Collaboration between Smads and a Hox protein in target gene repression.

Hox proteins control the differentiation of serially iterated structures in arthropods and chordates by differentially regulating many target genes. It is yet unclear to what extent Hox target gene selection is dependent upon other regulatory factors and how these interactions might affect target gene activation or repression. We find that two Smad proteins, effectors of the Drosophila Dpp/TGF-beta pathway, that are genetically required for the activation of the spalt (sal) gene in the wing, collaborate with the Hox protein Ultrabithorax (Ubx) to directly repress sal in the haltere. The repression of sal is integrated by a cis-regulatory element (CRE) through a remarkably conserved set of Smad binding sites flanked by Ubx binding sites. If the Ubx binding sites are relocated at a distance from the Smad binding sites, the proteins no longer collaborate to repress gene expression. These results support an emerging view of Hox proteins acting in collaboration with a much more diverse set of transcription factors than has generally been appreciated.