Genome structure and expression of a defective interfering mutant of the killer virus of yeast.

A large internal deletion in M1 double-stranded (ds) RNA from the killer virus of Saccharomyces cerevisiae generates a suppressive (S3) dsRNA molecule. Strains which harbor S3 dsRNA are defective in toxin production and immunity to the toxin. The biochemical defect in expression has been investigated and is apparently due to truncation of the protoxin polypeptide translation reading frame on S3 dsRNA. Transcription in vivo, and in isolated virions in vitro, results in the synthesis of a full-length positive polarity messenger RNA, denoted s. The s transcript contains no long poly(A) tracts as determined by its lack of affinity for oligo(dT)-cellulose, and as inferred by sequence analysis of approximately 87% of the S3 dsRNA genome. These data support a model for template coding of polyadenylate in transcripts derived from the wild-type M1 dsRNA. The orientation of the sequences conserved on S3 dsRNA with respect to M1 dsRNA has been determined. Some of the conserved sequences are likely to be required for the maintenance and replication of these viral dsRNA genomes in S. cerevisiae.