The eukaryotic transcription apparatus synthesizes a staggering diversity of RNA molecules. The labour of nuclear gene transcription is, therefore, divided among multiple DNA-dependent RNA polymerases. RNA polymerase I (Pol I) transcribes ribosomal RNA, Pol II synthesizes messenger RNAs and various non-coding RNAs (including long non-coding RNAs, microRNAs and small nuclear RNAs) and Pol III produces transfer RNAs and other short RNA molecules. Pol I, Pol II and Pol III are large, multisubunit protein complexes that associate with a multitude of additional factors to synthesize transcripts that largely differ in size, structure and abundance. The three transcription machineries share common characteristics, but differ widely in various aspects, such as numbers of RNA polymerase subunits, regulatory elements and accessory factors, which allows them to specialize in transcribing their specific RNAs. Common to the three RNA polymerases is that the transcription process consists of three major steps: transcription initiation, transcript elongation and transcription termination. In this Review, we outline the common principles and differences between the Pol I, Pol II and Pol III transcription machineries and discuss key structural and functional insights obtained into the three stages of their transcription processes.

真核转录装置合成了种类繁多的 RNA 分子。因此，核基因转录的工作在多个依赖于 DNA 的 RNA 聚合酶之间进行分配。RNA 聚合酶 I (Pol I) 转录核糖体 RNA，Pol II 合成信使 RNA 和各种非编码 RNA（包括长链非编码 RNA、微小 RNA 和小核 RNA），而 Pol III 产生转移 RNA 和其他短 RNA 分子。Pol I、Pol II 和 Pol III 是大型的多亚基蛋白复合物，它们与许多其他因子相关，以合成大小、结构和丰度差异很大的转录本。这三种转录机制具有共同的特征，但在各个方面差异很大，例如 RNA 聚合酶亚基的数量、调控元件和辅助因子、这使他们能够专注于转录其特定的 RNA。三种 RNA 聚合酶的共同点是转录过程由三个主要步骤组成：转录起始、转录延伸和转录终止。在这篇综述中，我们概述了 Pol I、Pol II 和 Pol III 转录机制之间的共同原则和差异，并讨论了对其转录过程的三个阶段获得的关键结构和功能见解。