Multiple cis-acting target sequences are required for orb mRNA localization during Drosophila oogenesis

The targeting of positional information to specific regions of the oocyte or early embryo is one of the key processes in establishing anterior-posterior and dorsal-ventral polarity. In many developmental systems, this is accomplished by localization of mRNAs. The germ line-specific Drosophila orb gene plays a critical role in defining both axes of the developing oocyte, and its mRNA is localized in a complex pattern during oogenesis. We have identified a 280-bp sequence from the orb 3 ' untranslated region capable of reproducing this complex localization pattern. Furthermore, we have found that multiple cis-acting elements appear to be required for proper targeting of orb mRNA. The localization of mRNAs to specific subcellular regions or compartments provides an important mechanism for targeting proteins to the sites where their activity is required. In neurons, MAP2 mRNA, which encodes a dendrite-specific microtubule-associated protein, accumulates in the dendrites but not in the cell bodies (7, 9). Similarly, in fibroblasts, actin mRNA is localized to the periphery of the cell close to the actin filament network (19). The use of mRNA targeting to localize protein