Late in their maturation, nascent small (40S) ribosomal subunits bind 60S subunits to produce 80S-like ribosomes. Because of the analogy of this translation-like cycle to actual translation, and because 80S-like ribosomes do not produce any protein, it has been suggested that this represents a quality control mechanism for subunit functionality. Here we use genetic and biochemical experiments to show that the essential ATPase Fap7 promotes formation of the rotated state, a key intermediate in translocation, thereby releasing the essential assembly factor Dim1 from pre-40S subunits. Bypassing this quality control step produces defects in reading frame maintenance. These results show how progress in the maturation cascade is linked to a test for a key functionality of 40S ribosomes: their ability to translocate the mRNA⋅tRNA pair. Furthermore, our data demonstrate for the first time that the translation-like cycle is a quality control mechanism that ensures the fidelity of the cellular ribosome pool.

在它们成熟的后期，新生的小 (40S) 核糖体亚基结合 60S 亚基产生 80S 样核糖体。由于这种翻译样循环与实际翻译的类比，并且因为 80S 样核糖体不产生任何蛋白质，有人认为这代表了亚基功能的质量控制机制。在这里，我们使用遗传和生化实验来证明必需的 ATPase Fap7 促进旋转状态的形成，旋转状态是易位的关键中间体，从而从 40S 前亚基释放必需的装配因子 Dim1。绕过这个质量控制步骤会在阅读框架维护中产生缺陷。这些结果显示了成熟级联反应的进展如何与 40S 核糖体的关键功能测试相关联：它们转移 mRNA⋅tRNA 对的能力。此外，