Autoinhibition of TFIID. (A) Decomposition of pol II transcription initiation into antirepression and net activation. (B) Two-step model for reversal of autoinhibitory effects of TFIID, involving formation of an intermediate complex composed of TBP and a transcriptional activator, such as VP16. (C) Ribbon drawing of saddle-shaped TBP, showing binding surface for portions of dTAF230, NC2, VP16, TFIIA, TFIIB, and the TATA element. Alpha helices H1, H2, H1', and H2', and the N and C termini of TBP are labeled.