Initiation by the yeast viral transcriptase in vitro.

All double-stranded RNA viruses have capsid-associated transcriptase activities. In the yeast viruses, as in reovirus, transcription appears to be the first stage of replication. We have found that the yeast viral transcriptase initiates RNA transcription in vitro and that the resultant plus strand RNA has the 5' terminus ppGp. No pre-existing primers are normally utilized in vitro. Like other double-stranded RNA viruses of eucaryotes, the yeast viruses have a primer-independent capsid-associated transcriptase. Unlike these viruses of higher eucaryotes, the yeast viruses synthesize uncapped mRNAs. Viral particles with only a single major capsid polypeptide are active in transcription and replication, while reovirus particles active in transcription have 5 or 6 polypeptides.