THYROID-HORMONE RECEPTOR FUNCTIONS AS MONOMERIC LIGAND-INDUCED TRANSCRIPTION FACTOR ON OCTAMERIC HALF-SITES - CONSEQUENCES ALSO FOR DIMERIZATION

The thyroid hormone (3,5,3'-triiodothyronine) receptor (T(3)R) belongs to the nuclear receptor superfamily of ligand-inducible transcription factors. T(3)Rs are known to bind as homodimers and heterodimers with retinoid X receptors (RXRs) to two hexameric half-sites in directly r epeated, palindromic, and inverted palindromic orientations. The bindi ng of T(3)R monomers to individual half-sites was often reported, but no clear ligand-induced transactivational activity has been shown. Her e, we analyzed interactions of T(3)R monomers with individual half-sit es of the sequence NNAGGTCA. We found that the two nucleotides 5' of t he AGGTCA core half-site strongly influence T(3)R binding and transcri ptional activity: octameric half sites of the consensus sequence (T/C) (A/G)AGGTCA were bound by T(3)Rs with the highest affinity. This sugge sts T(3)R functioning also as a monomeric transcription factor like th e orphan nuclear receptors NGFI-B and FTZ-F1. Moreover, we observed th at the function of T(3)R-RXR heterodimers on response elements compose d of two half-sites in a directly repeated orientation spaced by 4 nuc leotides is determined in major parts by the 5'-flanking sequence of t he upstream half-site. Consequently, we noted that the affinity of T(3 )R homodimers is influenced by both 5'-flanking sequences. Our finding s suggest that the binding of dimerizing receptors like T(3)R and othe r nuclear receptors to their cognate response elements is determined n ot only by the half-site core sequence, orientation, and number of spa cing nucleotides, but also by the nucleotide sequence preceding the ha lf-sites.