The transcription initiation factor TFIID, consisting of the TATA box-binding protein (TBP) and many TBP-associated factors (TAFs), plays a central role in both basal and activated transcription. An intriguing finding is that the 80-residue N-terminal region of Drosophila TAFII230 [dTAFII230-(2–81)] can bind directly to TBP and inhibit its function. Here, studies with mutated forms of TBP demonstrate that dTAFII230-(2–81) binds to the concave surface of TBP, which is important for TATA box binding. Previously, it was reported that a point mutation (L114K) on this concave surface destroys the ability of TBP to bind VP16 and to mediate VP16-dependent activation in vitro, but has no effect on basal transcription. Importantly, the same TBP mutation eliminates TBP binding to dTAFII230-(2–81). Consistent with these effects of the L114K mutation, dTAFII230-(2–81) and the VP16 activation domain compete for binding to wild-type TBP. These results indicate that transcriptional regulation may involve, in part, competitive interactions between transcriptional activators and TAFs on the TBP surface
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