The target of rapamycin (TOR) signaling pathway is an important mechanism by which cell growth is regulated by nutrient availability in eukaryotes. We provide evidence that the TOR signaling pathway controls mRNA turnover in Saccharomyces cerevisiae. During nutrient limitation (diauxic shift) or after treatment with rapamycin (a specific inhibitor of TOR), multiple mRNAs were destabilized, whereas the decay of other mRNAs was unaffected. Our findings suggest that the regulation of mRNA decay by the TOR pathway may play a significant role in controlling gene expression in response to nutrient depletion. The inhibition of the TOR pathway accelerated the major mRNA decay mechanism in yeast, the deadenylation-dependent decapping pathway. Of the destabilized mRNAs, two different responses to rapamycin were observed. Some mRNAs were destabilized rapidly, while others were affected only after prolonged exposure. Our data suggest that the mRNAs that respond rapidly are destabilized because they have short poly(A) tails prematurely either as a result of rapid deadenylation or reduced polyadenylation. In contrast, the mRNAs that respond slowly are destabilized by rapid decapping. In summary, the control of mRNA turnover by the TOR pathway is complex in that it specifically regulates the decay of some mRNAs and not others and that it appears to control decay by multiple mechanisms.
INTRODUCTIONIt is estimated that most of the microorganisms in the environment exist in conditions in which nutrients are limiting (Lewis and Gattie, 1991). Nutrient availability is very important in controlling cell division, growth, and physiology in microorganisms as well as in multicellular organisms. One critical response in yeast to glucose limitation is the switch from fermentation to respiration, which is termed the diauxic shift. At the diauxic shift, major changes in gene expression are induced (reviewed in Werner-Washburne et al., 1996) including a general repression of translation (Fuge et al., 1994) and extensive changes in the abundance of mRNAs (DeRisi et al., 1997). The target of rapamycin (TOR) signaling pathway senses external nutrient availability and is involved in mediating the changes in gene expression induced at the diauxic shift (reviewed in Cutler et al., 1999;Dennis et al., 1999;Thomas and Hall 1999;Cardenas et al., 1999). Rapamycin artificially induces a starvation-like state in yeast (Barbet et al., 1996) by first forming a complex with the yeast FK506 binding protein, FKBP. This complex then binds to and represses the activity of the TOR1 and TOR2proteins (Heitman et al., 1991). The TOR signaling transduction pathway is conserved among yeast, flies, and mammals. In mammalian cells, the TOR protein homolog mTOR/ FRAP/RAFT1 also coordinates nutrient and mitogenic signals to control cell growth and cell-cycle progression (reviewed in Cutler et al., 1999;Thomas and Hall, 1999). The Drosophila TOR homolog dTOR also senses nutrient availability (Oldham et al., 2000;Zhang et al., 2000). The TOR proteins are protein kinases...