Ribosome biogenesis and cell growth: mTOR coordinates transcription by all three classes of nuclear RNA polymerases

The target of rapamycin ( TOR) signal-transduction pathway is an important mechanism by which eucaryotic cells adjust their protein biosynthetic capacity to nutrient availability. Both in yeast and in mammals, the TOR pathway regulates the synthesis of ribosomal components, including transcription and processing of pre-rRNA, expression of ribosomal proteins and the synthesis of 5S rRNA. Expression of the genes encoding the numerous constituents of ribosomes requires transcription by all three classes of nuclear RNA polymerases. In this review, we summarize recent advances in understanding the interplay among nutrient availability, transcriptional control and ribosome biogenesis. We focus on transcription in response to nutrients, detailing the relevant downstream targets of TOR in yeast and mammals. The critical role of TOR in linking environmental cues to ribosome biogenesis provides an efficient means by which cells alter their overall protein biosynthetic capacity.