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. SLH1 is an RNA helicase homologous to SKI2. We showed that Ski2p and Slh1p have overlapping function, and together block the translation of non-poly(A) mRNA. A ski2 slh1 double mutant treats non-poly(A) mRNA the same as it treats poly(A)+ mRNA, with the same rate of translation and the same duration of translation. The ski2 slh1 double mutant has no detectable difference in mRNA turnover rate. Thus the 3' poly(A) structure of mRNA is only needed for translation because of the cooperating action of Ski2p and Slh1p (and other proteins that work with them). The ribosomes and translation factors are fully able to use non-poly(A) mRNAs even in the presence of a full complement of poly(A)+ mRNAs competing for the translation apparatus. The standard model of 3' poly(A) action in translation is that interaction of the poly(A) binding protein (Pab1p) with initiation factor 4G (eIF4G), by circularizing the mRNA, promotes 40S subunit recruitment. However, we find that elimination of the Pab1p - eIF4G interaction does not affect the requirement of translation for the 3' poly(A), showing that this model is incorrect. However, elimination of Fun12p, a protein involved in 60S ribosomal subunit joining, preferentially impairs translation of poly(A)+ mRNA without significantly affecting translation of poly(A)- mRNA. This indicates that the 3' poly(A) structure has a role in the 60S joining reaction. Our collaborators, Drs. Hisashi Naitoh and John E. Johnson, have crystallized the L-A virus and determined its structure to 3.4 angstroms resolution by X-ray crystallography. The L A dsRNA virus is 400 angstroms in diameter, and contains a single protein shell of 60 asymmetric dimers of the coat protein, a feature common among the inner protein shells of dsRNA viruses, and probably related to their unique mode of transcription and replication. The two identical Gag molecules in each dimer are in non-equivalent environments, and show substantially different conformations in specific surface regions. This virus decaps cellular mRNA in order to express its own uncapped mRNA. Our structure reveals a trench at the active site of the decapping reaction and suggests a role for nearby residues in the reaction.
