Transcriptional Activators Differ in Their Responses to Overexpression of TATA-Box-Binding Protein

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Abstract
We investigated how overexpression of human TATA-box-binding protein (TBP) affects the action of estrogen receptor (ER) and compared the response with that of other activators. When ER activates a simple promoter, consisting of a response element and either the collagenase or tk TATA box, TBP overexpression potentiates transcription. TBP potentiates only estrogen-induced and not basal transcription and does so independent of spacing between response element and TATA box. TBP overexpression also reduces autoinhibition by overexpressed ER, suggesting that one target of the autoinhibition may be TBP itself. Both AF-1 and AF-2 domains of ER are potentiated by TBP, and each domain binds TBP in vitro. Like ER, chimeric GAL4/VP16 and GAL4/Tat activators are also potentiated by TBP, as is the synergistic activation by ER and GAL4/VP16 on a complex promoter. Unlike ER, GAL4/Sp1 and GAL4/NF-I become less potent when TBP is overexpressed. Furthermore, synergy between ER and Sp1 or between ER and NF-I, whether these are supplied by transfected GAL4 fusions or by the endogenous genes, is inhibited by TBP overexpression. Thus, ER resembles VP16 in response to TBP overexpression and is different from Sp1 and NF-I, which predominate over ER in setting the response on complex promoters.