Molecular cloning and characterization of W double-stranded RNA, a linear molecule present in Saccharomyces cerevisiae. Identification of its single-stranded RNA form as 20 S RNA.

Most strains of the yeast Saccharomyces cerevisiae harbor a double-stranded RNA (dsRNA) molecule, called W. We obtained W cDNA clones by random priming of denatured W dsRNA followed by reverse transcription. Sequence data of W shows that only one strand ((+)-strand) has coding capacity for a protein with 829 amino acids which spans almost the entire length of the molecule (2.5 kilobases). Within this protein we found a sequence pattern characteristic of RNA dependent RNA polymerases of (+)-strand and double-stranded RNA viruses. W has no homology with other dsRNAs found in S. cerevisiae, such as L-A, L-BC or M1. However, a (+)-strand-specific probe for W hybridized with 20-S RNA. Furthermore, W (+)-strands comigrated with 20 S RNA in strand separation gels. These results suggest that 20 S RNA is a (+)-single-stranded RNA form of W dsRNA itself or a closely related molecule.