RETINOID-X RECEPTOR ALTERS THE DETERMINATION OF DNA-BINDING SPECIFICITY BY THE P-BOX AMINO-ACIDS OF THE THYROID-HORMONE RECEPTOR

Nuclear hormone receptors bind to hormone response elements in DNA con sisting of two half-sites of 6 base pairs. The P-box amino acids of ea ch receptor determine the identities of the central nucleotides of the half-site, 57 P-box variants of the human thyroid hormone receptor (h T3R beta) were used to demonstrate the relationship between P-box sequ ence and DNA binding specificity by homodimers and heterodimers formed with the retinoid X receptor (RXR). In general, the formation of hete rodimers relieved many of the constraints on the compatibility of hT3R beta 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 hT3R beta when heterodimers are bound to inverted or everted repeat elements, Th ese effects of RXR are most pronounced on AGGTCA half-sites but are sq uelched when the RXR partner of the heterodimer is bound to an AGGACA half-site. The influence of RXR on hT3R beta DNA binding specificity v aries 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 b inding domains of RXR-hT3R beta heterodimers are not positioned equiva lently on the various elements, affecting the manner in which the P-bo x amino acids of hT3R beta interact with base pairs within the half-si te.