ACTIVATION OF POSTERIOR GAP DENE EXPRESSION IN THE DROSOPHILA BLASTODERM

THE process of body prepatterning during Drosophila blastoderm formati on relies on the localized activities of zygotic segmentation genes, w hich are controlled by asymmetrically distributed maternal determinant s(1,2). The anterior determinant bicoid, a homeodomain transcription f actor(3,4), forms an anterior-to-posterior concentration gradient(1-4) . It interacts with the maternal transcription factor hunchback(5) to activate the anterior zygotic patterning genes, including the central gap gene Kruppel (Kr)(6). In contrast, the posterior maternal system(1 ,2) does not provide such a decisive transcription factor, but rather prevents the repressor hunchback from acting in the posterior half so that the gap genes giant (gt) and knirps (kni) are activated by an as yet unknown transcription factor(2,7). Here we show that caudal, a con served homeodomain protein that forms a posterior-to-anterior concentr ation gradient(8,9), and the anterior determinant bicoid cooperate to form a partly redundant activator system in the posterior region of th e embryo.