Département de Biologie et de Génomique Structurales, Institut de Génétique et de Biologie Moléculaire et Cellulaire, Illkirch 67400, France.
Nature. 2012 Mar 21;484(7393):256-9. doi: 10.1038/nature10913.
During protein synthesis, the ribosome accurately selects transfer RNAs (tRNAs) in accordance with the messenger RNA (mRNA) triplet in the decoding centre. tRNA selection is initiated by elongation factor Tu, which delivers tRNA to the aminoacyl tRNA-binding site (A site) and hydrolyses GTP upon establishing codon-anticodon interactions in the decoding centre. At the following proofreading step the ribosome re-examines the tRNA and rejects it if it does not match the A codon. It was suggested that universally conserved G530, A1492 and A1493 of 16S ribosomal RNA, critical for tRNA binding in the A site, actively monitor cognate tRNA, and that recognition of the correct codon-anticodon duplex induces an overall ribosome conformational change (domain closure). Here we propose an integrated mechanism for decoding based on six X-ray structures of the 70S ribosome determined at 3.1-3.4 Å resolution, modelling cognate or near-cognate states of the decoding centre at the proofreading step. We show that the 30S subunit undergoes an identical domain closure upon binding of either cognate or near-cognate tRNA. This conformational change of the 30S subunit forms a decoding centre that constrains the mRNA in such a way that the first two nucleotides of the A codon are limited to form Watson-Crick base pairs. When U·G and G·U mismatches, generally considered to form wobble base pairs, are at the first or second codon-anticodon position, the decoding centre forces this pair to adopt the geometry close to that of a canonical C·G pair. This by itself, or with distortions in the codon-anticodon mini-helix and the anticodon loop, causes the near-cognate tRNA to dissociate from the ribosome.
在蛋白质合成过程中，核糖体根据解码中心的信使 RNA（mRNA）三核苷酸准确选择转移 RNA（tRNA）。tRNA 的选择由延伸因子 Tu 启动，它将 tRNA 递送至氨酰基 tRNA 结合位点（A 位），并在解码中心建立密码子-反密码子相互作用后水解 GTP。在下一个校对步骤中，核糖体重新检查 tRNA，如果它与 A 密码子不匹配，则将其拒绝。有人认为，16S 核糖体 RNA 上普遍保守的 G530、A1492 和 A1493 对于 A 位的 tRNA 结合至关重要，它们积极监测同源 tRNA，并且识别正确的密码子-反密码子双链体诱导核糖体构象整体变化（结构域闭合）。在这里，我们提出了一种基于六个 70S 核糖体 X 射线结构的综合解码机制，这些结构在 3.1-3.4 Å 分辨率下确定，模拟了校对步骤中解码中心的同源或近同源状态。我们表明，30S 亚基在结合同源或近同源 tRNA 时都会经历相同的结构域闭合。30S 亚基的这种构象变化形成了一个解码中心，以这种方式限制 mRNA，使得 A 密码子的前两个核苷酸限制形成 Watson-Crick 碱基对。当 U·G 和 G·U 错配通常被认为形成摆动碱基对时，位于第一个或第二个密码子-反密码子位置，解码中心迫使该碱基对采用接近规范 C·G 对的几何形状。这种情况本身，或在密码子-反密码子小螺旋和反密码子环的扭曲，会导致近同源 tRNA 从核糖体上解离。
