The Mediator complex, which in humans is 1.4 MDa in size and includes 26 subunits, controls many aspects of RNA polymerase II (Pol II) function. Apart from its size, a defining feature of Mediator is its intrinsic disorder and conformational flexibility, which contributes to its ability to undergo phase separation and to interact with a myriad of regulatory factors. In this Review, we discuss Mediator structure and function, with emphasis on recent cryogenic electron microscopy data of the 4.0-MDa transcription preinitiation complex. We further discuss how Mediator and sequence-specific DNA-binding transcription factors enable enhancer-dependent regulation of Pol II function at distal gene promoters, through the formation of molecular condensates (or transcription hubs) and chromatin loops. Mediator regulation of Pol II reinitiation is also discussed, in the context of transcription bursting. We propose a working model for Mediator function that combines experimental results and theoretical considerations related to enhancer–promoter interactions, which reconciles contradictory data regarding whether enhancer–promoter communication is direct or indirect. We conclude with a discussion of Mediator’s potential as a therapeutic target and of future research directions.

介体复合体在人体中大小为 1.4 MDa，包含 26 个亚基，控制 RNA 聚合酶 II (Pol II) 功能的许多方面。除了其大小之外，Mediator 的一个决定性特征是其内在的无序性和构象灵活性，这有助于其进行相分离并与无数调节因子相互作用的能力。在这篇综述中，我们讨论了介体的结构和功能，重点是 4.0-MDa 转录前起始复合物的最新低温电子显微镜数据。我们进一步讨论介体和序列特异性 DNA 结合转录因子如何通过形成分子缩合物（或转录中心）和染色质环，实现远端基因启动子处 Pol II 功能的增强子依赖性调节。在转录爆发的背景下，还讨论了 Pol II 重新启动的介体调节。我们提出了一种中介功能的工作模型，该模型结合了与增强子-启动子相互作用相关的实验结果和理论考虑，它调和了关于增强子-启动子沟通是直接还是间接的矛盾数据。最后我们讨论了 Mediator 作为治疗靶点的潜力和未来的研究方向。