The helix-loop-helix proteins dAP-4 and Daughterless bind both in vitro and in vivo to SEBP3 sites required for transcriptional activation of the Drosophila gene Sgs-4

The expression of Sgs genes in the salivary gland of the third instar larva of Drosophila is a spatially restricted response to signalling by the ster oid hormone 20-hydroxyecdysone. For Sgs-4, we have previously demonstrated that its strictly tissue and stage-specific expression is the result of com bined action of the ecdysone receptor and secretion enhancer binding protei ns (SEBPs). One of these SEBPs, SEBP2, was shown to be the product of the h omeotic gene fork head. Together with SEEPS, SEBP2 appears to be responsibl e for the spatial restriction of the hormone response of Sgs-4. Here, we sh ow that SEEPS is a heterogeneous binding activity that consists of differen t helix-loop-helix (HLH) proteins. We cloned the Drosophila homologue of hu man transcription factor AP-4 (dAP-4) and identified it as one of these HLH proteins. The dAP-4 protein shows great similarity to its human and Caenor habditis counterparts within the bHLHZip domain, the second leucine zipper dimerization motif, and a third region of unknown function. The expression pattern of dAP-4 indicates that it is a ubiquitously expressed HLH protein in Drosophila. As a second component of SEEPS we identified the Daughterles s (Da) protein, which is also ubiquitously expressed and binds to SEEPS sit es independent of dAP-4. Since both dAP-4 and Da can be detected in situ at transposed Sgs-4 transcriptional control elements in polytene salivary gla nd chromosomes, we propose that each of the two proteins contributes to the transcriptional control of Sgs-4. (C) 1999 Academic Press.