RNA-directed RNA polymerase involved in transcription and genome replication. Following infection, catalyzes the synthesis of fully conservative plus strands. After core assembly, which consists in recruitment of one capped plus-strand for each genomic segments and polymerase complexes, the polymerase switches mode and catalyzes the synthesis of complementary minus-strands (By similarity). a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Interacts with VP4. Belongs to the reoviridae RNA-directed RNA polymerase family.