Further Development 9.6: Bicoid mRNA Localization in the Anterior Pole of the Oocyte

The Genetics of Axis Specification in Drosophila

The 3' untranslated region (3'UTR) of bicoid mRNA contains sequences that are critical for its localization at the anterior pole (Figure 1; Ferrandon et al. 1997; Macdonald and Kerr 1998; Spirov et al. 2009). These sequences interact with the Exuperantia and Swallow proteins while the messages are still in the nurse cells of the egg chamber (Schnorrer et al. 2000). Experiments in which fluorescently labeled bicoid mRNA was microinjected into living egg chambers of wild-type or mutant flies indicate that Exuperantia must be present in the nurse cells for anterior localization. But Exuperantia alone is not sufficient to bring the bicoid message into the oocyte (Cha et al. 2001; Reichmann and Ephrussi 2005). The bicoid-Exuperantia complex is transported out of the nurse cells and into the oocyte via microtubules, seeming to ride on a kinesin motor (Arn et al. 2003). Once inside the oocyte, bicoid mRNA attaches to dynein proteins that are maintained at the microtubule organizing center (the slower growing “minus end”) at the anterior of the oocyte (see Figure 1 in Further Development 9.3, online; Cha et al. 2001). About 90 percent of the bicoid mRNA is localized to the anterior 20 percent of the embryo (Little et al. 2011).

**Figure 1** The *bicoid* mRNA and protein gradients shown by in situ hybridization and confocal microscopy. (A) *bicoid* mRNA shows a steep gradient (here seen as red to blue) across the anterior portion of the oocyte. (B) When the mRNA is translated, the Bicoid protein gradient can be seen in the anterior nuclei. Anterior is to the left; the dorsal surface is upward.

Nanos mRNA localization in the posterior pole of the oocyte

The posterior organizing center is defined by the activities of the nanos gene (Lehmann and Nüsslein-Volhard 1991; Wang and Lehmann 1991; Wharton and Struhl 1991). While the bicoid message is actively transported and bound to the anterior end of the microtubules, the nanos message appears to get “trapped” in the posterior end of the oocyte by passive diffusion. The nanos message becomes bound to the cytoskeleton in the posterior region of the egg through its 3' UTR and its association with the products of several other genes (oskar, valois, vasa, staufen, and tudor).ⁱ If nanos (or any other of these maternal effect genes) is absent in the mother, no abdomen forms in the embryo (Lehmann and Nüsslein-Volhard 1986; Schüpbach and Wieschaus 1986). But before the nanos message can be localized in the posterior cortex, a nanos mRNA-specific “trap” has to be made; this trap is the Oskar protein (Ephrussi et al. 1991). The oskar message and the Staufen protein are transported to the posterior end of the oocyte by the kinesin motor protein (see Figure 1 in Further Development 9.3, online). There they become bound to the actin filaments of the cortex. Staufen allows the translation of the oskar message, and the resulting Oskar protein is capable of binding the nanos message (Brendza et al. 2000; Hatchet and Ephrussi 2004).

Most nanos mRNA, however, is not trapped. Rather, it is bound in the cytoplasm by the translation inhibitors Smaug and CUP. Smaug (yes, it’s named after the dragon in The Hobbit) binds to the 3'UTR of nanos mRNA and recruits the CUP protein that prevents the association of the message with the ribosome as well as recruiting other proteins that deadenylate the message and target it for degradation (Rouget et al. 2010). If the nanos-Smaug-CUP complex reaches the posterior pole, however, Oskar can dissociate CUP from Smaug, allowing the mRNA to be bound at the posterior and ready for translation (Forrest et al. 2004; Nelson et al. 2004).

ⁱ Like the placement of the bicoid message, localization of the nanos message is determined by its 3'UTR. If the bicoid 3'UTR is experimentally transferred to the protein-encoding region of nanos mRNA, the nanos message gets localized in the anterior of the egg. When this chimeric mRNA is translated, Nanos protein inhibits translation of hunchback and bicoid mRNAs and the embryo forms two abdomens-one in the anterior of the embryo and one in the posterior (Gavis and Lehmann 1992).

Literature Cited

Arn, E. A., B. J. Cha, W. E. Theurkauf and P. M. Macdonald. 2003. Recognition of a bicoid mRNA localization signal by a protein complex containing Swallow, Nod, and RNA binding proteins. Dev. Cell 4: 41–51.