The 7SK small nuclear RNA (7SKsnRNA) plays a key role in the regulation of RNA polymerase II by sequestrating and inhibiting the positive transcription elongation factor b (P-TEFb) in the 7SK ribonucleoprotein complex (7SKsnRNP), a process mediated by interaction with the protein HEXIM. P-TEFb is also an essential cellular factor recruited by the viral protein Tat to ensure the replication of the viral RNA in the infection cycle of the human immunodeficiency virus (HIV-1). Tat promotes the release of P-TEFb from the 7SKsnRNP and subsequent activation of transcription, by displacing HEXIM from the 5'-hairpin of the 7SKsnRNA. This hairpin (HP1), comprising the signature sequence of the 7SKsnRNA, has been the subject of three independent structural studies aiming at identifying the structural features that could drive the recognition by the two proteins, both depending on Arginine Rich Motifs (ARM). Interestingly, four distinct structures were determined. In an attempt to provide a comprehensive view of the structure-function relationship of this versatile RNA, we present here a structural analysis of the models, highlighting how HP1 is able to adopt distinct conformations with significant impact on the compactness of the molecule. Since these models are solved under different conditions by NMR and crystallography, the impact of the buffer composition on the conformational variation was investigated by complementary biophysical approaches. Finally, using Isothermal Titration Calorimetry, we determined the thermodynamic signatures of the Tat-ARM and HEXIM-ARM peptide interactions with the RNA, showing that they are associated with distinct binding mechanisms.

中文翻译：

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7SK 小核 RNA 的 5'-发夹覆盖的动态景观的不同视图

7SK 小核 RNA (7SKsnRNA) 通过隔离和抑制 7SK 核糖核蛋白复合体 (7SKsnRNP) 中的正转录延伸因子 b (P-TEFb) 在 RNA 聚合酶 II 的调节中起关键作用，该过程由与蛋白质HEXIM。P-TEFb 也是病毒蛋白 Tat 募集的重要细胞因子，以确保病毒 RNA 在人类免疫缺陷病毒 (HIV-1) 的感染周期中复制。通过从 7SKsnRNA 的 5'-发夹中置换 HEXIM，Tat 促进 P-TEFb 从 7SKsnRNP 释放和随后的转录激活。这个发夹 (HP1)，包含 7SKsnRNA 的特征序列，一直是三项独立结构研究的主题，旨在确定可以驱动两种蛋白质识别的结构特征，这两种蛋白质都取决于富含精氨酸的基序 (ARM)。有趣的是，确定了四种不同的结构。为了全面了解这种多功能 RNA 的结构-功能关系，我们在此展示了模型的结构分析，重点介绍了 HP1 如何能够采用不同的构象，并对分子的紧凑性产生重大影响。由于这些模型是通过 NMR 和晶体学在不同条件下求解的，因此通过补充生物物理方法研究了缓冲液成分对构象变化的影响。最后，使用等温滴定量热法，