Thyroid Hormone Alters in Vitro DNA Binding of Monomers and Dimers of Thyroid Hormone Receptors

T3 binds to intranuclear thyroid hormone receptors (TRs) on target DNA elements and exerts profound influences on gene expression by mechanisms not yet characterized. We used gel shift assays and cross-linking experiments to demonstrate that T3 greatly induced the monomeric binding of the hTR beta produced in Escherichia coli to DNA. T3 also increased the gel mobility of these monomer-DNA complexes suggesting they undergo a ligand-induced conformational change. This effect did not depend on the orientation and spacing of the half-site motifs within the DNA structure. In contrast, T3 had diverse effects on the dimeric interaction. T3 increased the dimeric interaction to the palindrome GGTCA.TGACC (an effect lost by spacing the half-sites with 3 base pairs) and decreased the dimeric interaction to the inverted palindrome containing the TGACC.GGTCA motif. Scatchard analyses indicated that the T3 enhancement on binding was due to an increase in the number of TR with high affinity DNA-binding activity and not by increasing the affinity of TR that could bind to DNA. The effects of various T3 analogs were directly related to their affinities for the TR. These ligand effects on in vitro TR-DNA binding may reflect mechanisms by which T3 regulates transcription in vivo.