Trans- and cis-acting requirements for blastodermal expression of the head gap gene buttonhead

The Drosophila gene buttonhead (btd) encodes a zinc-finger protein related to the human transcription factor Sp1. btd is expressed in the syncytial blastoderm embryo in a stripe covering the anlagen of the antennal, intercalary and mandibular head segments. btd has been characterized as a head gap gene, since these segments are deleted in btd mutant embryos. We report here that the cis-acting elements required for btd head stripe expression are contained in a 1 kb DNA fragment, located about 3 kb upstream of the promoter. The four maternal coordinate systems are necessary for correct btd head stripe expression, likely by acting through the 1 kb cis-acting control region. Expression of the btd head stripe depends on the anterior morphogen encoded by the gene bicoid (bcd). bcd-dependent activation also involves the activity of the morphogens of the posterior and dorsoventral systems, hunchback and dorsal, respectively, which act together to control the spatial limits of the expression domain. Finally, the terminal system takes part in the regulation of btd head stripe expression by enhancing activation at low levels of activity and repression at high levels of activity.