Coinfection of a fungal pathogen by two distinct double-stranded RNA viruses

Unsegmented double-stranded (ds)RNA viruses belonging to the family Totivir idae persistently infect protozoa and fungi. In this study, two totiviruses were found to coinfect the filamentous fungus Sphaeropsis sapinea, a well known pathogen of pines. Isometric, virus-like particles similar to 35 nm i n diameter were isolated from extracts of this fungus. The nucleotide seque nces of the genomes of the two S. sapinea RNA viruses named SsRV1 and SsRV2 were established. The linear genomes of 5163 and 5202 bp, respectively, ar e identically organized with two large, overlapping ORFs. The 5' located OR F1 probably encodes the coat protein, whereas the gene product of ORF2 show s the typical features of RNA-dependent RNA polymerases. The absence of a p seudoknot and a slippery site at the overlapping region between ORF1 and OR F2, as well as the shortness of that region, leads us to suggest that the t ranslation of ORF2 of both viruses is internally initiated. The mode of tra nslation and the genomic organization are similar to those of Helminthospor ium victoriae 190S virus (Hv190SV; Huang, S., and Ghabrial, S. A. (1996). P roc. Natl. Acad. Sci USA 93, 12541-12546). Hv190SV thus appears to be close ly related to the SsRVs. Interestingly, based on amino acid sequence homolo gy SsRV1 is more closely related to Hv190SV than to SsRV2. (C) 1998 Academi c Press.