Translational regulation in the ribosome tunnel

我们最近一直在研究SecA的表达调控，这是一种细胞质ATPase，在大肠杆菌中通过SecYEG复合体转运蛋白质起着重要作用。编码SecA的基因位于编码SECM的基因的下游，这两个基因形成了一个操纵子。当分泌受损时，SECM C末端附近的17个氨基酸基序导致核糖体隧道内的翻译停滞。结果，mRNA的结构发生了变化，SECA的翻译效率更高。我们使用了一种新的应用荧光共振能量转移(FRET)来深入了解SECM翻译阻止的机制。我们发现，SECM的C-末端在合成阻止基序时采用了紧凑的构象。这种构象变化不是自发发生的，而是由核糖体诱导的。翻译逮捕需要SECM C-末端的紧凑和逮捕基序中关键残基的存在。进一步的分析表明，核糖体在翻译停止期间也经历了构象变化，导致SECM多肽-tRNA的稳定。基于这些结果，我们提出翻译抑制是核糖体和新生SECM多肽的C末端之间一系列相互作用的结果。

We have recently been investigating the regulation of the expression of SecA, a cytoplasmic ATPase that plays a major role in the translocation of proteins through the SecYEG complex in E. coli. The gene that encodes SecA resides just downstream of the gene that encodes SecM, and the two genes form a single operon. When secretion is impaired, a 17 amino acid motif near the C-terminus of SecM induces a translation arrest from within the ribosome tunnel. As a consequence, the structure of the mRNA is altered and SecA is translated more efficiently. We have used a novel application of fluorescence resonance energy transfer (FRET) to gain insight into the mechanism of SecM translation arrest. We found that the SecM C-terminus adopts a compact conformation upon synthesis of the arrest motif. This conformational change does not occur spontaneously, but rather is induced by the ribosome. Translation arrest requires both compaction of the SecM C-terminus and the presence of key residues in the arrest motif. Further analysis revealed that the ribosome also undergoes a conformational change during translation arrest that leads to the stabilization of the SecM peptidyl-tRNA. Based on these results, we have proposed that translation arrest results from a series of reciprocal interactions between the ribosome and the C-terminus of the nascent SecM polypeptide.