Abstract
Double-stranded RNA viruses have a virion-associated RNA-dependent RNA polymerase activity which is involved in such critical steps of viral assembly as genome packaging and minus strand synthesis. In vitro studies of a bacterial dsRNA virus, ø6, and a yeast virus, L-A, have shed light on capsid formation as well as on the protein/RNA interactions and packaging of the viral genomes. In the ø6 system, an empty dodecahedral polymerase complex (procapsid) composed of four protein species is formed without the help of other viral proteins or RNA. This particle packages positive sense viral RNA genome segments in an ATP dependent reaction. The presence of all rNTPs allows the synthesis of complementary (-) strands within the particle. Self-assembly of an additional protein shell (composed of protein P8) around this particle takes place in the presence of Ca2+ ions. In vivo, these nucleocapsids obtain an envelope while still residing in the cell cytoplasm. L-A, in contrast, is not known to make a prohead structure. The Pol domain of L-A's Gag-Pol fusion protein is necessary for packaging of the (+) strand RNA and probably actually binds to the (+) strand packaging site (a stem-loop with a protruding A) insuring its packaging while the Gag domain primes polymerization of the coat protein. N-Acetylation of Gag by the host MAK3 N-acetyltransferase is necessary for proper assembly, and the ratio of Gag-Pol/Gag, determined by the efficiency of - 1 ribosomal frameshifting, is critical for propagation of the M1 satellite dsRNA.
Published Version
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