Reconstitution of template-dependent in vitro transcriptase activity of a yeast double-stranded RNA virus.

Isolated mature L-A viral particles from yeast have a transcriptase activity that uses endogenous L-A double-stranded RNA (dsRNA) as template. We have previously demonstrated that empty particles derived from mature L-A viral particles have replicase activity capable of synthesizing minus strand single-stranded RNA (ssRNA) on an added plus strand ssRNA template to form dsRNA. We report here that empty particles also have transcriptase activity that uses added viral dsRNA as template. The newly synthesized ssRNA was the plus strand, and some of these transcripts were converted to the dsRNA form by the replicase activity associated with the empty particles. This transcriptase activity, however, required a much higher concentration of polyethylene glycol than that used previously for the replicase activity. The mode of transcription was conservative. The enzyme transcribed ssRNA from L-A, M1, or X (a deletion mutant of L-A) dsRNAs but not from other yeast dsRNAs (L-BC, T, or W), bacteriophage Phi6 dsRNAs, or animal rotavirus dsRNAs, indicating the same template specificity as that expected for the in vivo reaction. This assay system, and the replicase assay system, will allow us to study in vitro all the enzymatic reactions essential for the viral replication cycle.