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RNA polymerase II (Pol II)–mediated transcription initiation requires assembly of a preinitiation complex (PIC), during which the 14-subunit transcription factor IID (TFIID) recognizes core promoters and recruits TFIIA, TFIIB, TFIIE, TFIIF, TFIIH, and Pol II to sequentially assemble core PIC (cPIC), intermediate PIC (mPIC), and holo PIC (hPIC). The textbook model of transcription initiation starts with binding of TBP (TATA box–binding protein, a TFIID subunit) to TATA box. However, up to 85% of coding genes lack consensus TATA box, and the TFIID complex is required for almost all Pol II–mediated gene transcription. Despite extensive structural studies of TBP-based PIC on TATA box promoters, it remains elusive how TFIID supports PIC assembly on different promoters.

We reconstituted and determined cryo–electron microcopy structures of the human TFIID-based PIC. Stepwise PIC …