Retinoid X Receptor Alters the Determination of DNA Binding Specificity by the P-box Amino Acids of the Thyroid Hormone Receptor*

By C. Nelson, S. Hendy, J. S. Faris, P. Romaniuk. 1996

Nuclear hormone receptors bind to hormone response elements in DNA consisting of two half-sites of 6 base pairs. The P-box amino acids of each receptor determine the identities of the central nucleotides of the half-site. 57 P-box variants of the human thyroid hormone receptor (hT3Rβ) were used to demonstrate the relationship between P-box sequence and DNA binding specificity by homodimers and heterodimers formed with the retinoid X receptor (RXR). In general, the formation of heterodimers relieved many of the constraints on the compatibility of hT3Rβ P-box sequences with DNA binding. Effects were most dramatic for heterodimers bound to a direct repeat spaced by four base pairs. RXR also overrides the P-box-derived DNA binding specificity of hT3Rβ when heterodimers are bound to inverted or everted repeat elements. These effects of RXR are most pronounced on AGGCA half-sites but are squelched when the RXR partner of the heterodimer is bound to an AGGCA half-site. The influence of RXR on hT3Rβ DNA binding specificity varies with the orientation of half-sites in the element, the identity of the fourth base pair of the half-site, and the spacing between the half-sites of direct repeats. These differences suggest that the DNA binding domains of RXR-hT3Rβ heterodimers are not positioned equivalently on the various elements, affecting the manner in which the P-box amino acids of hT3Rβ interact with base pairs within the half-site.