Fig. 6. Collaboration between Ubx and Smads in the selective repression of the sal gene in the Drosophila haltere disc. (Left) A depiction of the wing and haltere imaginal discs of the second and third thoracic segments (T2 and T3, respectively). Sal is expressed (yellow) where high levels of Dpp signaling emanate from the anterior-posterior compartment boundary. Brk (purple) represses sal and restricts its expression to the center of the wing imaginal disc. Ubx (red stripes) is expressed in the haltere disc, where it regulates target genes such as sal. (Right) The genetic circuitry and molecular mechanisms allowing sal expression in the wing and repressing sal in the haltere. In the wing, Sal is expressed in cells where the Dpp-responsive Mad-Med-Shn repressor complex binds to a high-affinity Smad site in a brk CRE to directly repress Brk expression. The sal1.1 CRE contains a low-affinity Smad site but this is insufficient to repress sal in the wing. However, in the haltere, sal is repressed via the collaborative action of Ubx and Mad-Med-Shn binding to the sal1.1 CRE. Both the affinity and topology of repressor binding sites are critical to the selective repression of sal in the T3 flight appendage.