THYROID-HORMONE RECEPTOR MONOMER, HOMODIMER, AND HETERODIMER (WITH RETINOID-X RECEPTOR) CONTACT DIFFERENT NUCLEOTIDE-SEQUENCES IN THYROID-HORMONE RESPONSE ELEMENTS

Thyroid hormone receptors (TRs) bind as monomers, homodimers, and heterodimers with nuclear proteins such as retinoid-X receptors (RXRs) to thyroid hormone response elements (TREs). However, it is not known which nucleotides each TR complex contacts in a particular TRE. To identify the precise contact sites on a synthetic DR4 (TRE half-sites arranged as a direct repeat with a four-nucleotide spacer) and a chick lysoenzyme TRE, F2 (half-sites arranged as an inverted palindrome with a six-nucleotide spacer), for various TR complexes, mobility shift assays, and dimethylsulfate and KMnO4 DNA modification interference assays were employed. First, TR alpha monomer bound to the down-stream half-site of these TREs, whereas TR alpha homodimer bound to both half-sites. TR alpha/RXR alpha heterodimer also bound to both half-sites, but ''preferred'' to contact the down-stream half-site. Second, the specific flanking and spacing sequences of DR4 influenced the contact sites and the binding of TR alpha monomer and homodimer, but not TR alpha/RXR alpha heterodimer. Finally, cotransfection studies, using reporter plasmids containing DR4 or F2 in both orientations with respect to the basal promoter, provide evidence that preferential contact with the down-stream half-site by TR/RXR heterodimer may be important for maximal transcriptional activation.