RNA dependent RNA polymerase synthesizes RNA polymer using RNA template, reaction central to life cycles of RNA viruses. Unlike DNA-dependent RNA polymerase transcribing DNA to RNA, RdRp reads single-stranded or double-stranded RNA via complementary base pairing producing new RNA strand. Active site contains conserved GDD motif coordinating magnesium ions catalyzing phosphodiester formation. Host mammalian cells generally lack RdRp except for limited RNA interference amplification pathways, whereas most RNA viruses encode own enzyme for replication. Enzymatic activity often membrane-associated
Ref:
Lodish Chapter 8: RNA dependent RNA polymerase uses RNA template definition; Fields Virology Chapter 6 RdRp mechanism