A turnover pathway for both stable and unstable mRNAs in yeast: evidence for a requirement for deadenylation.

To determine pathways of mRNA turnover in yeast, we have followed the poly(A) tail removal and degradation of a pulse of newly synthesized transcripts from four different genes. Before decay of both stable and unstable mRNAs initiated, there was a temporal lag during which the poly(A) tail was deadenylated to an oligo(A) length. Altering the deadenylation rate of an mRNA led to a corresponding change in the length of this lag. The rate of deadenylation and the stability of the oligo(A) species varied between mRNAs, explaining the differences in mRNA half-lives. To examine how the transcript body was degraded following deadenylation, we used the strategy of inserting strong RNA secondary structures, which can slow exonucleolytic digestion and thereby trap decay intermediates, into the 3' UTR of mRNAs. Fragments lacking the 5' portion of two different mRNAs accumulated after deadenylation as full-length mRNA levels decreased. Therefore, these results define an mRNA decay pathway in which deadenylation leads to either internal cleavage or decapping followed by 5'-->3' exonucleolytic degradation of the mRNA.