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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.
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