Regulation of Ribosome Biogenesis by the Rapamycin-sensitive TOR-signaling Pathway inSaccharomyces cerevisiae

Abstract

The TOR (target of rapamycin) signal transduction pathway is an important mechanism by which cell growth is controlled in all eucaryotic cells. Specifically, TOR signaling adjusts the protein biosynthetic capacity of cells according to nutrient availability. In mammalian cells, one branch of this pathway controls general translational initiation, whereas a separate branch specifically regulates the translation of ribosomal protein (r-protein) mRNAs. InSaccharomyces cerevisiae, the TOR pathway similarly regulates general translational initiation, but its specific role in the synthesis of ribosomal components is not well understood. Here we demonstrate that in yeast control of ribosome biosynthesis by the TOR pathway is surprisingly complex. In addition to general effects on translational initiation, TOR exerts drastic control over r-protein gene transcription as well as the synthesis and subsequent processing of 35S precursor rRNA. We also find that TOR signaling is a prerequisite for the induction of r-protein gene transcription that occurs in response to improved nutrient conditions. This induction has been shown previously to involve both the Ras-adenylate cyclase as well as the fermentable growth medium–induced pathways, and our results therefore suggest that these three pathways may be intimately linked.

The Target of Rapamycin Signaling Pathway Regulates mRNA Turnover in the YeastSaccharomyces cerevisiae

“…The fact that the majority of the mRNAs that we examined that are down-regulated at diauxic shift were destabilized strongly suggests that the regulation of mRNA turnover by TOR plays an important role in controlling yeast gene expression in response to nutrient limitation. Although the TOR signaling pathway has been demonstrated to regulate the transcription of specific genes, in particular genes for ribosomal proteins and proteins involved in nitrogen catabolite repression Hardwick et al, 1999;Powers and Walter, 1999), our observation that the CRY1 and GRC5 ribosomal protein mRNAs are destabilized illustrates that the regulation of both transcription and stability contribute to the overall changes in mRNA abundance seen when TOR signaling is blocked. Given the conservation of the TOR signaling pathway, the regulation of mRNA turnover may be an important mechanism in controlling the balance between cell growth and proliferation in multicellular eukaryotes as well.…”

“…The treatment of yeast with rapamycin causes changes in mRNA abundance that overlap with the changes induced at the diauxic shift (Hardwick et al, 1999). Some of the changes in mRNA abundance that occur when TOR signaling is inhibited are due to the regulation of transcription (Beck and Hall, 1999;Carde-nas et al, 1999;Hardwick et al, 1999;Powers and Walter, 1999). Translation also undergoes dramatic changes at diauxic shift with translation rates decreasing to 2-25% of log-phase values (Boucherie, 1985;Fuge et al,1994).…”

“…Likewise, the treatment of yeast with rapamycin reduces the translation initiation rates to Ͻ50% after only 15 min and to less that 20% after 60 min of exposure (Barbet et al, 1996). The translational down-regulation occurs at several levels, including decreased synthesis of rRNAs (Ju and Warner, 1994;Powers and Walter, 1999), the down-regulation of mRNAs for ribosomal proteins, translation initiation and elongation factors (DeRisi et al, 1997;Powers and Walter, 1999;Shamji et al, 2000), and degradation of the translation initiation factor eIF4Gp (Berset et al,1998). The regulation of mRNA turnover continues to emerge as an important way in which cells control gene expression.…”

Bms1p, a G-domain-containing protein, associates with Rcl1p and is required for 18S rRNA biogenesis in yeast

“…proteins generally act as regulators or effectors of diverse intracellular signal transduction processes in eukaryotes (reviewed by Fu et al+, 2000)+ Among pathways involving 14-3-3 proteins in yeast is the rapamycinsensitive TOR kinase signaling pathway, which (among other processes) targets initiation of translation and, significantly, ribosome biogenesis at several different levels (Powers & Walter, 1999;Cosentino et al+, 2000;Schmelzle & Hall, 2000, and references therein)+ Although the basis of the reported genetic interaction between Bms1p and Bmh1p is not understood, this observation raises the possibility that Bms1p may be the downstream target of the TOR kinase pathway in yeast+ We have found, however, that the Gal::bms1 strain does not show increased sensitivity to rapamycin when grown on YPD (our unpubl+ results)+ A role for Bms1p in pre-rRNA processing and its similarity to G proteins have been independently reported by Gelperin et al+ (2001, …”

Expanding the Functional Repertoire of CTD Kinase I and RNA Polymerase II: Novel PhosphoCTD-Associating Proteins in the Yeast Proteome

“…Under these conditions, RP gene transcription is greatly upregulated (68), and splicing of newly synthesized transcripts can be assayed. In contrast to wild type, in which unspliced precursor is never observed (e.g., Figure 9B, lanes 2 and 3), ctk1Δ cells consistently contain significant amounts of the unspliced RPS17A transcript (lanes 5 and 6).…”