Differential regulation of the two transcripts from the Drosophila gap segmentation gene hunchback.

The Drosophila gap gene hunchback (hb) is required for the establishment of the anterior segment pattern of the embryo, and also for a small region of the posterior segment pattern. The hb gene encodes two transcripts from two promoters which show a differential regulation, although they code for the same protein product. The 3.2-kb transcript is expressed during oogenesis and forms an anterior-posterior gradient during the early stages of development. The first zygotic expression of hb during cleavage stages 11-12 is due to the 2.9-kb transcript. Its expression is under the control of the anterior pattern organizer gene bicoid (bcd) and it appears to be necessary and sufficient for the anterior segmentation. The 3.2-kb transcript is expressed again at syncytial blastoderm stage in the anterior yolk nuclei, as well as in an anterior stripe which is posteriorly adjacent to the domain of the 2.9-kb transcript, and as a posterior stripe. Using hb-promoter/lacZ fusion gene constructs in combination with germ line transformation, we have delimited a regulatory region for the 2.9-kb transcript to approximately 300 bp upstream of the site of transcription initiation and show that this region is sufficient to confer the full regulation by bcd.