Fig. 9. Molecular switching of Dpp-responsive enhancer activities in the dorsal mesoderm. The schematic diagram summarizes the molecular basis for the reciprocal activities of the bap and eve enhancers in the A and P domains of the dorsal mesoderm. In the A domains, the differential activities are due to the Wg-dependent relief of dTCF-associated co-repressors at the eve enhancer and to the Wg-dependent loading of Slp/co-repressor complexes at the bap enhancer. Conversely, in the P domains where Wg signaling is absent slp is not induced, which allows the bap enhancer to be active, while dTCF/co-repressor complexes keep the eve enhancer `off'. Owing to the lack of Wg-responsive sequences, the tin enhancer is `on' in both domains of the dorsal mesoderm. For simplicity, only one site of each type is depicted and various sites binding yet unknown factors involved in activation or ectodermal repression are not depicted.