Reevaluation of transcriptional regulation by TATA-binding protein oligomerization: Predominance of monomers

The TATA-binding protein (TBP) plays an important role in transcriptional i nitiation by all three nuclear RNA polymerases. TBP contains a conserved C- terminal domain (cTBP) that binds DNA. Crystallographic studies of cTBP (i. e., TBP without the N-terminal domain) from various species and molecular b iology studies of cTBP and mixed cTBP/TBP species have led to the view that DNA binding by TBP is regulated by TBP dimerization. Using sedimentation e quilibrium, we show that yeast cTBP forms dimers in solution at 5 degrees C with a dissociation constant of 7 +/- 1 mu M. This observation of cTBP dim ers in solution is in accord with the dimeric state observed in crystal str uctures of cTBP. In contrast, physiologically relevant, full-length yeast T BP is monomeric at 5 degrees C and forms dimers at 30 degrees C with a diss ociation constant of 51 +/- 16 mu M. This dissociation constant precludes f ormation of stable full-length TBP dimers at physiological concentrations. In addition, we tested for yeast TBP oligomerization in the presence of TBP -associated factors in the context of TFIID. No evidence for TBP oligomers was found using immunoprecipitation techniques from yeast whole-cell extrac ts. We conclude that yeast TBP is predominantly monomeric under physiologic al conditions, arguing against a role for TBP dimerization in the regulatio n of transcriptional initiation.