Transcription factor IID (TFIID) recognizes core promoters and supports preinitiation complex (PIC) assembly for RNA polymerase II (Pol II)–mediated eukaryotic transcription. We determined the structures of human TFIID–based PIC in three stepwise assembly states and revealed two-track PIC assembly: stepwise promoter deposition to Pol II and extensive modular reorganization on track I (on TATA–TFIID-binding element promoters) versus direct promoter deposition on track II (on TATA-only and TATA-less promoters). The two tracks converge at an ~50-subunit holo PIC in identical conformation, whereby TFIID stabilizes PIC organization and supports loading of cyclin-dependent kinase (CDK)–activating kinase (CAK) onto Pol II and CAK-mediated phosphorylation of the Pol II carboxyl-terminal domain. Unexpectedly, TBP of TFIID similarly bends TATA box and TATA-less promoters in PIC. Our study provides structural visualization of stepwise PIC assembly on highly diversified promoters.

转录因子IID（TFIID）识别核心启动子并支持RNA聚合酶II（Pol II）介导的真核转录的预启动复合物（PIC）组装。我们确定了基于人类TFIID的PIC在三个逐步组装状态下的结构，并揭示了两个轨迹的PIC组装：逐步将启动子沉积到Pol II和在轨迹I上进行广泛的模块重组（在TATA-TFIID结合元件启动子上）与直接启动子沉积在第二轨（在仅TATA和无TATA的启动子上）。两条轨道以相同的构象汇聚在一个约50个亚基的完整PIC上，从而TFIID可以稳定PIC的组织并支持将细胞周期蛋白依赖性激酶（CDK）激活激酶（CAK）装载到Pol II和CAK介导的Pol II磷酸化上羧基末端结构域。不料，TFIID的TBP类似地弯曲了PIC中的TATA盒启动子和TATA-less启动子。我们的研究提供了高度多样化的启动子上逐步PIC组装的结构可视化。