ULTRABITHORAX PROTEIN IS NECESSARY BUT NOT SUFFICIENT FOR FULL ACTIVATION OF DECAPENTAPLEGIC EXPRESSION IN THE VISCERAL MESODERM

To elucidate the mechanisms by which homeotic selector (HOM) genes specify the unique features of Drosophila segments, we have analyzed the regulation of decapentaplegic (dpp), a transforming growth factor (TGF)-beta superfamily member, and have found that the Ultrabithorax (Ubx) HOM protein directly activates dpp expression in parasegment 7 (PS7) of the embryonic visceral mesoderm. Other factors are also required, including one that appears to act through homeodomain protein binding sites and may be encoded by extradenticle (exd). The exd protein binds in a highly co-operative manner to regulatory sequences mediating PS7-specific dpp expression, consistent with a genetic requirement for exd function in normal visceral mesoderm expression of dpp. A second mechanism contributing to PS7 expression of dpp appears not to require Ubx protein directly, and involves a general visceral mesoderm enhancer coupled to a spatially specific repression element. Thus, even in an apparently simple case where visceral mesoderm expression of the dpp target gene mirrors that of the Ubx HOM protein, full activation by Ubx protein requires at least one additional factor. In addition, a distinct regulatory mode not directly involving Ubx protein also appears to contribute to PS7-specific expression.