Double-stranded and single-stranded RNA viruses of Saccharomyces cerevisiae.

Yeast RNA viruses include L-A (and its toxin-encoding satellites M1, M2, ...) and L-BC dsRNA viruses and the single-stranded replicons 20S RNA and 23S RNA. L-A has a single-segment 4.6-kb linear genome encoding a major coat protein (gag) and its RNA-dependent RNA polymerase (pol), the latter expressed as a gag-pol fusion protein formed by a -1 ribosomal frameshift. In vitro replication, transcription, and binding systems for L-A have been used to define cis sites necessary for packaging and replication of viral RNA. Cellular functions that promote viral replication include the MAK3-encoded N-acetyltransferase whose modification of the gag N terminus is necessary for L-A virus assembly. The toxins encoded by the M satellite RNAs are processed by enzymes (KEX1 and KEX2, for killer expression) whose study led to discovery of mammalian hormone-processing enzymes. 20S RNA is an apparently naked circular RNA replicon (with a dsRNA form called W) encoding a RNA polymerase-like molecule. Its copy number is induced 10,000-fold in 1% potassium acetate, and it is subject to the same SKI antiviral system that represses L-A, L-BC, and M dsRNA copy number.