Autonomous concentration-dependent activation and repression of Krüppel by hunchback in the Drosophila embryo.

The subdivision of the anterior-posterior axis in Drosophila is achieved by a cascade of spatially regulated transcription factors which form short-range gradients at the syncytial blastoderm stage. These factors are assumed to have concentration-dependent regulatory effects on their target genes. However, there is so far little direct in vivo evidence that a single factor can autonomously activate and repress a given target gene. We have analysed here the regulatory capabilities of the gap gene hunchback by creating an artificial gradient of hunchback in the early embryo. This was achieved by providing the maternally expressed mRNA of hunchback with the anterior localization signal of the bicoid RNA. The effects of this artificial hunchback gradient were then studied in different types of mutant background. We show that under these conditions hb is autonomously capable of activating the target gene Krüppel at low concentrations and repressing it at high concentrations. In addition, we show that the artificially created hunchback gradient can organize a large part of the segment pattern, although it is expressed at a different position and in a different shape than the wild-type gradient of hunchback.