Head swivel on the ribosome facilitates translocation by means of intra-subunit tRNA hybrid sites

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?site. The translocation reaction is catalysed by elongation factor G (EF-G) in a GTP-dependent manner. 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.     

聚甲基乙撑碳酸酯的交联

利用过氧化物过氧化二异丙苯（DCP）和多官能团化合物三烯丙基异氰尿酸酯（TAIC），对聚甲基乙撑碳酸酯（PPC）进行了交联研究。研究发现，DCP和TAIC的用量对PPC的交联均有较大影响，PPC交联后，其凝胶含量、机械性能、弹性模量、玻璃化转变温度均有所增加，1％的DCP和4％的TAIC用量可得到较理想的交联PPC材料。