A SMALL MITOCHONDRIAL DOUBLE-STRANDED (DS) RNA ELEMENT ASSOCIATED WITH A HYPOVIRULENT STRAIN OF THE CHESTNUT BLIGHT FUNGUS AND ANCESTRALLY RELATED TO YEAST CYTOPLASMIC T-DSRNA AND W-DSRNA

A small double-stranded (ds) RNA element was isolated from a moderatel y hypovirulent strain of the chestnut blight fungus Cryphonectria para sitica (Murr.) Barr. from eastern New Jersey. Virulence was somewhat l ower in the dsRNA-containing strain than in a virulent dsRNA-free cont rol strain, but colony morphology and sporulation levels were comparab le. A library of cDNA clones was constructed, and overlapping clones r epresenting the entire genome were sequenced. The 2728-bp dsRNA was co nsiderably smaller than previously characterized C. parasitica dsRNAs, which are 12-13 kb and ancestrally related to the Potyviridae family of plant viruses. Sequence analysis revealed one large open reading fr ame, but only if mitochondrial codon usage (UGA = Trp) was invoked. Nu clease assays of purified mitochondria confirmed that the dsRNA was lo calized within mitochondria. Assuming mitochondrial translation, the d educed amino acid sequence had landmarks typical of RNA-dependent RNA polymerases. Alignments of the conserved regions indicate that this ds RNA is more closely related to yeast T and W dsRNAs and single-strande d RNA bacteriophages such as Q beta than to other hypovirulence-associ ated dsRNAs.