Half-site arrangement of hybrid glucocorticoid and thyroid hormone response elements specifies thyroid hormone receptor complex binding to DNA and transcriptional activity

Thyroid hormone receptors bind to thyroid hormone response elements (TREs) as heterodimers with 3,5,3'-L-triiodothyronine (T₃) receptor auxiliary protein (TRAP) and retinoid X receptors (RXRs). Currently, it is not known whether TR/TRAP or TR/RXR heterodimers need to bind to both TRE half-sites and whether there is a preferred orientation for TR/RXR heterodimer binding to TREs or transcriptional activation. Accordingly, we created a mutant TRα (TR-P box) by changing 3 amino acids in the P box region of the first zinc finger of the DNA-binding domain to that of the glucocorticoid receptor (GR), and we examined wild-type TRα and TR-P box complex binding to hybrid response elements containing TRE and glucocorticoid receptor element (GRE) half-sites arranged as a direct repeat with a four-nucleotide gap. TR-P box/RXR heterodimers selectively bound to the hybrid response element in which the GRE half-site was the downstream half-site, whereas TRα/RXR bound to hybrid response elements in which GREs were in either position. Additionally, TR/TRAP or TR/RXR heterodimer required two half-sites for binding to DNA, with strong binding to at least one of the half-sites. Last, co-transfection assays and methylation interference studies using the hybrid response elements suggest that the sequential arrangement of strong and weak half-sites in the TRE may be a critical determinant of TR/RXR heterodimer binding and transcriptional activation.