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Journal of Virology, June 2005, p. 7227-7238, Vol. 79, No. 11
0022-538X/05/$08.00+0     doi:10.1128/JVI.79.11.7227-7238.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Viral Class 1 RNase III Involved in Suppression of RNA Silencing

Jan F. Kreuze ,1,{dagger},{ddagger} Eugene I. Savenkov,1,{dagger} Wilmer Cuellar,2 Xiangdong Li,1,§ and Jari P. T. Valkonen1,2*

Department of Plant Biology and Forest Genetics, SLU, Box 7080, SE-750 07 Uppsala, Sweden,1 Department of Applied Biology, PO Box 27, FIN-00014 University of Helsinki, Finland2

Received 7 November 2004/ Accepted 17 January 2005

Double-stranded RNA (dsRNA)-specific endonucleases belonging to RNase III classes 3 and 2 process dsRNA precursors to small interfering RNA (siRNA) or microRNA, respectively, thereby initiating and amplifying RNA silencing-based antiviral defense and gene regulation in eukaryotic cells. However, we now provide evidence that a class 1 RNase III is involved in suppression of RNA silencing. The single-stranded RNA genome of sweet potato chlorotic stunt virus (SPCSV) encodes an RNase III (RNase3) homologous to putative class 1 RNase IIIs of unknown function in rice and Arabidopsis. We show that RNase3 has dsRNA-specific endonuclease activity that enhances the RNA-silencing suppression activity of another protein (p22) encoded by SPCSV. RNase3 and p22 coexpression reduced siRNA accumulation more efficiently than p22 alone in Nicotiana benthamiana leaves expressing a strong silencing inducer (i.e., dsRNA). RNase3 did not cause intracellular silencing suppression or reduce accumulation of siRNA in the absence of p22 or enhance silencing suppression activity of a protein encoded by a heterologous virus. No other known RNA virus encodes an RNase III or uses two independent proteins cooperatively for RNA silencing suppression.

* Corresponding author. Mailing address: Department of Applied Biology, P.O. Box 27, FIN-00014 University of Helsinki, Finland. Phone: 358-9-19158387. Fax: 358-9-19158727. E-mail: jari.valkonen{at}helsinki.fi.

{dagger} J.F.K. and E.I.S. contributed equally to the study.

{ddagger} Present address: International Potato Center (CIP), Apartado 1558, Lima 12, Peru.

§ Present address: Department of Plant Pathology, Shandong Agricultural University, Tai'an, Shandong 271018, China.

Journal of Virology, June 2005, p. 7227-7238, Vol. 79, No. 11
0022-538X/05/$08.00+0     doi:10.1128/JVI.79.11.7227-7238.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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