Department of Biological Sciences, 5722 Deering Hall, University of Maine, Orono 04469-5722, U.S.A.
We have recently shown that acquisition of a 6.4-kb, double-stranded (ds) RNA (M1) by hyphal anastomosis is associated with enhanced vigor and virulence, whereas its removal by hyphal tipping correlates with diminished virulence in the plant-pathogenic basidiomycete Rhizoctonia solani. The M1 dsRNA is not encapsidated by a typical nucleocapsid, has a circular and/or concatemeric form, and is associated with the mitochondrial and cytosolic fractions. M1 possesses six open reading frames (ORFs) the longest of which (ORF 2) is located on the (+) strand, and encodes a putative polypeptide consisting of 1,747 amino acids or 199.4 kDa. This polypeptide has a significant amino acid sequence similarity, including six conserved helicase domains and an ATP/GTP binding motif, with the 1A protein of broad bean mottle virus (BBMV) and other bromoviruses. ORF 5, which is located on the (-) strand of M1 and is complementary to a region of ORF 2, codes for a putative polypeptide that has a significant amino acid sequence similarity with the cytochrome c oxidase assembly factor. This complementarity provides direct evidence suggesting that the long-standing hypothesis of viruses evolving from cellular genes may be valid.