Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 276

Additional N- and C-terminal domains that surround the RdRP cores are often encoded in the genomes of larger RNA viruses (Fig. 6.13). The flavivirus RdRP has an extra N-terminal domain that has 5′-methyltransferase activity that contributes to mRNA capping. The core RdRPs of double-stranded RNA viruses are flanked by large N- and C-terminal domains. The former surrounds the fingers and thumb subdomains, closing the enzyme in a cage-like structure. The C-terminal domains are shaped like bracelets and resemble the sliding clamps that contribute to the efficiency of DNA polymerases. In contrast to other RdRPs, these enzymes have four channels. Two are in equivalent positions to the template and nucleotide entry channels of other RdRPs, but the other two serve as RNA exit pathways. One extends through the bracelet domain and is the pathway for release of new double-stranded RNA to the particle interior. The other serves to guide newly synthesized (+) single-stranded RNAs out of the core.

The RdRP of (–) strand RNA bunyaviruses has an N-terminal endonuclease domain that is essential for procuring capped mRNA primers. The minimal RdRP of vesicular stomatitis virus is surrounded by three globular domains with three enzymatic activities required for mRNA 5′-cap synthesis: 2′-O-methyltransferase, guanine-N7-methyltransferase, and polyribonucleotidyl transferase.

Not all RdRPs have other functions encoded in extra N- and C-terminal domains. The influenza virus RdRP consists of three individual polypeptides, PA, PB1, and PB2, each of which has the distinct activity described above.