An autoregulatory feedback loop directs the localized expression of the Drosophila CPEB protein Orb in the developing oocyte

The RRM-type RNA binding protein Orb plays a central role in the establishm ent of polarity in the Drosophila egg and embryo. In addition to its role i n the formation and initial differentiation of the egg chamber, orb is requ ired later in oogenesis for the determination of the dorsoventral (DV) and anteroposterior (AP) axes. In DV axis formation, Orb protein is required to localize and translate gurken mRNA at the dorsoanterior part of the oocyte . In AP axis formation, Orb is required for the translation of oskar mRNA. In each case, Orb protein is already localized at the appropriate sites wit hin the oocyte before the arrival of the mRNAs encoding axis determinants. We present evidence that an autoregulatory mechanism is responsible for dir ecting the on site accumulation of Orb protein in the Drosophila oocyte, Th is orb autoregulatory activity ensures the accumulation of high levels of O rb protein at sites in the oocyte that contain localized orb message.