How are RNA polymerase and transcription factors assembled at a promoter of eukaryotic cells? A. The formation of a closed complex begins when the TATA-binding protein (TBP) binds to the TATA box. At promoters lacking a TATA box, TBP arrives as part of a multi-subunit complex called TFIID; the sequence elements that direct the binding of TFIID at these TATA-less promoters are poorly understood. B. TBP is bound, in turn, by the transcription factor TFIIB. TFIIA then binds and, along with TFIIB, helps to stabilize the TBP-DNA complex. TFIIB provides an important link to DNA polymerase II, and the TFIIB-TBP complex is next bound by another complex consisting of TFIIF and Pol II. C. TFIIF helps target Pol II to its promoters, both by interacting with TFIIB and by reducing the binding of the polymerase to nonspecific sites on the DNA. D. Finally, TFIIE and TFIIH bind to create the closed complex. TFIIH has multiple subunits and includes a DNA helicase activity that promotes the unwinding of DNA near the RNA start site (a process requiring the hydrolysis of ATP), thereby creating an open complex. Counting all the subunits of the various factors (including TFIIA and the subunits of TFIID), this active initiation complex can have more than 50 polypeptides.