How ribosomes are made The formation of eukaryotic ribosomes is a complex process that starts with transcription of a large precursor RNA that assembles into a large 90S preribosome, which matures to finally give the 40S small subunit of the ribosome. Cheng et al. and Du et al. give insight into this process, using cryo–electron microscopy to look at intermediates along the pathway. Together, these studies reveal how a cast of molecular players act to coordinate the compositional and structural changes that transform the 90S preribosome into a pre-40S subunit. Science, this issue p. 1470, p. 1477 The steps that drive the stepwise dissociation of factors in the transition from the 90S to the 40S ribosome subunit are observed. Production of small ribosomal subunits initially requires the formation of a 90S precursor followed by an enigmatic process of restructuring into the primordial pre-40S subunit. We elucidate this process by biochemical and cryo–electron microscopy analysis of intermediates along this pathway in yeast. First, the remodeling RNA helicase Dhr1 engages the 90S pre-ribosome, followed by Utp24 endonuclease–driven RNA cleavage at site A1, thereby separating the 5′-external transcribed spacer (ETS) from 18S ribosomal RNA. Next, the 5′-ETS and 90S assembly factors become dislodged, but this occurs sequentially, not en bloc. Eventually, the primordial pre-40S emerges, still retaining some 90S factors including Dhr1, now ready to unwind the final small nucleolar U3–18S RNA hybrid. Our data shed light on the elusive 90S to pre-40S transition and clarify the principles of assembly and remodeling of large ribonucleoproteins.

中文翻译：

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90S 前核糖体转化为原始 40S 亚基

核糖体是如何制造的 真核生物核糖体的形成是一个复杂的过程，从一个大的前体 RNA 的转录开始，该前体 RNA 组装成一个大的 90S 前核糖体，它成熟最终形成核糖体的 40S 小亚基。程等人。和杜等人。深入了解这个过程，使用冷冻电子显微镜观察沿途的中间体。总之，这些研究揭示了一系列分子参与者如何协调将 90S 前核糖体转变为 40S 前亚基的组成和结构变化。科学，这个问题 p。1470 页。1477 观察到在从 90S 到 40S 核糖体亚基转变过程中驱动因子逐步解离的步骤。小核糖体亚基的产生最初需要形成 90S 前体，然后是重组为原始 40S 前亚基的神秘过程。我们通过对酵母中这条途径的中间体进行生化和冷冻电子显微镜分析来阐明这一过程。首先，重塑 RNA 解旋酶 Dhr1 与 90S 前核糖体结合，然后在 A1 位点进行 Utp24 核酸内切酶驱动的 RNA 切割，从而将 5'-外部转录间隔区 (ETS) 与 18S 核糖体 RNA 分开。接下来，5'-ETS 和 90S 组装因子被移出，但这是顺序发生的，而不是整体发生的。最终，原始的 pre-40S 出现了，仍然保留了一些 90S 因子，包括 Dhr1，现在准备展开最终的小核仁 U3-18S RNA 杂交。