The polymerase of negative-stranded RNA viruses

• Comparative structure and function analysis of RNA synthesis machinery of negative-sense RNA viruses.
• Unique biology of mRNA capping: cap snatching versus cap synthesis.
• New insights into how NP–RNA templates of negative-sense viruses are copied.
• Different cofactors requirements by different classes of polymerase.
• In vitro assays for polymerase activity.

Negative-sense (NS) RNA viruses deliver into cells a mega-dalton RNA–protein complex competent for transcription. Within this complex, the RNA is protected in a nucleocapsid protein (NP) sheath which the viral polymerase negotiates during RNA synthesis. The NP–RNA templates come as nonsegmented (NNS) or segmented (SNS), necessitating distinct strategies for transcription by their polymerases. Atomic-level understanding of the NP–RNA of both NNS and SNS RNA viruses show that the RNA must be transiently dissociated from NP during RNA synthesis. Here we summarize and compare the polymerases of NNS and SNS RNA viruses, and the current structural data on the polymerases. Those comparisons inform us on the evolution of related RNA synthesis machines which use two distinct mechanisms for mRNA cap formation.