EFFICIENT TRANSACTIVATION BY RETINOIC ACID RECEPTORS IN YEAST REQUIRES RETINOID-X RECEPTORS

All-trans and 9-cis retinoic acids are natural derivatives of vitamin A that modulate gene expression as a consequence of binding to nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs). RXRs form heterodimers with RARs in vitro, and such complexes display enhan ced binding affinities for cognate DNA response elements. As yeast is devoid of endogenous RARs and RXRs, we used this organism to investiga te whether transactivation in vivo requires RAR/RXR heterodimers. Usin g a domain-swapping approach, we demonstrate that chimeric RARalpha1 a nd RXRalpha containing the DNA-binding domain of the estrogen receptor activate transcription of a cognate reporter gene in yeast, independe ntly of each other. These activities result from an inducible transcri ption activation function located in the ligand-binding domains of RAR alpha1 and RXRalpha and a constitutive activation function located in the A/B region of RARalpha1. The inducible activation function of RXRa lpha is induced exclusively by 9-cis-retinoic acid in this system. Tra nsactivation of a reporter gene containing a retinoic acid response el ement by RARalpha was considerably increased by RXRalpha, even in the absence of ligand. Optimal induction was achieved with 9-cis-retinoic acid, which stimulates the activity of both receptors. This study illu strates the utility of yeast to investigate signal transduction by ret inoids in the absence of endogenous RARs, RXRs, and detectable retinoi c acid isomerization.