Lambdoid bacteriophage Q proteins are transcription antipausing and antitermination factors that enable RNA polymerase (RNAP) to read through pause and termination sites. Q proteins load onto RNAP engaged in promoter-proximal pausing at a Q binding element (QBE) and adjacent sigma-dependent pause element to yield a Q-loading complex, and they translocate with RNAP as a pausing-deficient, termination-deficient Q-loaded complex. In previous work, we showed that the Q protein of bacteriophage 21 (Q21) functions by forming a nozzle that narrows and extends the RNAP RNA-exit channel, preventing formation of pause and termination RNA hairpins. Here, we report atomic structures of four states on the pathway of antitermination by the Q protein of bacteriophage λ (Qλ), a Q protein that shows no sequence similarity to Q21 and that, unlike Q21, requires the transcription elongation factor NusA for efficient antipausing and antitermination. We report structures of Qλ, the Qλ-QBE complex, the NusA-free pre-engaged Qλ-loading complex, and the NusA-containing engaged Qλ-loading complex. The results show that Qλ, like Q21, forms a nozzle that narrows and extends the RNAP RNA-exit channel, preventing formation of RNA hairpins. However, the results show that Qλ has no three-dimensional structural similarity to Q21, employs a different mechanism of QBE recognition than Q21, and employs a more complex process for loading onto RNAP than Q21, involving recruitment of Qλ to form a pre-engaged loading complex, followed by NusA-facilitated refolding of Qλ to form an engaged loading complex. The results establish that Qλ and Q21 are not structural homologs and are solely functional analogs.

中文翻译：

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在 Qλ 的转录抗终止中，NusA 诱导 Qλ 的重新折叠，形成一个喷嘴，延伸 RNA 聚合酶 RNA 出口通道

Lambdoid 噬菌体 Q 蛋白是转录抗暂停和抗终止因子，可使 RNA 聚合酶 (RNAP) 读取暂停和终止位点。Q 蛋白加载到 RNAP 上，在 Q 结合元件 (QBE) 和相邻的 sigma 依赖性暂停元件处参与启动子近端暂停以产生 Q 加载复合体，并且它们与 RNAP 一起易位为暂停缺陷、终止缺陷 Q-加载复杂。在之前的工作中，我们发现噬菌体 21 (Q21) 的 Q 蛋白通过形成一个喷嘴来发挥作用，该喷嘴缩小和延伸 RNAP RNA 出口通道，防止形成暂停和终止 RNA 发夹。在这里，我们报告了噬菌体 λ (Qλ) 的 Q 蛋白抗终止通路中四种状态的原子结构，这是一种与 Q21 没有序列相似性的 Q 蛋白，而且与 Q21 不同，需要转录延伸因子 NusA 以进行有效的抗暂停和抗终止。我们报告了 Qλ、Qλ-QBE 复合体、无 NusA 的预接合 Qλ 加载复合体和包含 NusA 的接合 Qλ 加载复合体的结构。结果表明，Qλ 与 Q21 一样，形成一个喷嘴，使 RNAP RNA 出口通道变窄和延伸，从而防止 RNA 发夹的形成。然而，结果表明 Qλ 与 Q21 没有三维结构相似性，采用与 Q21 不同的 QBE 识别机制，并且采用比 Q21 更复杂的加载到 RNAP 的过程，涉及招募 Qλ 以形成预接合加载复合体，然后是 NusA 促进 Qλ 的重新折叠以形成接合的加载复合体。结果表明 Qλ 和 Q21 不是结构同系物，只是功能类似物。