The transcription of vitamin D (VD) responsive genes is regulated by three different nuclear signalling pathways mediated by homodimers of VD receptors (VDRs), heterodimers of VDRs and retinoid X receptors (RXRs) and heterodimers of VDRs with retinoic acid receptors (RARs). Here, the in vitro DNA-binding affinity of all three receptor complexes was shown to be enhanced by the presence of VD. However, the specificity of the three pathways was dictated by the differential affinities of the receptor complexes for VD response elements. Potential response elements were distinguished by the sequence, the separation and the relative orientation of the hexameric core binding motifs. It was found that both VDR-RAR and VDR-RXR heterodimers act functionally on all three response element configurations: direct repeats, palindromes and inverted palindromes. With direct repeats, neither heterodimer type showed a preference for any of the three principal core motifs, (A/G)GGTGA, (A/G)GGTCA and (A/G)GTTCA. However, while they did exhibit preferences for core motifs in palindromes, the spacing requirements were identical for both complexes. Inverted palindromes, however, formed the most specific response elements. A simple model explains a steric link between the optimal spacing of direct repeats and that of inverted palindromes. Taken together, the experimental data and the model provide further criteria for the screening of VD responsive genes.
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