5 Yeast RNA Virology The Killer Systems

Abstract

I. INTRODUCTION
Most strains of Saccharomyces cerevisiae harbor one or more double-stranded RNA (dsRNA) viruses. These viral species were first discovered as the agents responsible for the killer phenotype of yeast. Killer strains of yeast are those that secrete a protein toxin that is lethal to most nonkiller Saccharomyces strains. The viral particles are infectious only by cell-to-cell fusion, a route less commonly used by animal viruses. In other respects, these yeast viruses are quite similar to dsRNA viruses found in larger eukaryotic organisms. As a result of this similarity, the viruses of Saccharomyces provide a useful model for the central mechanisms of virus replication, such as RNA-dependent transcription, replication, packaging, ribosomal frameshifting, and the interrelationship of host and viral functions. The killer toxin, encoded by a satellite virus, has also been useful for the study of protein processing and secretion and toxin action and receptors.

Yeast RNA viruses include three families with dsRNA genomes (L-A, L-BC, and M), two other families of dsRNA replicating in yeast that may yet prove to be viral (T and W), and four families of retroviruses (Tyl–Ty4) (Table 1). This paper focuses on the dsRNA viruses and the single-stranded circular RNA replicon, 20S RNA; retroviruses are reviewed by Boeke and Sandmeyer (this volume), although several striking parallels between these systems will be mentioned here as well. This review also includes several speculative models designed to relate the molecular, biochemical, and genetic information on dsRNA viruses to their biological context. Detailed descriptions of the...