Dual requirement for the EcR USP nuclear receptor and the dGATAb factor inan ecdysone response in Drosophila melanogaster

The EcR/USP nuclear receptor controls Drosophila metamorphosis by activatin g complex cascades of gene transcription in response to pulses of the stero id hormone ecdysone at the end of larval development. Ecdysone release prov ides a ubiquitous signal for the activation of the receptor, but a number o f its target genes are induced in a tissue- and stage-specific manner. Litt le is known about the molecular mechanisms involved in this developmental m odulation of the EcR/USP-mediated pathway. Fbp1 is a good model of primary ecdysone response gene expressed in the fat body for addressing this questi on. We show here that the dGATAb factor binds to three target sites flankin g an EcR/USP binding site in a 70-bp enhancer that controls the tissue and stage specificity of Fbp1 transcription. We demonstrate that one of these s ites and proper expression of dGATAb are required for specific activation o f the enhancer in the fat body. In addition, we provide further evidence th at EcR/USP plays an essential role as a hormonal timer. Our study provides a striking example of the integration of molecular pathways at the level of a tissue-specific hormone response unit.