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

Abstract

Human transcription factor TFIID contains the TATA-binding protein (TBP) and several TBP-associated factors (TAF_IIs). To elucidate the structural organization and function of TFIID, we expressed and characterized the product of a cloned cDNA encoding human TAF_II135 (hTAF_II135). Comparative far Western blots have shown that hTAF_II135 interacts strongly with hTAF_II20, moderately with hTAF_II150, and weakly with hTAF_II43 and hTAF_II250. Consistent with these observations and with sequence relationships of hTAF_II20 and hTAF_II135 to histones H2B and H2A, respectively, TFIID preparations that contain higher levels of hTAF_II135 also contain higher levels of hTAF_II20, and the interaction between hTAF_II20 and hTAF_II135 is critical for human TFIID assembly in vitro. From a functional standpoint, hTAF_II135 has been found to interact strongly and directly with hTFIIA and (within a complex that also contains hTBP and hTAF_II250) to specifically cooperate with TFIIA to relieve TAF_II250-mediated repression of TBP binding and function on core promoters. Finally, we report a functional synergism between TAF_IIs and the TRAP/Mediator complex in activated transcription, manifested as hTAF_II-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 hTAF_II-enhanced repression state to an activator-mediated activated state on a promoter may be mediated in part through activator or coactivator interactions with hTAF_II135.

ACKNOWLEDGMENTS

We thank C.-M. Chiang for the pVL-HA-TAF_II20 expression plasmid and S. Malik for critical comments on the manuscript.

This work was supported by NIH grants CA 42567 and AI 37327 to R.G.R.