Semi-conservative replication of double-stranded RNA by a virion-associated RNA polymerase

5-Bromo-UTP was found to replace UTP efficiently as a substrate for the virion-associated double-stranded RNA replicase of $\text{Penicillium}\text{stoloniferum}$ virus PsV-S. The double-stranded RNA product of the replication reaction with 5-bromo-UTP as a substrate gave in equilibrium caesium sulphate density gradient centrifugation a single band with a buoyant density of 1.647 g/ml, consistent with that of a hybrid double-stranded RNA consisting of one brominated and one unbrominated strand. After the reaction none of the original unbrominated double-stranded RNA (buoyant density 1.606 g/ml) could be detected. It is concluded that replication of double-stranded RNA in virions of PsV-S takes place by a semi-conservative mechanism.