A single amino acid substitution in yeast eIF-5A results in mRNA stabilization

Most factors known to function in mRNA turnover are not essential for cell viability. To identify essential factors, approximately 4000 temperature-sensitive yeast strains were screened for an increase in the level of the unstable CYH2 pre-mRNA. At the non-permissive temperature, five mutants exhibited decreased decay rates of the CYH2 pre-mRNA and mRNA, 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 and shown to encode a serine to proline change within a predicted alpha-helical segment of the protein. ts1159 also exhibited an approximately 30% decrease in protein synthesis at the restrictive temperature. Measurement of amino acid incorporation in wild-type cells incubated with increasing amounts of cycloheximide demonstrated that a decrease in protein synthesis of this magnitude could not account for the full extent of the mRNA decay defects observed in ts1159. Interestingly, the ts1159 cells accumulated uncapped mRNAs at the non-permissive temperature. These results suggest that eIF-5A plays a role in mRNA turnover, perhaps acting downstream of decapping.