Binding characteristics of the thyroid hormone receptor homo- and heterodimers to consensus AGGTCA repeat motifs.

Previous studies have shown that thyroid hormone receptors can form homo- and heterodimeric complexes when binding to response elements. We report here the binding characteristics of thyroid hormone receptor (TR) homo- and heterodimers binding to synthetic oligonucleotides with directly and palindromically repeated consensus motifs (AGGTCA). Binding assays showed that TR homodimer formation on DNA had a low specificity and cooperativity, and very fast off rates. In contrast, TRs and retinoic acid receptors readily formed heterodimers with higher specificity and affinity on direct repeats of the AGGTCA motif spaced by four or five nucleotides, although these heterodimer/DNA complexes were only moderately stable when compared to DNA-bound TR/retinoid X receptor heterodimers. Also, TR/retinoic acid receptor heteromeric binding to other elements, including the synthetic T3RE-pal element, was of low specificity. These biochemical results suggest that TRs are unlikely to regulate transcription as homodimers in vivo, and that TR heterodimers mediate the effects of thyroid hormone.