Identification of a tissue-specific transcription factor required for ecdysone production in the prothoracic gland of Drosophila. Erik Thomas Danielsen¹, Morten E. Møller¹, Rachel Harder², Michael B. OConnor², Kim F. Rewtiz¹. 1) Department of Biology, Copenhagen University, Faculty of Science, Copenhagen, Denmark; 2) Department of Genetics, Cell Biology and Development, University of Minnesota, Minneapolis, USA.

   In the Drosophila larva, molting and metamorphosis depend on the steroid hormone producing prothoracic gland (PG). The PG regulates transcription of genes required for the biosynthesis of the molting hormone ecdysone in response to both developmental and environmental cues. Ecdysone is produced specifically in this endocrine tissue, and thus a PG-specific expression profile of genes encoding enzymes involved in ecdysone synthesis is required. However, the underlying transcriptional mechanism which dictates the cell specific program still remains to be elucidated. We have identified a transcription factor (TF) that is predominantly expressed in the PG from the embryonic formation and throughout larval development and, have investigated the consequence of reducing its expression specific in the PG. Interestingly, the larva arrests in the first instar and fails to undergo metamorphosis. The PG is intact in these larvae suggesting that the TF regulates aspects of steroid synthesis and not cell fate. We observed a reduced expression of the Halloween gene, phantom, and a reduced level of an ecdysone-responsive target gene suggesting that the TF is implicated in transcriptional regulation of the steroidogenic pathway. We further demonstrate binding-sites for the TF in the regulatory promoter-region of phantom in vitro and in vivo. In conclusion we believe this ís a key factor implicated in Drosophila steroid synthesis by regulating the expression of a PG-specific gene encoding an enzyme in the biosynthetic pathway of molting hormone, ecdysone.