Negative‑sense RNA viruses require which enzyme first?
Negative-sense RNA viruses carry genome complementary to mRNA, unable to be translated directly by host ribosomes. Influenza, rabies, Ebola and measles viruses package RNA-dependent RNA polymerase within virion to transcribe genome into positive-sense cRNAs and mRNAs immediately after entry. This polymerase synthesizes complementary strand serving as template for replication and as translatable message bearing 5' cap obtained via cap-snatching. Host RNA polymerase II cannot transcribe RNA genomes, precluding reliance on cellular enzyme. Requirement for preformed viral polymerase explains why purified genomic RNA alone is non-infectious.
Ref: Baltimore D., PNAS 1971: Negative-Sense RNA Viruses Require RNA-Dependent RNA Polymerase First