DIRECT REGULATION OF DECAPENTAPLEGIC BY ULTRABITHORAX AND ITS ROLE INDROSOPHILA MIDGUT MORPHOGENESIS

Drosophila homeotic genes encode transcription factors thought to cont rol segmental identity by regulating expression of largely unknown tar get genes. The formation of the second midgut constriction requires th e Ultrabithorax (Ubx) and abdominal-A (abd-A) homeotic genes and decap entaplegic (dpp), a gene encoding a member of the TGF beta family of p roteins. We identified a 674 bp enhancer of dpp controlling its expres sion in the second constriction domain of the visceral mesoderm (paras egment 7). Normal enhancer function requires positive regulation by Ub x and negative regulation by abd-A. This enhancer contains UBX- and AB D-A-binding sites defined in vitro. By generating complementary altera tions of the binding sites and the binding specificity of UBX, we show that Ubx directly regulates dpp expression. These regulatory interact ions are relevant to normal development, because a transgene made with this enhancer driving a dpp transcription unit rescues the second mid gut constriction and larval lethality phenotypes of dpp(s) mutations.