核糖體是活細(xì)胞的蛋白質(zhì)制造工廠，它們以細(xì)胞中核苷酸的遺傳密碼子進(jìn)行蛋白質(zhì)的生產(chǎn),，當(dāng)然,，信使RNA（mRNA）提供蛋白質(zhì)翻譯的遺傳密碼,，核糖體纏繞在信使RNA分子,，通過識別起始和終止信號進(jìn)行蛋白質(zhì)的生產(chǎn)。如果一個信號缺失,，蛋白質(zhì)的生產(chǎn)就不能完成,，這樣一來,，核糖體的生產(chǎn)模式就會被阻塞。
直到現(xiàn)在,，我們并不清楚核糖體是如何克服這種蛋白質(zhì)生產(chǎn)過程中的阻塞的,，在修復(fù)過程中，即反式翻譯過程中,，一種額外的核酸分子（tmRNA）可以將mRNA分子和轉(zhuǎn)移RNA分子（tRNA）聯(lián)合起來,，在蛋白質(zhì)產(chǎn)生的過程中，轉(zhuǎn)移RNA分子可以將正確的氨基酸分子轉(zhuǎn)移到mRNA上,，在蛋白質(zhì)產(chǎn)生過程中，如果信號終止,，tmRNA分子能夠偷偷竄進(jìn)來,，解除蛋白質(zhì)合成的封鎖。但是tmRNA分子是如何穿過核糖體將其信息運(yùn)輸?shù)絤RNA上的,，目前研究者并不清楚,。
目前，此過程可以用低溫電子顯微鏡進(jìn)行記錄,，這種方法可以記錄單一大分子組分之間的空間和時間上相互作用,，這都可以通過速凍核糖體在負(fù)192攝氏度的液態(tài)乙烷中完成，而且數(shù)以千計(jì)的蛋白顆粒二維投影可以被反投影成其三維結(jié)構(gòu),。運(yùn)用低溫電子顯微鏡技術(shù),，研究者可以詳細(xì)記錄核糖體、tmRNA,、特殊蛋白SmbP和延伸因子G之間的相互作用,。
在mRNA通道中，tmRNA可以悄悄地提供確實(shí)的信號信息,，該通道徑直穿過核糖體的中間,，在小核糖體亞基的頭部和身體結(jié)構(gòu)域之間。研究者通過進(jìn)行結(jié)構(gòu)分析展示了核糖體和tmRNA之間在修復(fù)過程中的相互協(xié)作和相互作用,。相關(guān)研究刊登在了近日的國際雜志Nature上,。（生物谷：T.Shen編譯）
doi:10.1038/nature11006
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The complex of tmRNA–SmpB and EF-G on translocating ribosomes
David J. F. Ramrath, Hiroshi Yamamoto, Kristian Rother, Daniela Wittek, Markus Pech, Thorsten Mielke, Justus Loerke, Patrick Scheerer, Pavel Ivanov, Yoshika Teraoka, Olga Shpanchenko, Knud H. Nierhaus & Christian M. T. Spahn
Bacterial ribosomes stalled at the 3′ end of malfunctioning messenger RNAs can be rescued by transfer-messenger RNA (tmRNA)-mediated trans-translation1, 2. The SmpB protein forms a complex with the tmRNA, and the transfer-RNA-like domain (TLD) of the tmRNA then enters the A site of the ribosome. Subsequently, the TLD–SmpB module is translocated to the P site, a process that is facilitated by the elongation factor EF-G, and translation is switched to the mRNA-like domain (MLD) of the tmRNA. Accurate loading of the MLD into the mRNA path is an unusual initiation mechanism. Despite various snapshots of different ribosome–tmRNA complexes at low to intermediate resolution3, 4, 5, 6, 7, it is unclear how the large, highly structured tmRNA is translocated and how the MLD is loaded. Here we present a cryo-electron microscopy reconstruction of a fusidic-acid-stalled ribosomal 70S–tmRNA–SmpB–EF-G complex (carrying both of the large ligands, that is, EF-G and tmRNA) at 8.3 Å resolution. This post-translocational intermediate (TIPOST) presents the TLD–SmpB module in an intrasubunit ap/P hybrid site and a tRNAfMet in an intrasubunit pe/E hybrid site. Conformational changes in the ribosome and tmRNA occur in the intersubunit space and on the solvent side. The key underlying event is a unique extra-large swivel movement of the 30S head, which is crucial for both tmRNA–SmpB translocation and MLD loading, thereby coupling translocation to MLD loading. This mechanism exemplifies the versatile, dynamic nature of the ribosome, and it shows that the conformational modes of the ribosome that normally drive canonical translation can also be used in a modified form to facilitate more complex tasks in specialized non-canonical pathways.
