Regulation of yeast development by mRNA methylation

Abstract

The internal methylation of mRNA post-transcriptionally is an essential component of the mRNA editing machinery in virtually every eukaryotic system. Despite this ubiquity, little is known about the relevance, consequences or machinery involved in this process. The recent demonstration of this modification in the brewers' yeast, Saccharomyces cerevisiae, has allowed the study of this modification using the vast array of genetic and biochemical tools available in the organism. In the second chapter of this thesis, we show that diploid cells of the yeast Saccharomyces cerevisiae experiencing nutrient limitation undergo a restriction of cellular potential and commitment in which the cells cease vegetative, mitotic growth and commit to meiosis. We show that the period prior to commitment can be divided further into two distinct phases: an early stage of initial starvation followed by a commitment to differentiation. Cells that are in the initial starvation phase revert to yeast-form mitotic growth if shifted to nutrientrich conditions. Cells that are in the commitment to differentiation phase are incapable of returning to yeast-form growth if shifted to nutrient-rich medium, but instead synchronously engage in pseudo-hyphal budding-a nutrient foraging response. Co-ordination of meiosis and PH development in the commitment to differentiation phase is regulated by mRNA methylation. We dissect this mRNA methylation upon nutrient starvation in the third chapter of this thesis. We identify Ime4, Mum2 and Slz1 as the components of a protein complex that catalyzes mRNA methylation in yeast. These components are necessary for m⁶A accumulation during nutrient starvation; mutation of any one of these components results in defects in meiotic and PH development. Furthermore, we find that ectopic expression of these components under nutrient-rich conditions is sufficient to catalyze this methylation of mRNA. Finally, we provide evidence that this modification is necessary for the activation of translation of genes under starvation conditions. These findings provide evidence for a method of finetuning translation under nutrient-stress conditions. Together, our results support the notion that the yeast starvation response is an extended process that progressively restricts cell fate and reveal a broad role of posttranscriptional RNA methylation in regulating these decisions.