Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 267

The genomes of (−) strand viruses are organized into nucleocapsids in which protein molecules, including the RdRP and accessory proteins, are bound to the genomic RNAs at regular intervals. These tightly wound ribonucleoproteins are very stable and generally resistant to RNase. The RdRPs of (−) strand viruses copy viral RNAs only when they are present in the nucleocapsid, such as that formed by the N protein of vesicular stomatitis virus bound to genomic RNA. In contrast, the genomes of many (+) strand RNA viruses are not coated with proteins in the virus particle [exceptions are the (+) strand RNA genomes of members of the Coronaviridae, Arteriviridae, and Retroviridae]. This difference is consistent with the fact that mRNAs are produced from the genomes of (−) strand RNA viruses upon cell entry, whereas the genomes of (+) strand RNA viruses are translated directly.

The viral nucleoproteins (NP) are cooperative, single-stranded-RNA-binding proteins, as are the single-stranded nucleic acid-binding proteins required during DNA-directed DNA synthesis. Their function during replication is to keep the RNA single stranded and prevent base pairing between the template and product, so that additional rounds of RNA synthesis can occur. The nucleoproteins of nonsegmented (−) strand RNA viruses have a two-lobe architecture that forms a positively charged groove that binds and shields the genomic RNA (Fig. 6.3). Interactions between nucleoproteins lock monomers tightly, resulting in rigid NP-RNA assemblies. The NP structures from segmented (−) strand RNA viruses are more varied and display less coordinated contacts between nucleoprotein subunits. These differences may explain why the NP-RNAs of these viruses are more susceptible to RNase digestion than those of nonsegmented (−) strand RNA viruses. The varied structures of the NP-RNA complexes also influence access of the viral RNA polymerase to the template. The RdRP of segmented (−) strand RNA viruses can bind the NP-RNA template directly, whereas those of nonsegmented (−) strand RNA viruses cannot: a phosphoprotein (P) is required to recruit the RdRP to the NP-RNA.

The genomes of many (+) strand RNA viruses encode helicases that serve functions similar to that of the nucleoproteins of (−) strand RNA viruses (see “Unwinding the RNA Template” below). In addition to its enzymatic activity, the poliovirus RdRP (3D^pol) is a cooperative single-stranded-RNA-binding protein and can unwind RNA duplexes without the hydrolysis of ATP, as is characteristic of helicase-mediated unwinding.

Figure 6.1 Strategies for replication and mRNA synthesis of RNA virus genomes are shown for representative virus families. Picornaviral genomic RNA is linked to VPg at the 5′ end. The (+) genomic RNA of some flaviviruses does not contain poly(A). Only one RNA segment is shown for segmented (−) strand RNA viruses.