P

back mRNA synthesized by nurse cells also is present in the early embryo. Even though this maternal hunchback mRNA is uniformly distributed throughout the embryo, its translation is prevented in the posterior region by another maternally derived protein called Nanos, which is localized to the posterior end of the embryo. Nanos protein not only blocks translation of maternal hunchback mRNA in the posterior region. The set of genes required for Nanos protein localization is also required for germ-line cells to form at the posterior end of the embryo. The evolutionary conservation of this Nanos function in flies and in worms may indicate an ancient system for forming germ-line cells. Related proteins exist in vertebrates, but their functions are not yet known.

Figure 15-21 illustrates how translational regulation by Nanos helps to establish the anterior ^ posterior Hunchback gradient needed for normal development. Translational repression of hunchback mRNA by Nanos depends on specific sequences in the 3 '-untranslated region of the mRNA, called Nanos-response elements (NREs). Along with two other RNA-binding proteins, Nanos binds to the NRE in hunchback mRNA. Although the precise mechanism by which repression is achieved is not known, repression inversely correlates with the length of the poly(A) tail in hunchback mRNA, which is determined by the balance between the opposite processes of polyadenylation and dead-enylation. In wild-type embryos, the length of the poly(A) tail increases immediately before translation of hunchback mRNA. The results of genetic and molecular studies suggest that Nanos promotes deadenylation of hunchback mRNA and thereby decreases its translation. In the absence of Nanos, an accumulation of maternal Hb protein in the posterior region leads to the failure of the posterior structures to form normally, and the embryo dies. Conversely, if Nanos is produced in the anterior, thereby inhibiting the production of Hb from both maternal and embryonic hunchback mRNA, anterior body parts fail to form, again a lethal consequence.

Nanos protein localization in the posterior embryo is intimately coupled to the regulation of translation of nanos mRNA. The nanos mRNA that is not located at the posterior is not translated due to a protein called Smaug that binds the 3' UTR of nanos mRNA. Localization of nanos mRNA at the posterior depends on other proteins as well. One of these is Oskar, whose maternally provided mRNA is transported to the posterior by kinesin, a motor protein that moves along microtubules (Chapter 20). Therefore the kinesin controls, after several intervening steps, the localized activity of a transcription factor (Hunchback).

Translational control due to the action of an inhibitor, mRNA localization, or both, may be widely used strategies for regulating development. For instance, specific mRNAs are localized during the development of muscle cells (see Figure 12-31) and during cell division in the budding yeast Saccharomyces cerevisiae (see Figure 22-22). Similar mechanisms operate during the development of C. elegans. Even more intriguing is the discovery that Bicoid protein binds

Fertilized egg

Early embryo

Fertilized egg

Maternal hunchback mRNA

nanos mRNA

Maternal hunchback mRNA

nanos mRNA

Hunchback protein (maternal and embryonic)

Nanos protein

Hunchback protein (maternal and embryonic)

Nanos protein

Anterior

Posterior

Nanos protein i

AAAAAAA 3'

5' hb mRNA

Translation of hb mRNA

Nanos promotes deadenylation

No hb translation

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