Ecdysteroid signaling in insects is mediated by the ecdysone receptor complex that is composed of a heterodimer of the ecdysone receptor and Ultraspiracle. The DNA binding specificity plays a critical role of defining the repertoire of target genes that respond to the hormone. We report here the determination of the preferred core recognition motif by a binding site selection procedure. The consensus sequence consists of a perfect palindrome of the heptameric half-site sequence GAGGTCA that is separated by a single A/T base pair. No binding polarity of the ecdysone receptor/Ultraspiracle heterodimer to the core recognition motif was observed. This core motif mediated the highest level of ligand-induced transactivation when compared to a series of synthetic ecdysone response elements and to the natural element of the Drosophila hsp27 gene. This is the first report of a palindromic sequence identified as the highest affinity DNA binding site for a heterodimeric nuclear hormone receptor complex. We further present evidence that the ligand of the ecdysone receptor preferentially drives Ultraspiracle from a homodimer into a heterodimer. This mechanism might contribute additionally to a tight control of target gene expression.
The use of reverse genetics has permitted a definition of the structural features within estrogen receptor required for its productive association with the transcription apparatus. These sequences, transactivation function 1 (TAF1) in the amino terminus and TAF2 at the carboxyl terminus, display distinct transcriptional functions. Using specific receptor mutations it has been shown that on some promoters both TAF1 and TAF2 are required for maximal transcriptional activity, whereas on others, additional factors bound to the target promoter can functionally substitute for TAF1 or TAF2. Estrogen functions as an ER agonist by promoting functional synergism between TAF1 and TAF2. Conversely, 4-OH-tamoxifen inhibits TAF2 activity and functions as an antagonist in cell contexts where TAF2 is required. Alternatively, if a 'TAF2 function' is supplied by another factor, 4-OH tamoxifen can manifest ER agonist activity. These data indicate that alterations in the cellular expression of proteins which mimic TAF1 or TAF2 activity can have a profound effect on the pharmacology of ER modulators. Thus the identification of the cellular proteins which interact with ER and its TAF regions will allow a definition of the mechanism used by the cell to distinguish between hormone- and antihormone-activated estrogen receptor.
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