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J. Virol. doi:10.1128/JVI.01230-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

The p122 subunit of Tobacco mosaic virus replicase is a potent silencing suppressor and compromises both siRNA and miRNA mediated pathways

Agricultural Biotechnology Center, Plant Biology Institute, P.O. Box 411, H-2101 Gödöllõ, Hungary; Department of Genetics, Eötvös Lóránd University, Budapest, Hungary; University of East Anglia, Norwich UK

^* To whom correspondence should be addressed. Email: T.Dalmay{at}uea.ac.uk. burgyan{at}abc.hu.

	Abstract

One of the functions of RNA silencing in plants is to defend against molecular parasites such as viruses, retrotranspozons, and transgenes. Plant viruses are inducers as well as targets of RNA silencing based antiviral defense. Replication intermediates or folded viral RNAs activate RNA silencing generating small interfering (si) RNAs, which are the key players in the antiviral response. Viruses are able to counteract RNA silencing by expressing silencing suppressor proteins. It has been shown that many of the identified silencing suppressor proteins bind long dsRNA or siRNAs and thereby prevent assembly of the silencing effector complexes. In this study we have shown that the 122 kDa replicase subunit (p122) of crucifer-infecting Tobacco mosaic virus (cr-TMV) is a potent silencing suppressor protein. We found that the p122 protein preferentially binds to double-stranded 21nt siRNA and micro (mi) RNA intermediates having 2 nt 3'overhangs inhibiting the incorporation of siRNA and miRNA into silencing related complexes (e.g.: RISC) both in vitro and in planta, but cannot interfere with the previously programmed RISC complexes. In addition, our results also suggest that the virus infection and/or sequestration of the siRNA and miRNA molecules by p122 enhance miRNAs accumulation despite preventing their methylation. However, the p122 silencing suppressor does not prevent the methylation of certain miRNAs in hst-15 mutants where the nuclear export of miRNAs is compromised.

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