Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 291

The genomic RNA of (+) strand viruses can be translated in the cell, and the translation products include the viral RNA polymerase. At a certain point in infection, the RNA polymerase copies the RNA in a 3′ → 5′ direction while ribosomes traverse it in an opposite direction (Fig. 6.26), raising the question of whether the viral polymerase avoids collisions with ribosomes. When ribosomes are frozen on polioviral RNA by using inhibitors of protein synthesis, replication is blocked. In contrast, when ribosomes are run off the template, replication of the RNA increases. These results suggest that ribosomes must be cleared from viral RNA before it can serve as a template for (−) strand RNA synthesis; in other words, replication and translation cannot occur simultaneously.

Figure 6.26 Ribosome-RNA polymerase collisions. A strand of viral RNA is shown, with ribosomes translating in the 5′ ′ 3′ direction and RNA polymerase copying the RNA chains in the 3′ ′ 5′ direction. Ribosome-polymerase collisions would occur in cells infected with (+) strand RNA viruses unless mechanisms exist to avoid simultaneous translation and replication.

The interactions of viral and cellular proteins with the polioviral 5′ untranslated region might determine whether the genome is translated or replicated. In this model, binding of cellular poly(rC)-binding protein 2 within the 5′ untranslated region initially stimulates translation. Once the viral protease has been synthesized, it cleaves poly(rC)-binding protein, and binding of the cellular protein is reduced. However, cleaved poly(rC)-binding protein can still bind to a different segment of the 5′ untranslated region (the cloverleaf) (Fig. 6.10) and promote viral genome synthesis.

Restricting translation and RNA synthesis to distinct compartments may prevent collisions of ribosomes and polymerases. Viral mRNA synthesis takes place in the reovirus capsid, where the enzymes responsible for this process are located. The viral mRNAs are exported to the cytoplasm for translation. Retroviral RNAs are synthesized in the cell nucleus, where translation does not take place. The architecture of membranous replication complexes of (+) strand RNA viruses may favor RNA synthesis and exclude translation.

Even if mechanisms exist for controlling whether the genomes of RNA viruses are translated or replicated, some ribosome-RNA polymerase collisions are likely to occur. The isolation of a polioviral mutant with a genome that contains an insertion of a 15-nucleotide sequence from 28S ribosomal RNA (rRNA) is consistent with this hypothesis. The RNA polymerase apparently copied 15 nucleotides of rRNA after colliding with a ribosome.