MCB 141. Lecture 5. AP patterning and Hox genes

1. The Bicoid gradient establishes differential patterns of giant and Hunchback expression in the anterior half of the embryo.

2. It is possible that these distinct expression patterns depend on the quality of Bicoid binding sites in the giant and Hunchback regulatory DNAs. Hunchback is activated by high and low levels of Bicoid due to the occurrence of several optimal sites (TAATCC), while giant expression is restricted to anterior regions containing peak levels of Bicoid due to low affinity, suboptimal binding sites. We will discuss this type of “threshold affinity” model in more detail next week when we consider dorsal-ventral patterning of the embryo.

3. Distinct Giant and Hunchback expression patterns in the anterior half of the embryo lead to sequential patterns of Kruppel and knirps expression in the thorax and abdomen. Giant represses Kruppel and Hunchback represses knirps.

4. The differential activation of Giant and Hunchback by the Bicoid gradient, followed by the sequential repression of Kruppel and knirps, leads to a series of gap gene expression patterns across the length of the embryo.

5. These four gap genes, Giant, Hunchback, Kruppel and Knirps, constitute the segmentation prepattern seen in early embryos, approximately 90 min following fertilization. These gap patterns produce pair-rule stripes of gene expression in a rapid period of just 30-45 min. Consequently, by 2.5 hrs after fertilization the Drosophila embryo contains a series of stripes that foreshadow the subdivision of the embryo into a repeating series of body segments.

6. The gap proteins mainly function as repressors, which “carve-out” individual stripes of gene expression. Consider the even-skipped or eve gene. It contains a series of 5 separate enhancers, located both upstream and downstream of the eve transcription unit, that together generate 7 pair-rule stripes of expression. Each stripe enhancer directs the expression of one or two of the stripes .

7. The stripe 2 enhancer contains 5 binding sites for Bicoid, which initially activates expression in the entire anterior half of the embryo. However, the enhancer also contains 3 binding sites for the Giant repressor and 3 binding sites for Kruppel. These repress the enhancer and define the anterior and posterior borders of the stripe.

8. A similar principle—broad activation and localized repression—applies to the other eve stripe enhancers. For example, the stripe 3 + 7 enhancer is activated by ubiquitous factors (dStat and Zelda), which are distributed throughout the embryo. The borders of the stripes are established by the Hunchback and Knirps repressors. Hunchback establishes the anterior border of stripe 3 and the posterior border of stripe 7, while Knirps defines the posterior border of stripe 3 and the anterior border of stripe 7.

9. Short-range repression is essential for the additive activities of the 5 different eve enhancers. For example, stripe 3 evades repression by Kruppel since the stripe 3 + 7 enhancer lacks Kruppel binding sites and the binding of Kruppel to the stripe 2 enhancer does not interfere with the activators bound to the 3 + 7 enhancer.

10. Homeotic mutants produce spectacular phenotypes: antennae to legs (Antennapedia) and halters to wings (Ubx). Antp and Ubx represent two of the 8 homeotic genes in the Drosophila genome. Antp is the founding member of the Antennapedia gene complex (ANT-C), which contains 5 homeotic genes. Ubx is the founding member of the bithorax complex, which contains 3 homeotic genes.

11. Antp homeotic transformations are due to the misexpression of the gene in the head. It is normally expressed in the developing mesothorax, which forms the middle legs. Ubx homeotic transformations are due to the loss of activity in the metathorax, where the gene normally suppresses the development of wings.

12. Localized stripes of Antp and Ubx expression are established by the Hunchback gap repressor, similar to its role in forming the anterior border of eve stripe 3. High levels of Hunchback are required to repress Antp, while lower levels are sufficient to repress Ubx.

13. Other homeotic genes are regulated by different combinations of gap repressors. For example, Hunchback + Kruppel repress the abdA homeotic gene, thereby restricting its expression to regions posterior of the Antp and Ubx patterns (the future anterior abdomen).

14. Once collinear patterns of homeotic gene expression are established, they are maintained by posterior prevalence, whereby posterior homeotic genes repress the expression of more anterior genes.