Control of tailless expression by bicoid, dorsal and synergistically interacting terminal system regulatory elements

Three different maternal morphogen gradients regulate expression of the gap gene tailless (tll), which is required to establish the acron and telson of the Drosophila embryo. To identify elements in the tll promoter that respond to these different maternal systems, we have generated promoter-lacZ fusions and transformed them into the germline. Expression of these constructs in both wild type and mutant embryos revealed the presence of at least two separate but synergistically interacting regions that mediate tll expression by the terminal system. This functional synergism between regulatory elements may play a role in the translation of the torso (tor) morphogen gradient into the sharp boundary of tll gene activity. In addition to regions mediating activation by the terminal system, regions mediating both activation and repression by bicoid (bcd), and repression by dorsal (dl) were identified. Binding sites of bcd protein in a 0.5 kb region, revealed by DNaseI footprinting, could be crucial for the bcd-dependent activation of tll expression in the anterior stripe.