MECHANISMS AND CONTROL OF MESSENGER-RNA TURNOVER IN SACCHAROMYCES-CEREVISIAE

mRNA turnover makes significant contributions to the control of gene e xpression. Recent studies of mRNA turnover in the simple eukaryote Sac charomyces cerevisiae have led to a better understanding of this proce ss. These studies suggest that half-lives of individual transcripts ar e determined in large part by the interplay between specific sequences within mRNAs and less defined features of an mRNA, such as translatio n rate. These studies also indicate that yeast mRNAs are degraded thro ugh a diverse set of decay pathways. The most common of these pathways is deadenylation-dependent decapping, in which transcripts are deaden ylated, thereby promoting decapping and exposing the transcript to 5'- to-3' exonucleolytic digestion. Additional pathways of mRNA decay incl ude deadenylation-independent decapping 3'-to-5' degradation and possi bly endonucleolytic cleavage. Recent experiments have also begun to id entify some of the trans-acting factors responsible for both performin g and regulating the nucleolytic events in these various pathways. Fur ther identification of additional trans-acting factors that function i n mRNA decay and the development of yeast cell-free systems capable of reproducing in vivo mRNA turnover are major challenges to be addresse d.