Regulation of activity of the yeast TATA-binding protein through intra-molecular interactions.

Dimerization is proposed to be a regulatory mechanism for TATA-binding protein (TBP) activity both in vitro and in vivo. The reversible dimer-monomer transition of TBP is influenced by the buffer conditions in vitro. Using in vitro chemical cross-linking, we found yeast TBP (yTBP) to be largely monomeric in the presence of the divalent cation Mg2+, even at high salt concentrations. Apparent molecular mass of yTBP at high salt with Mg2+, run through a gel filtration column, was close to that of monomeric yTBP. Lowering the monovalent ionic concentration in the absence of Mg2+, resulted in dimerization of TBP. Effect of Mg2+ was seen at two different levels: at higher TBP concentrations, it suppressed the TBP dimerization and at lower TBP levels, it helped keep TBP monomers in active conformation (competent for binding TATA box), resulting in enhanced TBP-TATA complex formation in the presence of increasing Mg2+. At both the levels, activity of the full-length TBP in the presence of Mg2+ was like that reported for the truncated C-terminal domain of TBP from which the N-terminus is removed. Therefore for full-length TBP, intra-molecular interactions can regulate its activity via a similar mechanism.

The transcriptional activator GAL4-VP16 regulates the intra-molecular interactions of the TATA-binding protein.

Binding characteristics of yeast TATA-binding protein (yTBP) over five oligomers having different TATA variants and lacking a UASGAL, showed that TATA-binding protein (TBP)-TATA complex gets stabilized in the presence of the acidic activator GAL4-VP16. Activator also greatly suppressed the non-specific TBP-DNA complex formation. The effects were more pronounced over weaker TATA boxes. Activator also reduced the TBP dimer levels both in vitro and in vivo, suggesting the dimer may be a direct target of transcriptional activators. The transcriptional activator facilitated the dimer to monomer transition and activated monomers further to help TBP bind even the weaker TATA boxes stably. The overall stimulatory effect of the GAL4-VP16 on the TBP-TATA complex formation resembles the known effects of removal of the N-terminus of TBP on its activity, suggesting that the activator directly targets the N-terminus of TBP and facilitates its binding to the TATA box.