Bacterial ribosomes require three initiation factors IF1, IF2, and IF3 during the initial steps of translation. These IFs ensure correct base pairing of the initiator tRNA anticodon with the start codon in the mRNA located at the P-site of the 30S ribosomal subunit. IF3 is one of the first IFs to bind to the 30S and plays a crucial role in the selection of the correct start codon and codon: anticodon base pairing. IF3 also prevents the premature association of the 50S subunit of ribosomes and aids in ribosome recycling. IF3 is reported to change binding sites and conformation to ensure translation initiation fidelity. A recent study suggested an initial binding of IF3 CTD away from the P-site and that IF1 and IF2 promote the movement of CTD to the P-site and concomitant movement of NTD. Hence, to visualize the position of IF3 in the absence of any other IFs, we determined cryo-EM structure of the 30S-IF3 complex. The map shows that IF3 is present in an extended conformation with CTD present at the P-site and NTD near the platform even in the absence of IF1 and IF2. Hence, IF3 CTD binds at the P-site and moves away during the accommodation of the initiator tRNA at the P-site in the later steps of translation initiation. Overall, we report the structure of 30S-IF3 which demystifies the starting binding site and conformation of IF3 on the 30S ribosomal subunit.

中文翻译：

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在翻译的初始步骤中，起始因子 3 与 30S 核糖体亚基结合

在翻译的初始步骤中，细菌核糖体需要三个起始因子 IF1、IF2 和 IF3。这些 IF 确保起始 tRNA 反密码子与位于 30S 核糖体亚基 P 位点的 mRNA 中的起始密码子正确碱基配对。IF3 是最早与 30S 结合的 IF 之一，在选择正确的起始密码子和密码子：反密码子碱基配对中起着至关重要的作用。IF3 还可以防止核糖体 50S 亚基过早结合，并有助于核糖体回收。据报道，IF3 会改变结合位点和构象，以确保翻译起始保真度。最近的一项研究表明，IF3 CTD 最初与 P 位点结合，IF1 和 IF2 促进 CTD 向 P 位点的移动以及 NTD 的伴随移动。因此，为了在没有任何其他 IF 的情况下可视化 IF3 的位置，我们确定了 30S-IF3 复合物的冷冻电镜结构。该图显示 IF3 以延伸构象存在，即使在不存在 IF1 和 IF2 的情况下，CTD 也存在于 P 位点，NTD 存在于平台附近。因此，IF3 CTD 在 P 位点结合，并在翻译起始的后续步骤中在 P 位点处起始 tRNA 的调节过程中移走。总的来说，我们报告了 30S-IF3 的结构，揭开了 30S 核糖体亚基上 IF3 的起始结合位点和构象的神秘面纱。