Co-transcriptional assembly is an integral feature of the formation of RNA-protein complexes that mediate translation. For ribosome synthesis, prior studies have indicated that the strict order of transcription of rRNA domains may not be obligatory during bacterial ribosome biogenesis, since a series of circularly permuted rRNAs are viable. In this work, we report the insights into assembly of the bacterial ribosome large subunit (LSU) based on cryo-EM density maps of intermediates that accumulate during in vitro ribosome synthesis using a set of circularly permuted (CiPer) rRNAs. The observed ensemble of twenty-three resolved ribosome large subunit intermediates reveals conserved assembly routes with an underlying hierarchy among cooperative assembly blocks. There are intricate interdependencies for the formation of key structural rRNA helices revealed from the circular permutation of rRNA. While the order of domain synthesis is not obligatory, the order of domain association does appear to proceed with a particular order, likely due to the strong evolutionary pressure on efficient ribosome synthesis. This work reinforces the robustness of the known assembly hierarchy of the bacterial large ribosomal subunit, and offers a coherent view of how efficient assembly of CiPer rRNAs can be understood in that context.

中文翻译：

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用循环排列的 rRNA 组装细菌核糖体

共转录组装是介导翻译的 RNA-蛋白质复合物形成的一个重要特征。对于核糖体合成，先前的研究表明，在细菌核糖体生物发生过程中，rRNA 结构域的严格转录顺序可能不是必需的，因为一系列循环排列的 rRNA 是可行的。在这项工作中，我们基于使用一组循环排列 (CiPer) rRNA进行体外核糖体合成过程中积累的中间体的冷冻电镜密度图，报告了对细菌核糖体大亚基 (LSU) 组装的见解。观察到的二十三个解析核糖体大亚基中间体的整体揭示了保守的组装路线，以及协作组装块之间的底层层次结构。 rRNA 的环状排列揭示了关键结构 rRNA 螺旋的形成存在复杂的相互依赖性。虽然结构域合成的顺序不是强制性的，但结构域关联的顺序似乎确实以特定的顺序进行，这可能是由于有效核糖体合成的强大进化压力。这项工作增强了已知的细菌大核糖体亚基组装层次结构的稳健性，并提供了如何在该背景下理解 CiPer rRNA 的有效组装的连贯观点。