Inhibition of 5' to 3' mRNA degradation under stress conditions in Saccharomyces cerevisiae: from GCN4 to MET16

After deadenylation, most cytoplasmic mRNAs are decapped and digested by 5' to 3' exonucleases in Saccharomyces cerevisiae. Capped and deadenylated mRNAs are degraded to a lesser extent by 3' to 5' exonucleases. We have used a method, based on the electroporation of in vitro synthetised mRNAs, to study the relative importance of these two exonucleolytic pathways under stress conditions. We show that derepression of GCN4 upon amino acid starvation specifically limits the 5'-to-3'-degradation pathway. Because adenosine 3'-5' biphosphate (pAp), which is produced by Met16p, inhibits this degradation pathway to a comparable extent, we were prompted to analyse the role of Met16p in this phenomenon. We show that the inhibitory effects of amino acid limitation on 5' to 3' mRNA degradation are absent in a met16 mutant. We therefore conclude that the GCN4 dependence of MET16 expression is responsible for the decrease in 5' to 3' digestion under stress conditions and that cells use pAp as a signal to limit 5' to 3' RNA degradation under stress conditions. Because 3' to 5' mRNA degradation is unaffected, the relative importance of this pathway in the decay of certain RNAs may be increased under stress conditions.