The association of monomeric TATA binding protein with promoter DNA is an essential first step in many current models of eukaryotic transcription initiation. This step is followed by others in which additional transcription factors, and finally RNA polymerase, assemble at the promoter. Here we characterize the quaternary interactions of the Saccharomyces cerevisiae TATA-binding protein (yTBP), in the absence of other proteins or DNA. The data reveal a robust pattern in which yTBP monomers equilibrate with tetramers and octamers over a broad span of temperatures (4 °C ≤ T ≤37 °C) and salt concentrations (60 mM ≤ [KC1] ≤1 M), that includes the physiological range. Association is highly cooperative, with octamer formation favored by ~9 kcal/mol over tetramer formation. Changes in association constant with [KC1] are consistent with an assembly-linked release of ions at low salt and an assembly-linked uptake of ions at high salt, for both monomer ⇆ tetramer and tetramer ⇆ octamer reaction steps. Fluorescence emission spectra and steady-state anisotropies reveal that the amino-terminal domain changes conformation and dynamics at both association steps and that the polarity of the environment near tryptophan 26 is sensitive to changes in [KC1] in the monomeric and tetrameric states but not the octameric state. These results are consistent with a [salt]-dependent change in the assembly mechanism near 300 mM KC1 and suggest that the amino- terminal domain may modulate the self-association of the full-length protein. TBP self-association may regulate many of its cellular functions, including transit of the nuclear membrane and participation in transcription initiation.
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