Novel Cofactors and TFIIA Mediate Functional Core Promoter Selectivity by the Human TAF(II)150-Containing TFIID Complex

TATA-binding protein-associated factors (TAF(II)s) within TFIID control differential gene transcription through interactions with both activators and core promoter elements. In particular, TAF(II)150 contributes to initiator-dependent transcription through an unknown mechanism. Here, we address whether TAF(II)s within TFIID are sufficient, in conjunction with highly purified general transcription factors (GTFs), for differential core promoter-dependent transcription by RNA polymerase II and whether additional cofactors are required. We identify the human homologue of Drosophila TAF(II)150 through cognate cDNA cloning and show that it is a tightly associated component of human TFIID. More importantly, we demonstrate that the human TAF(II)150-containing TFIID complex is not sufficient, in the context of all purified GTFs and RNA polymerase II, to mediate transcription synergism between TATA and initiator elements and initiator-directed transcription from a TAF(II)-dependent TATA-less promoter. Therefore, TAF(II)-promoter interactions are not sufficient for the productive core promoter-selective functions of TFIID. Consistent with this finding, we have partially purified novel cofactor activities (TICs) that potentiate the TAF(II)-mediated synergism between TATA and initiator elements (TIC-1) and TAF(II)-dependent transcription from TATA-less promoters (TIC-2 and -3). Furthermore, we demonstrate an essential function for TFIIA in TIC- and TAF(II)-dependent basal transcription from a TATA-less promoter. Our results reveal a parallel between the basal transcription activity of TAF(II)s through core promoter elements and TAF(II)-dependent activator function.