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 and SKI8 gene products block the translation of non-poly(A) mRNAs. 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 have now cloned the SKI6 and SKI7 genes. Ski6p has homology with RNase PH, an enzyme that removes 3' nucleotides from tRNA precursors in a phosphorolytic reaction (like polynucleotide phosphorylase). Ski6-2 mutants are hypersensitive to hygromycin, an indication of involvement of Ski6p in translation. These ski6-2 strains translate non-poly(A) mRNAs 10x better than isogenic wild type cells. They also accumulate a 38S particle with a partially degraded 25S rRNA, providing direct evidence for altered 60S subunits as the cause of the derepressed translation of non-poly(A) mRNAs. Ski7p is highly homologous to translation factor EF1alpha. The ski7 mutants are derepressed for translation of non-poly(A) mRNAs and overexpression of Ski7p results in loss of M dsRNA.
