RNA-directed RNA polymerase that is involved in transcription and genome replication. Following infection, it 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) Belongs to the reoviridae RNA-directed RNA polymerase family.