Specificity and mechanism of thyroid hormone induction from an octamer response element.

Thyroid hormone response elements are specific DNA sequences that allow thyroid hormone receptors to confer ligand-dependent regulation of gene expression. The response elements characterized to date have been composed of varying arrangements of multiple copies of a conserved hexameric sequence. The traditional consensus half-site of these response elements is the sequence 5'-AGGTCA, although we have demonstrated recently that the optimal thyroid hormone receptor monomer binding site is 2 base pairs larger, 5'-TAAGGTCA. Since other members of this family of nuclear receptors also have been shown to use varying arrangements of the traditional hexamer sequence as response elements, we examined whether the octamer sequence was specific as a thyroid hormone response element. The studies reported here demonstrate that only thyroid hormone receptors confer ligand responsiveness to a reporter gene containing a single copy of the octamer sequence as a response element and that qualitative and quantitative differences in the binding of related nuclear receptors to this sequence can account for this functional specificity. We also have shown that thyroid hormone induction from the octamer response element occurs independently of retinoid X receptors, in contrast to the induction from traditional complex thyroid hormone response elements.