Gene expression stripes in Drosophila melanogaster embryos provide a model for how eukaryotic promoters are turned on and off in response to combinations of transcriptional regulators. Genetic studies suggested that even-skipped (eve) stripe 2 is controlled by three gap genes, hunchback (hb), Kruppel (Kr), and giant (gt), and by the maternal morphogen bicoid (bcd). A direct link is established between binding sites for these regulatory proteins in the stripe 2 promoter element and the expression of the stripe during early embryogenesis. The bcd and hb protein binding sites mediate activation, whereas neighboring gt and Kr protein sites repress expression and establish the stripe borders. The stripe 2 element has the properties of a genetic on-off switch.
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