The complementary strands of reovirus double-stranded ribonucleic acid (ds RNA) are synthesized sequentially in vivo and in vitro. In both cases, preformed plus strands serve as templates for the synthesis of the complementary minus strands. The in vitro synthesis of dsRNA is catalyzed by a large particulate fraction from reovirus-infected cells. Treatment of this fraction with chymotrypsin or with detergents which solubilize cellular membranes does not alter its capacity to synthesize dsRNA. The enzyme or enzymes responsible for dsRNA synthesis remain sedimentable at 10,000 × g after these enzyme or detergent treatments, indicating their particulate nature. Pretreatment of this fraction with ribonuclease, however, abolishes its ability to catalyze dsRNA synthesis, emphasizing the single-stranded nature of the template and its location in a structure permeable to ribonuclease. In contrast, the newly formed dsRNA is resistant to ribonuclease digestion at low salt concentrations and hence is thought to reside within a ribonuclease-impermeable structure
