In Escherichia coli, elevated levels of free l-tryptophan (l-Trp) promote translational arrest of the TnaC peptide by inhibiting its termination. However, the mechanism by which translation-termination by the UGA-specific decoding release factor 2 (RF2) is inhibited at the UGA stop codon of stalled TnaC-ribosome-nascent chain complexes has so far been ambiguous. This study presents cryo-EM structures for ribosomes stalled by TnaC in the absence and presence of RF2 at average resolutions of 2.9 and 3.5 Å, respectively. Stalled TnaC assumes a distinct conformation composed of two small α-helices that act together with residues in the peptide exit tunnel (PET) to coordinate a single L-Trp molecule. In addition, while the peptidyl-transferase center (PTC) is locked in a conformation that allows RF2 to adopt its canonical position in the ribosome, it prevents the conserved and catalytically essential GGQ motif of RF2 from adopting its active conformation in the PTC. This explains how translation of the TnaC peptide effectively allows the ribosome to function as a L-Trp-specific small-molecule sensor that regulates the tnaCAB operon.

中文翻译：

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TnaC阻滞肽对释放因子2的l-色氨酸依赖性抑制的结构基础

在大肠杆菌中，游离 l-色氨酸 (l-Trp) 水平升高通过抑制 TnaC 肽的终止来促进其翻译停滞。然而，到目前为止，在停滞的 TnaC-核糖体-新生链复合物的 UGA 终止密码子处抑制 UGA 特异性解码释放因子 2 (RF2) 的翻译终止的机制尚不明确。本研究展示了在没有和存在 RF2 的情况下被 TnaC 停滞的核糖体的冷冻电镜结构，平均分辨率分别为 2.9 和 3.5 Å。停滞的 TnaC 假定由两个小的 α-螺旋组成的独特构象，它们与肽出口隧道 (PET) 中的残基一起作用以协调单个 L-Trp 分子。此外，虽然肽基转移酶中心 (PTC) 被锁定在允许 RF2 在核糖体中采用其规范位置的构象中，它阻止 RF2 的保守和催化必需的 GGQ 基序在 PTC 中采用其活性构象。这解释了 TnaC 肽的翻译如何有效地使核糖体充当调节 tnaCAB 操纵子的 L-Trp 特异性小分子传感器。