TBP facilitates RNA Polymerase I transcription following mitosis

The TATA-box binding protein (TBP) is the sole transcription factor common in the initiation complexes of the three major eukaryotic RNA Polymerases (Pol I, II and III). Although TBP is central to transcription by the three RNA Pols in various species, the emergence of TBP paralogs throughout evolution has expanded the complexity in transcription initiation. Furthermore, recent studies have emerged that questioned the centrality of TBP in mammalian cells, particularly in Pol II transcription, but the role of TBP and its paralogs in Pol I transcription remains to be re-evaluated. In this report, we show that in murine embryonic stem cells TBP localizes onto Pol I promoters, whereas the TBP paralog TRF2 only weakly associates to the Spacer Promoter of rDNA, suggesting that it may not be able to replace TBP for Pol I transcription. Importantly, acute TBP depletion does not fully disrupt Pol I occupancy or activity on ribosomal RNA genes, but TBP binding in mitosis leads to efficient Pol I reactivation following cell division. These findings provide a more nuanced role for TBP in Pol I transcription in murine embryonic stem cells.

TATA盒结合蛋白（TATA-box binding protein，TBP）是三类主要真核RNA聚合酶（RNA聚合酶I、II和III，以下简称Pol I、Pol II、Pol III）起始复合物中唯一共有的转录因子。尽管在诸多物种中，TBP均为三类RNA聚合酶介导转录的核心因子，但进化过程中TBP旁系同源蛋白的出现，进一步提升了转录起始过程的复杂性。此外，近期已有研究对TBP在哺乳动物细胞中的核心地位提出了质疑，尤以Pol II转录过程中为甚，但TBP及其旁系同源蛋白在Pol I转录中的作用仍有待重新评估。本研究显示，在小鼠胚胎干细胞中，TBP可靶向结合Pol I启动子区域；而TBP旁系同源蛋白TRF2仅能微弱结合核糖体DNA（ribosomal DNA，rDNA）的间隔启动子，这提示其无法替代TBP参与Pol I转录过程。值得注意的是，急性TBP耗竭并不会完全破坏核糖体RNA基因上的Pol I占位或转录活性，但有丝分裂阶段TBP的结合可在细胞分裂完成后有效介导Pol I的重新激活。上述研究结果为小鼠胚胎干细胞中TBP在Pol I转录中的作用提供了更为精细的阐释。