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Journal of Virology, November 1999, p. 9478-9484, Vol. 73, No. 11
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Mapping of a Hypovirus p29 Protease Symptom Determinant Domain with Sequence Similarity to Potyvirus HC-Pro Protease

Nobuhiro Suzuki, Baoshan Chen, and Donald L. Nuss*

Center for Agricultural Biotechnology, University of Maryland Biotechnology Institute, University of Maryland, College Park, Maryland 20742

Received 9 April 1999/Accepted 6 July 1999

Hypovirus infection of the chestnut blight fungus Cryphonectria parasitica results in a spectrum of phenotypic changes that can include alterations in colony morphology and significant reductions in pigmentation, asexual sporulation, and virulence (hypovirulence). Deletion of 88% [Phe(25) to Pro(243)] of the virus-encoded papain-like protease, p29, in the context of an infectious cDNA clone of the prototypic hypovirus CHV1-EP713 (recombinant virus Delta p29) partially relieved virus-mediated suppression of pigmentation and sporulation without altering the level of hypovirulence. We now report mapping of the p29 symptom determinant domain to a region extending from Phe(25) through Gln(73) by a gain-of-function analysis following progressive repair of the Delta p29 deletion mutant. This domain was previously shown to share sequence similarity [including conserved cysteine residues Cys(38), Cys(48), Cys(70), and Cys(72)] with the N-terminal portion of the potyvirus-encoded helper component-proteinase (HC-Pro), a multifunctional protein implicated in aphid-mediated transmission, genome amplification, polyprotein processing, long-distance movement, and suppression of posttranscriptional silencing. Substitution of a glycine residue for either Cys(38) or Cys(48) resulted in no qualitative or quantitative changes in virus-mediated symptoms. Unexpectedly, mutation of Cys(70) resulted in a very severe phenotype that included significantly reduced mycelial growth and profoundly altered colony morphology. In contrast, substitution for Cys(72) resulted in a less severe symptom phenotype approaching that observed for Delta p29. The finding that p29-mediated symptom expression is influenced by two cysteine residues that are conserved in the potyvirus-encoded HC-Pro raises the possibility that these related viral-papain-like proteases function in their respective fungal and plant hosts by impacting ancestrally related regulatory pathways.

* Corresponding author. Mailing address: Center for Agricultural Biotechnology, University of Maryland Biotechnology Institute, Plant Sciences Bldg., Rm. 5115C, College Park, MD 20742-4450. Phone: (301) 405-0334. Fax: (301) 314-9075. E-mail: nuss{at}umbi.umb.edu.

Journal of Virology, November 1999, p. 9478-9484, Vol. 73, No. 11
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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