Tor1 and CK2 kinases control a switch between alternative ribosome biogenesis pathways in a growth-dependent manner

Ribosome biogenesis is a major energy-consuming process in the cell that has to be rapidly down-regulated in response to stress or nutrient depletion. The target of rapamycin 1 (Tor1) pathway regulates synthesis of ribosomal RNA (rRNA) at the level of transcription initiation. It remains unclear whether ribosome biogenesis is also controlled directly at the posttranscriptional level. We show that Tor1 and casein kinase 2 (CK2) kinases regulate a rapid switch between a productive and a non-productive pre-rRNA processing pathways in yeast. Under stress, the pre-rRNA continues to be synthesized; however, it is processed differently, and no new ribosomes are produced. Strikingly, the control of the switch does not require the Sch9 kinase, indicating that an unrecognized Tor Complex 1 (TORC1) signaling branch involving CK2 kinase directly regulates ribosome biogenesis at the posttranscriptional level.

核糖体生物发生（Ribosome biogenesis）是细胞内一项主要的能量消耗过程，需在应激或营养匮乏时快速下调。雷帕霉素靶蛋白1（Tor1）信号通路可在转录起始层面调控核糖体RNA（rRNA）的合成。目前尚不清楚核糖体生物发生是否也可在转录后层面受到直接调控。本研究证实，Tor1与酪蛋白激酶2（CK2）可调控酵母中功能性与非功能性前核糖体RNA加工通路之间的快速切换。在应激状态下，前核糖体RNA仍会持续合成，但其加工方式发生改变，无法产生新的核糖体。值得注意的是，该切换的调控并不依赖Sch9激酶，这表明存在一条尚未被阐明的、涉及CK2激酶的雷帕霉素复合物1（TORC1）信号分支，可在转录后层面直接调控核糖体生物发生。