| TATA box-binding protein (TBP) is required by all three eukaryotic RNA polymerases for correct initiation of transcription of ribosomal, messenger, small nuclear and transfer RNAs. Since the first gene encoding a TBP was cloned, it has been the object of considerable biochemical and genetic study. Substantial progress has also been made on structural and mechanistic studies, including our three-dimensional crystal structures of TBP, two TBPs bound to consensus TATA elements, and the ternary complex of transcription factor IIB (TFIIB) recognizing TBP bound to a TATA element. The structure of apo TBP was determined at 2.1A resolution. This highly symmetric {dollar}alpha/beta{dollar} structure represents a new DNA- binding fold, which resembles a molecular "saddle" that sits astride the DNA. The DNA-binding surface is a novel curved, antiparallel {dollar}beta{dollar}-sheet. The structures of two different TBPs complexed with the TATA element of the Adenovirus major late promoter were determined at 1.9A resolution. Binding of the proteins induces a dramatic conformational change in the DNA, by tracking the minor groove and inducing two sharp kinks at either end of the sequence TATAAAAG. Between the kinks, the right-handed double helix is smoothly curved and partially unwound, presenting a widened minor groove to TBP's concave, antiparallel {dollar}beta{dollar}-sheet. Side chain-base interactions are completely restricted to the minor groove, and include hydrogen bonds, van der Waals contacts and phenylalanine-base stacking interactions. The structure of a TFIIB/TBP/TATA element ternary complex was determined at 2.7A resolution. Core TFIIB resembles cyclinA, and recognizes the preformed TBP-DNA complex via protein-protein and protein-DNA interactions. The N-terminal domain of core TFIIB forms the downstream surface of the ternary complex, where it could fix the transcription start site. The remaining surfaces of TBP and the TFIIB can interact with TBP-associated factors, other class II initiation factors, and transcriptional activators and coactivators. |
