Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 284

The genes of RNA viruses with a nonsegmented (−) strand RNA genome are expressed by the production of subgenomic mRNAs in infected cells (Fig. 6.19). An RdRP composed of one molecule of L protein associated with four molecules of P protein is thought to carry out vesicular stomatitis virus mRNA synthesis. Individual mRNAs are produced by a series of initiation and termination reactions as the RdRP moves down the viral genome (Fig. 6.20). This start-stop mechanism accounts for the observation that 3′-proximal genes must be copied before downstream genes (Box 6.3). The viral RdRP is unable to initiate synthesis of each mRNA independently.

Figure 6.19 Vesicular stomatitis viral RNA synthesis. Viral (−) strand genomes are templates for the production of either subgenomic mRNAs or full-length (+) strand RNAs. The switch from mRNA synthesis to genomic RNA replication is mediated by two RdRPs and by the N protein. mRNA synthesis initiates at the beginning of the N gene, near the 3′ end of the viral genome. Poly(A) addition is a result of reiterative copying of a sequence of seven U residues present in each intergenic region. Chain termination and release occur after approximately 150 A residues have been added to the mRNA. The RNA polymerase then initiates synthesis of the next mRNA at the conserved start site 3′-UUGUC … 5′. This process is repeated for all five viral genes. Synthesis of the full-length (+) strand begins at the exact 3′ end of the viral genome and is carried out by the assembled RdRP L-N-(P)4. The (+) strand RNA is bound by the viral nucleocapsid (N) protein, which is associated with the P protein in a 1:1 molar ratio. The N-bound assembled RdRP-L-(P)4 complexes bind to the nascent (+) strand RNA, allowing the RdRP to read through the intergenic junctions at which polyadenylation and termination take place during mRNA synthesis.

Figure 6.20 Stop-start model of vesicular stomatitis virus mRNA synthesis. The RdRP (Pol) initiates RNA synthesis at the 3′ end of the N gene. After synthesis of the N mRNA, RNA synthesis terminates at the intergenic region, followed by reinitiation at the 3′ end of the P gene. This process continues until all five mRNAs are synthesized. Reinitiation does not occur after the last mRNA (the L mRNA) is synthesized, and, as a consequence, the 59 5′-terminal nucleotides of the vesicular stomatitis virus genomic RNA are not copied. Only a fraction of the polymerase molecules successfully make the transition from termination to reinitiation of mRNA synthesis at each intergenic region.