RNA polymerase activity of Ustilago maydis virus

Ustilago maydis virus has an RNA polymerase enzyme which is associated with virion capsids. In the presence of Mg2+ ion and ribonucleotide triphosphate, the enzyme catalyzes the in vitro synthesis of mRNA by using dsRNA as a template. The products of the UmV RNA polymerase were both ssRNA and dsRNA. The dsRNA was determined by characteristic mobilities in gel electrophoresis, lack of sensitivity to RNase, and specific hybridization tests. The ssRNAs were identified by elution from a CF-11 column and by their RNase sensitivity. On the basis of the size of ssRNAs, it was concluded that partial transcripts were produced from H dsRNA segments, and full length transcripts were produced from M and L dsRNA segments. The following observations indicates that transcription occurs by strand displacement; (1) Only the positive strand of M2 dsRNA was labeled by the in vitro reaction. (2) The M2 dsRNA which had been labeled with 32''P-UTP in vitro could be chased from dsRNA with unlabeled UTP. The transcription products of three UmV strains were compared, and the overall pattern of transcription was very similar among them