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oskar RNA in pole cells c early oocyte nucleus.

anterior posterior

oskar mRNA

mature egg mature egg posterior plasm

FIGURE 18-16 Localization of maternal mRNAs in the Drosophila egg and embryo, (a) The unfertilized Drosophila egg contains two localised mRNAs, bicoid in anterior regions and oskar in posterior regions, (b) Use Oskar protein helps coordinate the assembly of the polar granules rn the postenoi cytoplasm. Nudet that enter this region bud-off the poste rior end of the embryo and form the pote cefls (c) During the formation of (he Drosophila egg, polarized microtubules are formed thai extend from the oocyte nudeus and grow toward the posterior plasm. The o$t-.ar mRNA binds adapter proteins that interact with the microtubules, and thereby transport the RNA to the posterior plasm The *—" and "4 " symbols indicate the direction of the grnwtng strands of the microtubules.

seen how the 3' UTR of the ushl mRNA mediates its localization to the daughter cell of budding yeast by interacting with the growing ends of microtubules. A remarkably similar process controls the localization of the osiwmRNA in the Drosophila oocyte.

The Drosophila oocyte is highly polarized. The nucleus is located in anterior regions; growing microtubules extend from the nucleus into the posterior cytoplasm. The oskur mKNA interacts with adapter proteins that are associated with the growing + ends of the microtubules and are thereby transported away from anterior regions of the egg. where the nucleus resides, into the posterior plasm. After fertilization, the cells that inherit the localized oskar mRNA (and polar granules) form the pole cells.

The localization of the bicoid mRNA in anterior regions of the unfertilized egg also depends on sequences contained within its 3' UTR. The nucleotide sequences of the oskar and bicoid mRNAs are distinct. As a result, they interact with different adapter proteins and become localized to different regions of the egg. The importance of the 3' UTRs in determining where each mRNA becomes localized is revealed by the following experiment. Jf the 3' UTR from the oskur mRNA is replaced with that from bicoid, the hybrid oskar mRNA is located to anterior regions (just as bicoid normally is). This mislocalization is sufficient to induce the formation of pole cells at abnormal locations in the early embryo (see Figure 18-17). In addition, the mislocalized polar granules suppress the expression of genes required for the differentiation of head tissues. As a result, embryonic cells that normally form head tissues are transformed into germ cells.

bicoid mRNA i_

osksr m RNA

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