RETINOID X RECEPTOR HOMODIMERS FUNCTION AS TRANSCRIPTIONAL ACTIVATORSIN YEAST

The possibility that different retinoids activate transcription from a specific retinoic acid (RA)-responsive element known as site A via di fferent home and heterodimeric versions of RA receptors cannot be eval uated in mammalian cells because they contain endogenous RA receptors (RAR). However, this limitation can be overcome by using yeast cells, which do not contain endogenous RAR, to study retinoid signaling pathw ays. Here, we describe heterologous expression of the human retinoid X receptor (RXR alpha) in yeast and hormone-dependent activation of a r eporter construct containing site A upstream from a yeast promoter fus ed to the lacZ gene of Escherichia coli. Western blot analysis of yeas t extracts containing RXR alpha revealed a distinct immunoreactive pol ypeptide co-migrating with the mammalian-produced RXR alpha. Electroph oretic mobility shift assays demonstrated that RXR alpha. produced in yeast binds efficiently to site A in the absence of 9-cis-RA. However, transcription activation experiments showed that RXR alpha transactiv ates a yeast basal promoter linked to site A only in the presence of 9 -cis-RA. We conclude that RXR alpha homodimers bind to site A in the a bsence of 9-cis-RA, but function as ligand-dependent transactivators i n yeast cells. This retinoid-responsive transcription unit created in yeast cells provides a powerful genetic tool for the systematic unrave ling of the synergistic interactions between RXR alpha and its heterod imeric partners.