Wispy and Orb cooperate in the cytoplasmic polyadenylation of localized gurken mRNA

In Drosophila, the dorsal-ventral (D-V) axis of the oocyte is dependent on Gurken (Grk) protein distribution. This is achieved through the cytoplasmic localization of grk mRNA and regulation of its translation. During mid-late stages of oogenesis, grk mRNA and protein are localized to the dorsal-anterior of the oocyte, while unlocalized grk transcripts are translationally silenced. As females carrying mutations in the gene encoding the CPEB protein Orb lay ventralized eggs due to insufficient Grk levels, it seemed likely that cytoplasmic polyadenylation of grk transcripts may play a role in their translational regulation.

We have found that grk is polyadenylated throughout oogenesis, with poly(A) tails of approximately 30-50 A residues. Hyperadenylated grk transcripts, with poly(A) tails of 50-90 As, are detected in late stage egg chambers, but they fail to accumulate in oocytes deficient in Orb or the poly(A) polymerase Wispy (Wisp). wisp females also lay weakly ventralized eggs, demonstrating that they produce inadequate amounts of Grk. Finally, unlocalized grk transcripts are also not appropriately hyperadenylated.

Localized cytoplasmic polyadenylation of grk mRNA by Wisp and Orb is necessary to achieve appropriate Grk protein accumulation in the D/A corner of the oocyte during mid to late oogenesis.