Eukaryotic transcriptional activators may stimulate RNA polymerase II activity by promoting assembly of preinitiation complexes on promoters through their interactions with one or more components of the basal machinery. On the basis of its central role in initiating transcription-complex formation upon binding to the TATA box, the general transcription factor TFIID, which includes the TATA-binding protein (TBP) and several TBP-associated factors, has been implicated as a target for activators. Consistent with this idea, an increasing number of activators have been reported to bind directly to TBP. To assess the functional importance of these in vitro interactions for transcriptional regulation in vivo, we made use of a novel strategy in yeast to show that a physical interaction with TBP is sufficient for a sequence-specific DNA-binding protein to increase initiation of transcription by RNA polymerase II. These results imply that binding of TFIID to promoter elements is a limiting step in transcription complex assembly in vivo.