Homeotic genes of the Bithorax complex repress limb development in the abdomen of the Drosophila embryo through the target gene Distal-less.

Homeotic genes encode transcription factors that are thought to specify segmental identity by regulating expression of subordinate genes. Limb development is repressed in the abdominal segments of the Drosophila embryo by the hometic genes of the Bithorax complex (BX-C). Localized expression of the homeobox gene Distal-less (DII) is required for leg development in thoracic segments. We have identified a minimal cis-regulatory enhancer element that directs DII expression in the larval leg primordia. We present evidence that the BX-C proteins repress DII expression in abdominal segments by binding to a small number of specific sites in this element. Mutating these sites eliminates BX-C protein binding and renders the element insensitive to BX-C-mediated repression in vivo. Repression of limb development in the abdomen appears to be controlled at the DII enhancer. Thus DII may serve as a downstream target gene through which the homeotic genes control abdominal segment identity in the Drosophila embryo.