BOMBYX ECR (BMECR) AND BOMBYX USP (BMCF1) COMBINE TO FORM A FUNCTIONAL ECDYSONE RECEPTOR

The Drosophila ecdysone receptor (DmEcR) is a member of the nuclear re ceptor superfamily; it functions as an obligate heterodimer with anoth er nuclear receptor, DmUSP. EcR homologs have now been cloned from sev eral other insects, We report here that one such homolog, BmEcR from t he commercial silkmoth, Bombyx mori, is a functional ecdysone receptor . Upon dimerization with BmCF1, the silkmoth homolog of DmUSP, BmEcR b inds the radiolabeled steroid ligand I-125-iodoponasterone A with K-d = 1.1 nM, indistinguishable from that exhibited by DmEcR/DmUSP. BmEcR/ BmCF1 forms a specific complex with an ecdysone response element (EcRE ) derived from the heat shock protein 27 (hsp27) gene promoter of Dros ophila; and, as with DmEcR/DmUSP, formation of this complex is stimula ted by the presence of 20-hydroxyecdysone, Finally, BmEcR can substitu te for DmEcR in an EcR-deficient Drosophila tissue culture line, stimu lating trans-activation of an ecdysone-inducible reporter gene constru ct, Thus, BmEcR and BmCF1 are the functional counterparts of DmEcR and DmUSP, respectively and, despite considerable sequence divergence bet ween the Drosophila and Bombyx proteins, the counterparts are-at least qualitatively-functionally equivalent.