The control ofmRNA decay in Saccharomyces cerevisiae

Abstract

Control of mRNA degradation is an important step in the regulation of gene expression. In Saccharomyces cerevisiae, pathways of mRNA decay have been determined and have provided a framework for understanding how mRNA decay is controlled. I have studied how the process of translation affects the decay mechanism of a yeast transcript and I have isolated and characterized yeast mutants that exhibit reduced rates of mRNA decay. The process of translation has been shown to affect mRNA decay rates in eukaryotes. However, using a MFA2 mRNA that cannot be translated due to insertion of secondary structure in its 5' untranslated region, I have determined that translation of the MFA2 mRNA is not required for its degradation. This observation demonstrates that translation of an mRNA, per se, is not required for the normal kinetics or mechanism of mRNA decay. Additionally, I have demonstrated that the translational inhibitor, cycloheximide, reduces the rate at which the MFA2 transcript is decapped. Inhibition of decapping occurs even on MFA2 transcripts that cannot be translated due to insertion of secondary structure. This result suggests that the general stabilizing effects of translational inhibitors on mRNAs may not be due to the inhibition of translation of these transcripts. The identification of mRNA decay pathways in yeast, deadenylation-dependent decapping and deadenylation-independent decapping, provided a basis by which gene products required for mRNA decay through these pathways could be identified. To this end, a screen of mutant yeast strains was undertaken. I have isolated and characterized two mutants, mrt1 and mrt3, that exhibit reduced rates of deadenylation-dependent decapping on several yeast transcripts. This result suggests that the MRT1 and MRT3 gene products promote deadenylation-dependent mRNA decapping. A third mutant, dcp1, was also isolated, and the wild-type DCP1 gene was identified. Characterization of dcp1/ mutants by myself and others revealed that the DCP1 gene encodes the decapping enzyme, or an essential component of the decapping enzyme, required for both deadenylation-dependent and deadenylation-independent mRNA decapping. This result demonstrates that the DCP1 gene product, Dcp1p, is required for all known mRNA decapping in yeast.