TFIID is a general transcription factor which initiates preinitiation complex assembly through direct interaction with the TATA promoter element. It is a multisubunit complex consisting of a small TATA-binding polypeptide and other TBP-associated factors (TAFs). Although native TFIID can mediate both activator-independent (basal) and activator-dependent transcription in reconstituted systems, TBP can mediate only basal transcription. The largest subunit (TAF) of TFIID is a protein designated TAF II p250. B-TFIID is an initial factor composed of TBP and TAF II p170 that has been identified as a pol II transcription factor. TAF II p170 has been shown to have potent (d)ATPase activity.