A chimeric thyroid hormone receptor constitutively bound to DNA requires retinoid X receptor for hormone-dependent transcriptional activation in yeast

T₃ receptors (TRs) regulate transcription by binding to specific DNA response elements as heterodimers with the retinoid X receptors (RXRs). To study the consequences of this heterodimerization for transcriptional regulation in the absence of complications associated with its effects on DNA binding affinity, we expressed in the yeast Saccharomyces cerevisiae a chimeric protein consisting of the rat TRβ1 ligand-binding domain fused to the DNA-binding domain of the bacterial repressor lexA (lexATR). LexATR is a weak, T₃-responsive activator of a β-galactosidase reporter gene controlled by upstream lexA-binding sites (lexA-β-gal). In contrast, coexpression of human RXRα (hRXRα) strongly enhances both the basal and ligand-induced transcriptional activities. Both the N-terminal activation domain of RXR and sequences at the extreme C terminus of lexATR are required for this T₃- and RXR-dependent transcriptional activation. The lexATR chimera was also used to characterize receptor-receptor interactions using the two-hybrid system. Coexpression of B42RXR, a fusion protein of the human RXRα ligand-binding domain and the B42 transcriptional activation domain, strongly increases the transcriptional activity of lexATR in the absence of T₃ or 9-cis-retinoic acid. We conclude that RXR is essential for full, T₃-dependent transcriptional activity of the TR in yeast, and that protein-protein interaction of TR and RXR in vivo is ligand-independent.