HOMODIMER AND HETERODIMER DNA-BINDING AND TRANSCRIPTIONAL RESPONSIVENESS TO TRIIODOTHYRONINE (T-3) AND 9-CIS-RETINOIC ACID ARE DETERMINED BY THE NUMBER AND ORDER OF HIGH-AFFINITY HALF-SITES IN A T-3 RESPONSE ELEMENT

T-3 (triiodothyronine) response elements (TREs) consist of pairs of st rong and weak (S and W), 10-nucleotide Ts receptor (TR) monomer bindin g sites (half-sites). We report that the number and order of S and W h alf-sites in a direct repeat TRE determines whether it mediates ligand -dependent or independent transcriptional activation or inhibition in the presence of TR or TR and 9-cis-retinoic acid receptor (RXR); and w hether a TRE is preferentially bound by TR homodimers, TR-RXR heterodi mers, or CV1 cell TR accessory protein (TRAP)-TR heterodimers. TR homo dimers bound equally to TREs composed of the 5'-S and 3'-W (SW) and th e opposite (WS) arrangement of half-sites. TR-RXR gamma heterodimers b ound SW better than WS. TR-TRAP heterodimers bound WS better than SW. Transcription of a reporter gene cis-linked to WS responded to unligan ded TR and RXR, and either ligand stimulated expression 2-fold more. R eporter expression cis-linked to SW was not altered by unliganded rece ptors, and Ts stimulated transcription only in the presence of both TR and RXR. SS was strongly activated by liganded, but not by unliganded TR. SS was activated by unliganded TR and RXR gamma together, and Ts further stimulated transcription a-fold. Under these conditions, trans cription was inhibited 60% by 9-cis-retinoic acid.