Downregulation of the tissue-specific transcription factor Fork head by Broad-Complex mediates a stage-specific hormone response.

Drosophila development is coordinated by pulses of the steroid hormone 20-hydroxyecdysone (20E). During metamorphosis, the 20E-inducible Broad-Complex ( BR-C ) gene plays a key role in the genetic hierarchies that transduce the hormone signal, being required for the destruction of larval tissues and numerous aspects of adult development. Most of the known BR-C target genes, including the salivary gland secretion protein (Sgs) genes, are terminal differentiation genes that are thought to be directly regulated by BR-C -encoded transcription factors. Here, we show that repression of Sgs expression is indirectly controlled by the BR-C through transcriptional down-regulation of fork head , a tissue-specific gene that plays a central role in salivary gland development and is required for Sgs expression. Our results demonstrate that integration of a tissue-specific regulatory gene into a 20E-controlled genetic hierarchy provides a mechanism for hormonal repression. Furthermore, they suggest that the BR-C is placed at a different position within the 20E-controlled hierarchies than previously assumed, and that at least part of its pleiotropic functions are mediated by tissue-specific regulators.