Regensburg Center for Biochemistry, University of Regensburg, Regensburg, Germany.
Nat Commun. 2020 Mar 5;11(1):1206. doi: 10.1038/s41467-020-15052-y.
Transcription of the ribosomal RNA precursor by RNA polymerase (Pol) I is a prerequisite for the biosynthesis of ribosomes in eukaryotes. Compared to Pols II and III, the mechanisms underlying promoter recognition, initiation complex formation and DNA melting by Pol I substantially diverge. Here, we report the high-resolution cryo-EM reconstruction of a Pol I early initiation intermediate assembled on a double-stranded promoter scaffold that prevents the establishment of downstream DNA contacts. Our analyses demonstrate how efficient promoter-backbone interaction is achieved by combined re-arrangements of flexible regions in the 'core factor' subunits Rrn7 and Rrn11. Furthermore, structure-function analysis illustrates how destabilization of the melted DNA region correlates with contraction of the polymerase cleft upon transcription activation, thereby combining promoter recruitment with DNA-melting. This suggests that molecular mechanisms and structural features of Pol I initiation have co-evolved to support the efficient melting, initial transcription and promoter clearance required for high-level rRNA synthesis.
核糖体 RNA 前体的转录是真核生物核糖体生物合成的前提。与 Pol II 和 Pol III 相比，Pol I 启动子识别、起始复合物形成和 DNA 解链的机制有很大的不同。在这里，我们报告了在双链启动子支架上组装的 Pol I 早期起始中间产物的高分辨率冷冻电镜重建，该支架可防止下游 DNA 接触的建立。我们的分析表明，通过“核心因子”亚基 Rrn7 和 Rrn11 中的柔性区域的组合重排，如何实现有效的启动子-骨架相互作用。此外，结构-功能分析说明了 DNA 解链区域的失稳如何与转录激活时聚合酶裂隙的收缩相关联，从而将启动子募集与 DNA 解链相结合。这表明 Pol I 起始的分子机制和结构特征共同进化，以支持高效的解链、初始转录和启动子清除，从而实现高水平的 rRNA 合成。
