Glucose-regulated turnover of mRNA and the influence of poly(A) tail length on half-life

Glucose repression in Saccharomyces cerevisiae can now be seen to operate at two levels: regulation of transcription of certain genes and control of the half-life of the corresponding mRNAs (Scheffler, I. E., de la Cruz, B. J., and Prieto, S. (1998) Int. J. Biochem Cell Biol. 30, 1175-1193). For example, the steady state levels of SDH2 mRNA and SUC2 mRNA are significantly determined by their differential rates of turnover. A current model for the mechanism of mRNA turnover includes three distinct steps: a rate-limiting deadenylation, removal of the 5'-7-methyl-G (decapping), and 5'-3' exonuclease digestion. We have investigated the same three reactions during glucose-induced degradation of these transcripts. Our results indicate that while decapping (by Dcp1p) and 5'-3' exonuclease digestion (by Xrn1p) are obligatory steps for the rapid degradation of these mRNAs, the dependence on deadenylation is more complicated. At steady state in glycerol these transcripts have very short poly(A) tails but are nevertheless very stable; the addition of glucose causes immediate decapping and degradation without further deadenylation; in contrast, newly made SUC2 mRNA (after a shift from glucose to glycerol) has significantly longer poly(A) tails, and such transcripts are not rapidly degraded upon addition of glucose. A constitutive deadenylation reaction that is independent of the carbon source eventually makes the stability of these transcripts very sensitive to glucose. These results are interpreted in terms of a working hypothesis proposing a competition between translational initiation and decapping influenced by the carbon source. The presence of a long poly(A) tail may also affect this competition in favor of translational initiation and mRNA stabilization.

Control of mRNA turnover as a mechanism of glucose repression in Saccharomyces cerevisiae

The phenomenon of glucose repression in yeast is concerned with the repression of a large number of genes when glucose is an abundant carbon source and almost all of the energy requirements of the cell can be satisfied from glycolysis. Prominent among the repressed genes are those encoding mitochondrial proteins required for respiration and oxidative phosphorylation. Past studies have characterized a pathway by which a signal generated from extracellular glucose is transmitted to the nucleus. The ultimate outcome is the repression of transcription of numerous genes, but also the induction of a limited number of others. The emphasis has been almost exclusively on transcriptional control mechanisms. A discovery made originally with the transcript of the SDH2 gene prompted an investigation of post-transcriptional mechanisms, and more specifically a study of the turnover rate of this mRNA in the absence and presence of glucose. SDH2 mRNA has a very short half-life in medium with glucose (YPD) and a significantly longer half-life in medium with glycerol (YPG). Experimental evidence and recent progress in understanding of (1) mRNA turnover in yeast and (2) initiation of translation on the 5' untranslated region of mRNAs, lead to a working hypothesis with the following major features: the carbon source, via a signaling pathway involving kinase/phosphatase activities, controls the rate of initiation, and thus influences a competition between eukaryotic initiation factors (prominently eIF4E, eIF4G, eIF3) binding to the capped mRNA and a decapping activity (DCP1) which is one of the rate limiting activities in the turnover of such mRNAs.