Proline-rich activator CTF1 targets the TFIIB assembly step during transcriptional activation.

Activators can stimulate transcription through direct or indirect interactions with general initiation factors. We show here that the proline-rich activation domain of CTF1 (CCAAT-box-binding transcription factor 1) selectively interacts with TFIIB but not with the TATA-binding protein (TBP), whereas previous studies have shown that the acidic activation domain of viral VP16 interacts directly with both TBP and TFIIB. In addition, consistent with studies of acidic activation domains, we demonstrate that the activation domain of CTF1 facilitates the recruitment (or stabilization) of TFIIB within TBP-DNA complexes during preinitiation complex assembly. CTF1-enhanced TFIIB recruitment was observed in both human and yeast systems. The results indicate that the proline-rich activation domain enhances transcription, at least in part, through direct interactions with TFIIB and, with previous observations, suggest models involving either quantitative or qualitative changes in TFIIB-TFIID-promoter interactions that lead to increased utilization of downstream initiation factors.