Transcriptional activator proteins (activators) function, at least in part, by increasing preinitiation complex assembly. Previous studies have shown that an acidic activator forms a contact with the general transcription factor TFIIB and recruits it into the preinitiation complex. Mutational studies indicate that this interaction between the acidic activator and TFIIB is required for transcriptional stimulation. We show here that the acidic activator-TFIIB interaction has an additional function in preinitiation complex assembly. We provide evidence that in native TFIIB the amino- and carboxy-terminal domains are engaged in an intramolecular interaction. The acidic activator disrupts this intramolecular interaction to expose binding sites for general transcription factors that enter the preinitiation complex through association with TFIIB. Thus, the acidic activator induces a conformational change in TFIIB that drives preinitiation complex assembly forward.