在翻译过程中，转移RNA (tRNA)进入核糖体，然后在其将它们所携带的氨基酸转移到增长中的肽链上时，顺序通过被称为A、P和E的三个点。然而，核糖体是怎样帮助tRNA在这些点之间的转位的在很大程度上却不清楚。Christian Spahn及其同事利用结合到“翻译伸长因子”EF-G上的一个核糖体的“多粒子冷电子显微镜”图像来获取关于tRNA运动的信息。他们识别出两个新的亚状态， 并且得出这样的结论：在亚单元之间自发的“棘轮作用”之后，转位是30S核糖体亚单元的头部转动和“去棘轮作用”的直接结果。（生物谷Bioon.com）
生物谷推荐原文出处：
Nature doi:10.1038/nature09547
Head swivel on the ribosome facilitates translocation by means of intra-subunit tRNA hybrid sites
Andreas H. Ratje,Justus Loerke,Aleksandra Mikolajka,Matthias Brünner,Peter W. Hildebrand,Agata L. Starosta,Alexandra D?nh?fer,Sean R. Connell,Paola Fucini,Thorsten Mielke,Paul C. Whitford,José N. Onuchic,Yanan Yu,Karissa Y. Sanbonmatsu,Roland K. Hartmann,Pawel A. Penczek,Daniel N. Wilsonwilson@lmb.uni-muenchen.de& Christian M. T. Spahnchristian.spahn@charite.de
The elongation cycle of protein synthesis involves the delivery of aminoacyl-transfer RNAs to the aminoacyl-tRNA-binding site (A?site) of the ribosome, followed by peptide-bond formation and translocation of the tRNAs through the ribosome to reopen the A?site1, 2. The translocation reaction is catalysed by elongation factor G (EF-G) in a GTP-dependent manner3. Despite the availability of structures of various EF-G–ribosome complexes, the precise mechanism by which tRNAs move through the ribosome still remains unclear. Here we use multiparticle cryoelectron microscopy analysis to resolve two previously unseen subpopulations within Thermus thermophilus EF-G–ribosome complexes at subnanometre resolution, one of them with a partly translocated tRNA. Comparison of these substates reveals that translocation of tRNA on the 30S subunit parallels the swivelling of the 30S head and is coupled to unratcheting of the 30S body. Because the tRNA maintains contact with the peptidyl-tRNA-binding site (P?site) on the 30S head and simultaneously establishes interaction with the exit site (E?site) on the 30S platform, a novel intra-subunit ‘pe/E’ hybrid state is formed. This state is stabilized by domain?IV of EF-G, which interacts with the swivelled 30S-head conformation. These findings provide direct structural and mechanistic insight into the ‘missing link’ in terms of tRNA intermediates involved in the universally conserved translocation process.
