Saccharomyces cerevisiae virus L-A

Saccharomyces cerevisiae virus L-A is a double-stranded RNA (dsRNA) virus of the yeast Saccharomyces cerevisiae. This virus has a single 4.6 kb genomic segment that encodes its major coat protein, Gag (76 kDa) and a Gag-Pol fusion protein (180 kDa) formed by a -1 ribosomal frameshift. L-A can support the replication and encapsidation in separate viral particles of any of several satellite dsRNAs, called M dsRNAs, each of which encodes a secreted protein toxin (the killer toxin) and immunity to that toxin. L-A and M are transmitted from cell to cell by the cytoplasmic mixing that occurs in the process of mating. Neither is naturally released from the cell or enters cells by other mechanisms, but the high frequency of yeast mating in nature results in the wide distribution of these viruses in natural isolates. Moreover, the structural and functional similarities with dsRNA viruses of mammals has made it useful to consider these entities as viruses.^[1]

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Viral replication cycle

Both the (+) and the (–) strands of L-A are synthesized within viral particles. Mature particles containing L-A dsRNA synthesize (+) strands (transcription) by a conservative mechanism, and extrude the new strand from the particles. These (+) strands serve as mRNA, to make coat proteins, and as the species encapsidated by those proteins to make new nucleoprotein particles. Encapsidated (+) strands are converted to dsRNA form (replication). The absence of both 5′ cap and 3′ poly(A) on the viral mRNA is the basis of complex interactions of virus and host in translation.^[1]

Similarity of L-A to animal dsRNA viruses

Animal and plant dsRNA viruses have up to three capsid layers, with structures adapted to protect the virus in its extracellular travels, to facilitate uptake by specific target cells and to engineer the uncoating of the outer layers after uptake. The yeast L-A virus can dispense with much of this apparatus because of its strictly intracellular life cycle. However, both L-A and plant and animal dsRNA viruses synthesize both (+) and (–) viral RNA strands within particles. The core particles in the case of the animal viruses and the structures of these inner nucleoprotein particles are similar.^[1]

See also

• Double-stranded RNA viruses
• Virology
• RNA virus
• Virus classification
• List of viruses

References