A SINGLE AMINO-ACID SUBSTITUTION IN YEAST EIF-5A RESULTS IN MESSENGER-RNA STABILIZATION

Most factors known to function in mRNA turnover are not essential for cell viability. To identify essential factors, similar to 4000 tempera ture-sensitive yeast strains were screened for an increase in the leve l of the unstable CYH2 pre-mRNA. At the non-permissive temperature, fi ve mutants exhibited decreased decay rates of the CYH2 pre-mRNA and mR NA, and the STE2, URA5 and PAB1 mRNAs. Of these, the mutant ts1159 had the most extensive phenotype. Expression of the TIF51A gene (encoding eIF-5A) complemented the temperature-sensitive growth and mRNA decay phenotypes of ts1159, The tif51A allele was rescued from these cells a nd shown to encode a serine to proline change within a predicted alpha -helical segment of the protein. ts1159 also exhibited an similar to 3 0% decrease in protein synthesis at the restrictive temperature. Measu rement of amino acid incorporation in wild-type cells incubated with i ncreasing amounts of cycloheximide demonstrated that a decrease in pro tein synthesis of this magnitude could not account for the full extent of the mRNA decay defects observed in ts1159. interestingly, the ts11 59 cells accumulated uncapped mRNAs at the non-permissive temperature. These results suggest that eIF-5A plays a role in mRNA turnover, perh aps acting downstream of decapping.