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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.
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