Gene expression mediated by cis-acting sequences of the Krüppel gene in response to the Drosophila morphogens bicoid and hunchback.

The initial expression of the gap gene Krüppel (Kr) occurs in a precisely bounded central region of the Drosophila blastoderm embryo. According to genetic analysis, the spatial limits of the Kr expression domain are controlled by the morphogenetic activities of the anterior organizer gene bicoid (bcd) and the anterior gap gene hunchback (hb). Using gene fusion analysis, we assayed for cis-acting sequences of the Kr gene which mediate transcriptional activation and localized gene expression in response to trans-acting factors. A 730 bp Kr control element drives gene expression in place of the endogenous Kr central domain. This cis-acting element, Kr730, is composed of bcd and hb responsive sequences. They map into regions of multiple hb and bcd protein in vitro binding sites. A 142 bp core fragment containing one low affinity hb and five medium to strong bcd protein binding sites drives gene expression in a Kr-like location in the centre of the embryo. Our results show that this fragment represents a target for the redundant activator/repressor system provided by the anterior morphogens bcd and hb.