ringae and P. s. pv. tomato, hrpB, hrpC, hrpD, and hrpE showed varying levels of similarity to those of yscI, yscJ, yscK, and yscL. These ar e colinearly arranged genes in the virC locus of Yersinia spp., which are involved in the secretion of the Yop virulence proteins via the ty pe III pathway. The similarity of the Ysc proteins was generally stron ger in comparisons with the P. s. pv. tomato Hrp proteins. The HrpZ pr oteins were purified by heat denaturation of contaminating proteins fo llowed by ammonium sulfate fractionation, hydrophobic chromatography, and gel electrophoresis. All three HrpZ proteins elicited the IIR in t omato, whereas none of them elicited significant necrosis in soybean. The results indicate that HrpZ is encoded in an operon containing some of the genes involved in its own secretion and suggest that HrpZ stru cture does not directly determine bacterial host range.