Division of Structural Biology, Henry Wellcome Building for Genomic Medicine, University of Oxford, Roosevelt Drive, Oxford OX3 7BN, United Kingdom.
Institut de Genetique et Microbiologie, Universite Paris-Sud, batiment 400, 91405 Orsay Cedex, France.
Structure. 2010 Feb 10;18(2):257-264. doi: 10.1016/j.str.2009.12.007.
Transfer RNAs (tRNAs) link the genetic code in the form of messenger RNA (mRNA) to protein sequence. Translocation of tRNAs through the ribosome from aminoacyl (A) site to peptidyl (P) site and from P site to exit site is catalyzed in eukaryotes by the translocase elongation factor 2 (EF-2) and in prokaryotes by its homolog EF-G. During tRNA movement one or more "hybrid" states (A/P) is occupied, but molecular details of them and of the translocation process are limited. Here we show by cryo-electron microscopy that a population of mammalian ribosomes stalled at an mRNA pseudoknot structure contains structurally distorted tRNAs in two different A/P hybrid states. In one (A/P'), the tRNA is in contact with the translocase EF-2, which induces it. In the other (A/P''), the translocase is absent. The existence of these alternative A/P intermediate states has relevance to our understanding of the mechanics and kinetics of translocation.
转移 RNA（tRNA）将信使 RNA（mRNA）形式的遗传密码与蛋白质序列联系起来。在真核生物中，tRNA 通过核糖体从氨酰基（A）位转移到肽酰基（P）位，从 P 位转移到出口位，由移位因子延伸因子 2（EF-2）催化，而在原核生物中则由其同源物 EF-G 催化。在 tRNA 运动过程中，一个或多个“杂交”状态（A/P）被占据，但它们和移位过程的分子细节是有限的。在这里，我们通过冷冻电子显微镜显示，在含有 mRNA 假结结构的哺乳动物核糖体种群中，存在两种不同的 A/P 杂交状态的结构扭曲的 tRNA。在一种状态（A/P'）中，tRNA 与移位酶 EF-2 接触，EF-2 诱导它进入该状态。在另一种状态（A/P''）中，不存在移位酶。这些替代的 A/P 中间状态的存在对于我们理解移位的力学和动力学具有重要意义。
