Dimerization of the TATA binding protein.

The TATA binding protein (TBP) is a central component of all eukaryotic transcription machineries. The recruitment of TBP to the promoter is slow and possibly rate limiting in transcription complex assembly. In an effort to understand the nature of this potential rate-limiting step, we have investigated the physical state of TBP prior to DNA binding. By chemical cross-linking, gel filtration chromatography, and protein affinity chromatography, we find that the conserved carboxyl-terminal DNA binding domain of human TBP dimerizes when not bound to DNA. The data completely support the proposed dimeric structure of plant TBP, previously determined by x-ray crystallography. TBP dimers are quite stable, having an approximate equilibrium dissociation constant (KD) in the low nanomolar range. The dimerization interface appears to be dominated by hydrophobic forces, as predicted by the crystal structure. TBP dimers do not bind DNA, but they must dissociate into monomers before stably binding to the TATA box. Dissociation of TBP dimers appears to be relatively slow, and as such has the potential to dictate the kinetics of DNA binding.