Originally published as Genetics Published Articles Ahead of Print on September 9, 2008.

Genetics, Vol. 180, 1039-1049, October 2008, Copyright © 2008
doi:10.1534/genetics.108.091363

From Hypo- to Hypersuppression: Effect of Amino Acid Substitutions on the RNA-Silencing Suppressor Activity of the Tobacco etch potyvirus HC-Pro

Clara Torres-Barceló, Susana Martín, José-Antonio Daròs and Santiago F. Elena1

Instituto de Biología Molecular y Celular de Plantas, Consejo Superior de Investigaciones Científicas-UPV, 46022 València, Spain

1 Corresponding author: Instituto de Biología Molecular y Celular de Plantas (CSIC-UPV), Avenida de los naranjos s/n, 46022 València, Spain.
E-mail: sfelena{at}ibmcp.upv.es

RNA silencing participates in several important functions: from the regulation of cell metabolism and organism development to sequence-specific antiviral defense. Most plant viruses have evolved proteins that suppress RNA silencing and that in many cases are multifunctional. Tobacco etch potyvirus (TEV) HC-Pro protein suppresses RNA silencing and participates in aphid-mediated transmission, polyprotein processing, and genome amplification. In this study, we have generated 28 HC-Pro amino acid substitution mutants and quantified their capacity as suppressors of RNA silencing in a transient expression assay. Most mutations either had no quantitative effect or completely abolished silencing suppression (10 in each class), 3 caused a significant decrease in the activity, and 5 significantly increased it, revealing an unexpected high frequency of mutations conferring hypersuppressor activity. A representative set of the mutant alleles, containing both hypo- and hypersuppressors, was further analyzed for their effect on TEV accumulation and the strength of induced symptoms. Whereas TEV variants with hyposuppressor mutants were far less virulent than wild-type TEV, those with hypersuppressor alleles induced symptoms that were not more severe than those characteristic of the wild-type virus, suggesting that there is not a perfect match between suppression and virulence.