Gap genes are activated by specific maternally provided proteins

The Bicoid protein activates zygotic expression of the hunchback gene. (a) Bicoid protein expression is graded along the anteroposterior axis. The hunchback gap gene is expressed in the anterior half of the zygote. (b) The Bicoid protein (blue) binds to three sites 5′ of the hunchback gene. When this 5′ DNA is placed upstream of a reporter gene, reportergene expression recapitulates the pattern of hunchback expression (top right). However, progressive deletion of one, two, or all three Bicoid-binding sites either leads to more restricted expression of the reporter gene or abolishes it altogether. These observations show that the level and pattern of hunchback expression are controlled by Bicoid through its binding to hunchback DNA regulatory sequences.