Functioning of the Drosophila orb gene in gurken mRNA localization and translation

The orb gene encodes an RNA recognition motif (RRM)-type RNA-binding protei n that is a member of the cytoplasmic polyadenylation element binding prote in (CPEB) family of translational regulators. Early in oogenesis, orb is re quired for the formation and initial differentiation of the egg chamber, wh ile later in oogenesis it functions in the determination of the dorsoventra l (DV) and anteroposterior axes of egg and embryo. In the studies reported here, we have examined the role of the orb gene in the gurken (grk)-Drosoph ila epidermal growth factor receptor (DER) signaling pathway. During the pr evitellogenic stages of oogenesis, the grk-DER signaling pathway defines th e posterior pole of the oocyte by specifying posterior follicle cell identi ty. This is accomplished through the localized expression of Grk at the ver y posterior of the oocyte. Later in oogenesis, the grk-DER pathway is used to establish the DV axis. Grk protein synthesized at the dorsal anterior co rner of the oocyte signals dorsal fate to the overlying follicle cell epith elium. We show that orb functions in both the early and late grk-DER signal ing pathways, and in each case is required for the localized expression of Grk protein. We have found that orb is also required to promote the synthes is of a key component of the DV polarity pathway, K(10). Finally, we presen t evidence that Orb protein expression during the mid- to late stages of oo genesis is, in turn, negatively regulated by K(10).