The E75 ecdysone inducible gene, believed to be a key regulator of ecdysone triggered genetic hierarchies in Drosophila development, encodes three different protein products which are members of the steroid receptor superfamily and which share conserved structural features corresponding to the DNA and hormone binding domains. Genetic analysis shows that these proteins carry out distinct functions at different stages of development. The E75 A protein has been localized immunocytochemically to the chromosomal sites of many ecdysone regulated genes, including E75. In functional studies, this protein has been shown to be capable of negatively regulating its own promoter. The similarity of E75 sequences to the hormone binding domains of known receptor suggests that E75 is also a receptor. This hypothesis is supported by evolutionary comparison and preliminary functional analysis of the putative hormone binding domain. No known steroid, thyroid, retinoid hormone or insect juvenile hormone activates the E75 proteins, suggesting that the E75 ligand is a previously unidentified hormone. In the studies propose here, these observations will be extended to further define the role of the different E75 proteins and the putative E75 ligand in the regulation of gene expression and Drosophila development. Studies will be focused in three areas: 1) developmental genetic analysis of E75 to further define the mutant phenotypes, to understand the role of each of the E75 proteins in the generation of these phenotypes and the role of different E75 domains involved in the repression and activation of target genes, 2) characterization of its role in development, and 3) characterization of target gene regulation by developmentally characterized E75 mutant proteins.
