Structural basis of preinitiation complex assembly on human Pol II promoters

Transcription initiation requires the assembly of a preinitiation complex ( PIC), which is nucleated through binding of the TATA-box binding protein (T BP) to the promoter. Biochemical studies have shown, however, that TBP reco gnizes the TATA-box in both orientations and, therefore, cannot account for the directionality of PIC assembly. Transcription factor IIB (TFIIB) is es sential for transcription initiation from RNA polymerase II promoters. Rece nt functional studies have identified a specific 7 bp TFIIB recognition ele ment (BRE) immediately upstream of the TATA-box. We present here the 2.65 A ngstrom resolution crystal structure of a human TFIIBc-TBPc complex bound t o an idealized and extended adenovirus major late promoter. This structure now reveals that human TFIIBc binds to the promoter asymmetrically through base-specific contacts in the major groove upstream and in the minor groove downstream of the TATA-box. Binding of TFIIBc is, therefore, synergistic w ith TBPc requiring the distortion of the TATA-box. Thus, the newly describe d TFIIBc-DNA interface is likely to be a key determinant for the unidirecti onal assembly of a functional PIC.