Association of yeast viral 23 S RNA with its putative RNA-dependent RNA polymerase

T double-stranded RNA (dsRNA) and its single-stranded RNA (ssRNA) counterpart, 23 S RNA, are present in some strains of the yeast Saccharomyces cerevisiae. 23 S RNA is identical with the T (+)-strand, and both are cytoplasmically inherited. The T (+)-strand encodes a protein of 104 kilodaltons (kDa) (p104) that has sequences diagnostic of RNA-dependent RNA polymerases from (+)-strand and double-stranded RNA viruses. Here we show that p104 is expressed only in yeast strains carrying T dsRNA (and 23 S RNA) and that the ability to produce p104 is co-transmitted from one strain to another by cytoplasmic mixing (cytoduction) along with T dsRNA. Neither T nor 23 S RNA is encapsidated into viral particles; however, the latter was found to be non-covalently associated with p104. This finding was based on the evidence that: (i) both 23 S RNA and p104 cosedimented in sucrose gradients and that (ii) 23 S RNA was immunoprecipitated by a p104-specific polyclonal antiserum. Consistent with this association, p104 shows single-stranded RNA-binding activity in Northwestern blots. We have localized the RNA-binding domain of p104 within its C terminus 234 amino acids.