The fork head product directly specifies the tissue-specific hormone responsiveness of the Drosophila Sgs-4 gene.

Here we describe the identification of four binding sites of secretion enhancer binding protein 2 (SEBP2) in the regulatory region of the Drosophila salivary gland secretion protein gene 4 (Sgs-4) and show that despite these sites' correspondence with previously described Broad-Complex protein binding sites, SEBP2 is a Broad-Complex-independent factor encoded by the region-specific homeotic gene fork head (fkh). Two of the Fork head/SEBP2 binding sites are located within an ecdysone response unit which controls the tissue- and stage-specific responses of Sgs-4 to the steroid hormone 20-hydroxyecdysone. We demonstrate that these binding sites are relevant to the transcriptional activation of Sgs-4 and show that Fork head also binds to the Sgs-4 ecdysone response unit in vivo. Aside from being involved in the control of decisions during embryonic development, fkh thus participates directly in the control of specialized functions of differentiated cells at later stages of development.