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The C-Terminal Half of the HrpN Virulence Protein of the Fire Blight Pathogen Erwinia amylovora Is Essential for Its Secretion and for Its Virulence and Avirulence Activities

November 2008 , Volume 21 , Number  11
Pages  1,387 - 1,397

Judith P. Sinn,1 Chang-Sik Oh,2 Philip J. Jensen,1 Sara C. D. Carpenter,2 Steven V. Beer,2 and Timothy W. McNellis1

1Department of Plant Pathology, The Pennsylvania State University, University Park, PA 16802, U.S.A.; 2Department of Plant Pathology and Plant-Microbe Biology, Cornell University, Ithaca, NY, 14853, U.S.A.

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Accepted 11 July 2008.

The HrpN (harpin) protein of the fire blight pathogen Erwinia amylovora is an essential virulence factor secreted via the bacterial type III secretion system. HrpN also has avirulence activity when delivered to tobacco by E. amylovora and has defense elicitor activity when applied to plants as a cell-free protein extract. Here, we characterize a series of random mutations in hrpN that altered the predicted amino acid sequence of the protein. Amino acid substitutions and deletions in the highly conserved, C-terminal portion of HrpN disrupted the virulence and avirulence activities of the protein. Several of these mutations produced a dominant-negative effect on E. amylovora avirulence on tobacco. None of the mutations clearly separated the virulence and avirulence activities of HrpN. Some C-terminal mutations abolished secretion of HrpN by E. amylovora. The results indicate that the C-terminal half of HrpN is essential for its secretion by E. amylovora, for its virulence activity on apple and pear, and for its avirulence activity on tobacco. In contrast, the C-terminal half of HrpN was not required for cell-free elicitor activity. This suggests that the N-terminal and C-terminal halves of HrpN mediate cell-free elicitor activity and avirulence activity, respectively.

Additional keywords:DspA/E, HrpW, HrpJ, hypersensitive response.

© 2008 The American Phytopathological Society