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Two mechanisms for PIC formation, which are not mutually exclusive. (a) In the stepwise model, Pol II and the GTFs assemble in a particular order facilitated by one factor recruiting the subsequent factor via protein–protein interactions. (b) In the holoenzyme model, minimally TFIID and TFIIA assemble at the promoter while Pol II and remaining GTFs form a subcomplex that binds to the promoter, completing PIC assembly.
