SUMMARY

It is shown that the virion RNA polymerase of Aspergillus foetidus virus S, which has a genome consisting of three double-stranded (ds)RNA species, is a transcriptase. Synthesis of single-stranded RNA (ssRNA) in vitro continues for up to 48 h, during which time 6 to 8 full length transcripts are produced, on average, per molecule of dsRNA2, i.e. re-initiation of transcription occurs in this in vitro system. Full length ssRNA transcripts of dsRNAI are also produced in smaller quantity, but no transcripts of dsRNA3 could be detected. In reactions containing 3H-UTP, label is also incorporated into dsRNA2 reaching maximum levels after 6 h, but no incorporation oflabelled precursor into dsRNAI or dsRNA3 could be detected. The ssRNA transcripts produced were released into the medium, but labelled dsRNA remained within the virions. The locations, in the AfV-S multicomponent system, of RNA polymerase activities promoting incorporation of 3H-UMP into the two ssRNA transcripts and into dsRNA2 have been deter-mined