High-resolution mapping of nucleoprotein complexes by site-specific protein-DNA photocrosslinking: organization of the human TBP-TFIIA-TFIIB-DNA quaternary complex.

We have used a novel site-specific protein-DNA photocrosslinking procedure to define the positions of polypeptide chains relative to promoter DNA in binary, ternary, and quaternary complexes containing human TATA-binding protein, human or yeast transcription factor IIA (TFIIA), human transcription factor IIB (TFIIB), and promoter DNA. The results indicate that TFIIA and TFIIB make more extensive interactions with promoter DNA than previously anticipated. TATA-binding protein, TFIIA, and TFIIB surround promoter DNA for two turns of DNA helix and thus may form a "cylindrical clamp" effectively topologically linked to promoter DNA. Our results have implications for the energetics, DNA-sequence-specificity, and pathway of assembly of eukaryotic transcription complexes.