THE PSEUDOMONAS-SYRINGAE PV. TOMATO HRPW PROTEIN HAS DOMAINS SIMILAR TO HARPINS AND PECTATE LYASES AND CAN ELICIT THE PLANT HYPERSENSITIVE RESPONSE AND BIND TO PECTATE

The host-specific plant pathogen Pseudomonas syringae elicits the hype rsensitive response (HR) in nonhost plants and secretes the HrpZ harpi n in culture via the Hrp (type III) secretion system. Previous genetic evidence suggested the existence of another harpin gene in the P. syr ingae genome. hrpW was found in a region adjacent to the hrp cluster i n P. syringae pv, tomato DC3000, hrpW encodes a 42.9-kDa protein with domains resembling harpins and pectate lyases (Pels), respectively. Hr pW has key properties of harpins. It is heat stable and glycine rich, lacks cysteine, is secreted by the Hrp system, and is able to elicit t he HR when infiltrated into tobacco leaf tissue. The harpin domain (am ino acids 1 to 186) has six glycine-rich repeats of a repeated sequenc e found in HrpZ, and a purified HrpW harpin domain fragment possessed HR elicitor activity. In contrast, the HrpW Pel domain (amino acids 18 7 to 425) is similar to Pels from Nectria haematococca, Erwinia caroto vora, Erwinia chrysanthemi, and Bacillus subtilis, and a purified Pel domain fragment did not elicit the HR, Neither this fragment nor the f ull-length HrpW showed Pel activity in A(230) assays under a variety o f reaction conditions, but the Pel fragment bound to calcium pectate, a major constituent of the plant cell wall, The DNA sequence of the P. syringae pv. syringae B728a hrpW was also determined. The Pel domains of the two predicted HrpW proteins were 85% identical, whereas the ha rpin domains were only 53% identical. Sequences hybridizing at high st ringency with the P. syringae pv. tomato hrpW were found in other P. s yringae pathovars, Pseudomonas viridiflava, Ralstonia (Pseudomonas) so lanacearum, and Xanthomonas campestris. Delta hrpZ::nptII or hrpW::Ome ga Sp(r) P. syringae pv. tomato mutants were little reduced in HR elic itation activity in tobacco, whereas this activity was significantly r educed in a hrpZ hrpW double mutant. These features of hrpW and its pr oduct suggest that P. syringae produces multiple harpins and that the target of these proteins is in the plant cell wall.