Transcription activation by cyclic AMP (cAMP) receptor protein (CAP) is the classic paradigm of transcription regulation in bacteria. CAP was suggested to activate transcription on class-II promoters via a recruitment and isomerization mechanism. However, whether and how it modifies RNA polymerase (RNAP) to initiate transcription remains unclear. Here, we report cryo-electron microscopy (cryo-EM) structures of an intact Escherichia coli class-II CAP-dependent transcription activation complex (CAP-TAC) with and without de novo RNA transcript. The structures reveal two distinct architectures of TAC and raise the possibility that CAP binding may induce substantial conformational changes in all the subunits of RNAP and transiently widen the main cleft of RNAP to facilitate DNA promoter entering and formation of the initiation open complex. These structural changes vanish during further RNA transcript synthesis. The observations in this study may reveal a possible on-pathway intermediate and suggest a possibility that CAP activates transcription by inducing intermediate state, in addition to the previously proposed stabilization mechanism.

中文翻译：

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II类CAP依赖性转录激活中两种体系结构的可视化。

环状AMP（cAMP）受体蛋白（CAP）激活的转录是细菌中转录调控的经典范例。建议通过募集和异构化机制激活CAP类启动子上的转录。但是，是否以及如何修饰RNA聚合酶（RNAP）以启动转录仍不清楚。在这里，我们报告完整的大肠杆菌II类CAP依赖的转录激活复合体（CAP-TAC）有和没有从头RNA转录本的低温电子显微镜（cryo-EM）结构。该结构揭示了TAC的两种截然不同的结构，并提高了CAP结合可能在RNAP的所有亚基中诱导实质性构象变化并暂时扩大RNAP的主要裂隙以促进DNA启动子进入并形成起始开放复合物的可能性。这些结构变化在进一步的RNA转录物合成过程中消失。在这项研究中的观察结果可能揭示了可能的途中中间体，并暗示了除先前提出的稳定机制外，CAP还可以通过诱导中间体状态来激活转录。