Properties of a Mycovirus from Rhizoctonia solani and Its Virion-associated RNA Polymerase

Department of Botany and Plant Pathology, University of Maine, Orono, Maine 04469-0102, U.S.A. Isometric virus particles, 33 nm in diameter, were purified to apparent homogeneity from a dsRNA-containing isolate (Rhs 717) of anastomosis group 2 of the basidiomycete Rhizoctonia solani . The dsRNA segments isolated from purified particles had the same M r (1.5 x 10 6 and 1.4 x 10 6 ) as those isolated directly from mycelial tissue of Rhs 717. Purified virus particles had a buoyant density of 1.37 g/ml in CsCl, and an A 260 / A 280 ratio of 1.43. The melting curve of the mixture of dsRNA segments from Rhs 717 particles had a sharp thermal transition with a melting temperature of 95 °C in standard saline citrate buffer. The RNA polymerase activity associated with the purified virus was characterized. Enzyme activity was dependent on the presence of Mg 2+ and was insensitive to actinomycin D. Virus particles and RNA polymerase activity cosedimented as a single peak in isopycnic CsCl gradients. The major reaction products were two dsRNA segments of M r 1.5 x 10 6 and 1.4 x 10 6 , which corresponded to those of the two viral dsRNAs. The [ 32 P]UMP-labelled products hybridized to the two viral dsRNAs. The RNA synthesized in vitro remained in association with virus particles subjected to agarose gel electrophoresis. Keywords: mycovirus, RNA polymerase, Rhizoctonia solani Received 22 September 1987; accepted 14 March 1988.