Secondary pair-rule ^
Segment polarity ^
The segmentation genetic regulatory hierarchy
(left) The expression patterns of five classes of anteroposterior axis patterning genes are depicted in embryos at different stages. (right) Selected members of these classes are shown and the regulatory interactions between these genes are indicated. An arrow indicates a positive regulatory interaction; a line crossed at its end indicates a negative repressive regulatory relationship.
regulatory interactions between gene products translated from maternal mRNAs deposited in the egg, transcriptional activation of zygotic genes by certain maternal activators, and combinatorial action of segmentation gene products to refine the expression patterns of many zygotic segmentation genes into iterated domains. This hierarchy has four key features:
1. Generation of gradients of maternal transcription factor proteins
2. Transcriptional activation of and cross-regulation by the gap genes
3. Transcriptional regulation of individual pair-rule gene stripes by combinations of maternal and gap proteins
4. Regulation of segment polarity gene expression by pair-rule and segment polarity proteins Generation of maternal transcription factor gradients
The Bicoid (Bcd) and Caudal (Cad) proteins are homeodomain-type transcription factors, and the Hunchback (Hb) protein is a zinc-finger-type transcription factor. The mRNAs encoding these proteins are deposited in the egg and translated during early embryonic development. Because the early embryo is a syncytium, lacking any cell membranes that would impede the diffusion of protein molecules, these transcription factors can move freely throughout the cytoplasm. The concentration of each protein is graded along the A/P axis of the embryo (Fig. 3.6). These maternal gradients are important because several downstream segmentation genes are regulated by Bcd, Cad, and Hb (depending on their concentrations). Early gene regulation in the fly embryo provides a general model for the concentration-dependent control of gene expression by gradients of regulatory proteins.
In Drosophila, these three maternal gradients are formed by different mechanisms. The Bicoid gradient results from the localization of maternal mRNA at the anterior pole of the egg. Translation of this mRNA and diffusion of the Bicoid protein toward the posterior creates a concentration gradient. The Hunchback gradient is generated by selective inhibition of the translation of the ubiquitous hb mRNA in posterior regions (Fig. 3.6b). This inhibition is regulated by the product of the nanos gene, which binds to the hb mRNA. The Nanos (Nos) protein is in a posterior-to-anterior concentration gradient, generated by the localization of its mRNA to the posterior end of the egg. The caudal mRNA, like hunchback, is distributed evenly throughout the egg, although selective inhibition of its translation by the Bicoid protein generates a posterior-to-anterior Caudal protein gradient (Fig. 3.6b).
Transcriptional activation of and cross-regulation by gap genes
The gap genes are the first zygotic genes to be expressed in discrete regions along the A/P axis. The Bcd, Hb, and Cad proteins are involved in the initial regulation of these genes. One key regulatory interaction is the activation of the second phase of hunchback expression by the Bicoid protein in the anterior half of the embryo. This activation occurs through a direct interaction of the Bicoid protein with a cis-regulatory element of the hb gene. DNA sequences 5' to the hb promoter contain several binding sites for the Bicoid protein that are necessary for the Bcd-dependent activation of the hb gap domain (Fig. 3.6c). Bcd binds to these sites cooperatively—that is, the binding of one Bcd protein molecule to one site facilitates the binding of other Bcd molecules to nearby sites. More than one occupied Bcd site is necessary a b mRNA
hunchback ' caudal bicoid nanos
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