The site of oskar RNA and protein localization within the oocyte determines where in the embryo primordial germ cells form and where the abdomen develops. Initiation of oskar RNA localization requires the activity of several genes. We show that ovaries mutant for any of these genes lack Oskar protein. Using various transgenic constructs we have determined that sequences required for oskar RNA localization and translational repression map to the oskar 3′UTR, while sequences involved in the correct temporal activation of translation reside outside the oskar 3′UTR. Upon localization of oskar RNA and protein at the posterior pole, Oskar protein is required to maintain localization of oskar RNA throughout oogenesis. Stable anchoring of a transgenic reporter RNA at the posterior pole is disrupted by oskar nonsense mutations. We propose that initially localization of oskar RNA permits translation into Oskar protein and that subsequently Oskar protein regulates its own RNA localization through a positive feedback mechanism.

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