Embryonic enhancers help transmit positional information to the initiator cores that control Drosophila Abd-B regulatory domains.

Drosophila homeotic gene Abdominal-B (Abd-B) is controlled throughout development by four infraabdominal (iab) regulatory domains, the active or repressed state of which is determined by initiators that have parasegment-specific enhancer activity at an early stage of embryonic development. For this reason, it has long been assumed that the enhancer activity and initiation function of these elements are synonymous. Here, we studied two initiators that regulate the activity of the iab-5 and iab-6 domains responsible for Abd-B expression in embryonic parasegment PS10 (adult segment A5) and PS11(A6), respectively. In both initiators, core regions were identified that do not stimulate reporter gene transcription, but retain the ability to establish the appropriate activity state of the corresponding iab domains. Other initiator sequences are responsible for parasegment-specific reporter activation in early embryos and enhance the activity of the core initiators. Taken together, our results indicate that initiators represent a new type of regulatory element that function as on/off switches for regulatory domains controlling segment-specific Abd-B expression during Drosophila development.