We have described two dsRNA viruses (L-A and L-BC) and two ssRNA replicons (20S RNA and 23S RNA) in the yeast Saccharomyces cerevisiae. M dsRNA is a satellite of L-A encoding the killer toxin. We discovered 7 chromosomal genes, SKI1, 2, 3, 4, 6, 7, and 8, by their ability to prevent these replicons from causing pathogenicity to yeast cells. These four RNA replicons all make uncapped mRNAs and lack a 3 poly(A)structure. SKI1 is an exoribonuclease specific for uncapped mRNAs, while we showed that the SKI2, SKI3, SKI6, SKI7 and SKI8 gene products block the translation of non-poly(A) mRNAs. We showed that Ski2p is an RNA helicase, Ski6p has homology to a tRNA - processing RNAse, and Ski7p is similar to translation factor EF1alpha. We showed that mutations in 20 chromosomal genes resulting in loss of M dsRNA are deficient in 60S ribosomal subunits. These mutations are suppressed by ski mutations without restoration of the 60S subunit deficiency. We proposed that SKI2, SKI3 and SKI8 block translation of non-poly(A) mRNA by an effect on ribosome biogenesis affecting the interaction of 60S subunits with the 3 poly(A) structure of the mRNA.We find that a ski7 mutation increases both the initial rate and the duration of translation of electroporated non-polyA mRNAs, indicating effects of the gene on both translation and mRNA stability. The effect on mRNA stability may be secondary to the effect on translation. The ski7 deletion mutant is also hypersensitive to hygromycin, again suggesting an effect on translation. Overexpression of Ski7p lowers expression of non-polyA mRNAs and cures M dsRNA. We are now studying another yeast gene homologous to SKI2. We find that mutations of this gene also specifically derepress expression of non-polyA mRNAs. We are studying the mechanism of this effect and the interaction of this gene with SKI2. - double-stranded RNA, virus, replication, assembly, Saccharomyces cerevisiae, poly(A), translation, SKI6, SKI7, SKI2, SKI3, SKI8, FUN12
