The polymerase subunit of a dsRNA virus plays a central role in the regulation of viral RNA metabolism

Bacteriophage phi6 has a three-segmented double-stranded (ds) RNA genome, w hich resides inside a polymerase complex particle throughout the entire lif e cycle of the virus. The polymerase subunit P2, a minor constituent of the polymerase complex, has previously been reported to replicate both phi6-sp ecific and heterologous single-stranded (ss) RNAs, giving rise to dsRNA pro ducts. In this study, we show that the enzyme is also able to use dsRNA tem plates to perform semi-conservative RNA transcription in vitro without the assistance of other proteins. The polymerase synthesizes predominantly plus -sense copies of phi6 dsRNA, medium and small segments being more efficient templates than the large one. This distribution of the test-tube reaction products faithfully mimics viral transcription in vivo. Experiments with ch imeric ssRNAs and dsRNAs show that short terminal nucleotide sequences can account for the difference in efficiency of RNA synthesis. Taken together, these results suggest a model explaining important aspects of viral RNA met abolism regulation in terms of enzymatic properties of the polymerase subun it.