Individual somatic muscles and heart progenitors are specified at defined positions within the mesodermal layer of Drosophila. The expression of the homeobox gene even-skipped (eve) identifies one specific subset of cells in the dorsal mesoderm, which give rise to particular pericardial cells and dorsal body wall muscles. Genetic analysis has shown that the induction of eve in these cells involves the combined activities of genes encoding mesoderm-intrinsic factors, such as Tinman (Tin), and spatially restricted signaling activities that are largely derived from the ectoderm, particularly those encoded by wingless (wg) and decapentaplegic (dpp). Here we show that a Dpp-activated Smad protein, phosphorylated Mad, is colocalized in eve-expressing cells during an extended developmental period. We demonstrate further that a mesodermally active enhancer of eve contains several Smad and Tin binding sites that are essential for enhancer activity in vivo. This enhancer also contains a number of binding sites for the Wg-effector Pangolin (Pan/Lef-1), which are required for full levels of enhancer activity. However, we find that their main function is to prevent ectopic enhancer activity in the dorsal mesoderm. This suggests that, in the absence of Wg signaling, Pan binding serves to abrogate the synergistic activities of Smads and Tin in eve activation while, in cells that receive Wg signals, Pan is converted into a coactivator that promotes eve induction. Together, these data show that the eve enhancer integrates several regulatory pathways via the combinatorial binding of the mesoderm-intrinsic regulator Tin and the effectors of the Dpp and Wg signals.
Copyright 2001 Academic Press.