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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 × 106 and 1.4 × 106) 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 Mg2+ 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 × 106 and 1.4 × 106, which corresponded to those of the two viral dsRNAs. The [32P]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.
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