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Journal of Bacteriology, October 1998, p. 5211-5217, Vol. 180, No. 19
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

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

Amy O. Charkowski,1 James R. Alfano,1,dagger Gail Preston,1,Dagger Jing Yuan,2 Sheng Yang He,2 and Alan Collmer1,*

Department of Plant Pathology, Cornell University, Ithaca, New York 14853-4203,1 and Department of Energy Plant Research Laboratory, Michigan State University, East Lansing, Michigan 48824-13122

Received 14 April 1998/Accepted 21 July 1998

The host-specific plant pathogen Pseudomonas syringae elicits the hypersensitive response (HR) in nonhost plants and secretes the HrpZ harpin in culture via the Hrp (type III) secretion system. Previous genetic evidence suggested the existence of another harpin gene in the P. syringae genome. hrpW was found in a region adjacent to the hrp cluster in P. syringae pv. tomato DC3000. hrpW encodes a 42.9-kDa protein with domains resembling harpins and pectate lyases (Pels), respectively. HrpW 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 the HR when infiltrated into tobacco leaf tissue. The harpin domain (amino acids 1 to 186) has six glycine-rich repeats of a repeated sequence found in HrpZ, and a purified HrpW harpin domain fragment possessed HR elicitor activity. In contrast, the HrpW Pel domain (amino acids 187 to 425) is similar to Pels from Nectria haematococca, Erwinia carotovora, Erwinia chrysanthemi, and Bacillus subtilis, and a purified Pel domain fragment did not elicit the HR. Neither this fragment nor the full-length HrpW showed Pel activity in A230 assays under a variety of 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 harpin domains were only 53% identical. Sequences hybridizing at high stringency with the P. syringae pv. tomato hrpW were found in other P. syringae pathovars, Pseudomonas viridiflava, Ralstonia (Pseudomonas) solanacearum, and Xanthomonas campestris. Delta hrpZ::nptII or hrpW::Omega Spr P. syringae pv. tomato mutants were little reduced in HR elicitation activity in tobacco, whereas this activity was significantly reduced in a hrpZ hrpW double mutant. These features of hrpW and its product suggest that P. syringae produces multiple harpins and that the target of these proteins is in the plant cell wall.

* Corresponding author. Mailing address: Department of Plant Pathology, Cornell University, Ithaca, NY 14853-4203. Phone: (607) 255-7843. Fax: (607) 255-4471. E-mail: arc2{at}cornell.edu.

dagger Present address: Department of Biological Sciences, University of Nevada, Las Vegas, NV 89154-4004.

Dagger Present address: Department of Plant Sciences, University of Oxford, Oxford, Oxfordshire, OX1 3RB, United Kingdom.

Journal of Bacteriology, October 1998, p. 5211-5217, Vol. 180, No. 19
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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