DROSOPHILA ECDYSONE RECEPTOR FUNCTIONS AS A CONSTITUTIVE ACTIVATOR INYEAST

Transcriptional activation of the Drosophila ecdysone receptor (EcR) w as studied in yeast cells, which carry a reporter plasmid containing t he ecdysone response element in the absence or presence of its heterod imeric partners, ultraspiracle protein (USP) or human retinoid X recep tor (RXR alpha). High constitutive transcriptional activation was dete cted in the yeast strain expressing EcR, but not USP or RXR alpha in t he absence of ponasterone or muristerone A. Incubation of these Ligand s with yeast cells coexpressing EcR and USP or RXR alpha did not enhan ce the constitutive transcriptional activity. However, specific ligand binding using [H-3]ponasterone A as a radioactive ligand was detected only in yeast extracts prepared from the yeast strain coexpressing Ec R and USP, but not from yeast strains expressing only EcR or USP. The ligand binding characteristics of the EcR/USP complexes were similar t o those reported in an insect cell Line with a K-d value of 1.8 nM for [H-3]ponasterone A. These data are in contrast to mammalian cell tran sfection studies, and indicate that the EcR is the only member of the nuclear receptor superfamily of ligand-activated transcription factors which functions as a constitutive transcriptional activator in yeast, although the EcR/USP complexes exhibit normal ligand binding properti es. (C) 1997 Elsevier Science Ltd.