Positive and Negative TAFII Functions That Suggest a Dynamic TFIID Structure and Elicit Synergy with TRAPs in Activator-Induced Transcription

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Abstract
Human transcription factor TFIID contains the TATA-binding protein (TBP) and several TBP-associated factors (TAFIIs). To elucidate the structural organization and function of TFIID, we expressed and characterized the product of a cloned cDNA encoding human TAFII135 (hTAFII135). Comparative far Western blots have shown that hTAFII135 interacts strongly with hTAFII20, moderately with hTAFII150, and weakly with hTAFII43 and hTAFII250. Consistent with these observations and with sequence relationships of hTAFII20 and hTAFII135 to histones H2B and H2A, respectively, TFIID preparations that contain higher levels of hTAFII135 also contain higher levels of hTAFII20, and the interaction between hTAFII20 and hTAFII135 is critical for human TFIID assembly in vitro. From a functional standpoint, hTAFII135 has been found to interact strongly and directly with hTFIIA and (within a complex that also contains hTBP and hTAFII250) to specifically cooperate with TFIIA to relieve TAFII250-mediated repression of TBP binding and function on core promoters. Finally, we report a functional synergism between TAFIIs and the TRAP/Mediator complex in activated transcription, manifested as hTAFII-mediated inhibition of basal transcription and a consequent TRAP requirement for both a high absolute level of activated transcription and a high and more physiological activated/basal transcription ratio. These results suggest a dynamic TFIID structure in which the switch from a basal hTAFII-enhanced repression state to an activator-mediated activated state on a promoter may be mediated in part through activator or coactivator interactions with hTAFII135.