RNA LOCALIZATION IN DEVELOPMENT

Cytoplasmic RNA localization is an evolutionarily ancient mechanism fo r producing cellular asymmetries. This review considers RNA localizati on in the context of animal development. Both mRNAs and non-protein-co ding RNAs are localized in Drosophila, Xenopus, ascidian, zebrafish, a nd echinoderm oocytes and embryos, as well as in a variety of developi ng and differentiated polarized cells from yeast to mammals. Mechanism s used to transport and anchor RNAs in the cytoplasm include vectorial transport out of the nucleus, directed cytoplasmic transport in assoc iation with the cytoskeleton, and local entrapment at particular cytop lasmic sites. The majority of localized RNAs are targeted to particula r cytoplasmic regions by cis-acting RNA elements; in mRNAs these are a lmost always in the 3'-untranslated region (UTR). A variety of trans-a cting factors-many of them RNA-binding proteins-function in localizati on. Developmental functions of RNA localization have been defined in X enopus, Drosophila, and Saccharomyces cerevisiae. In Drosophila, local ized RNAs program the anteroposterior and dorso-ventral axes of the oo cyte and embryo. In Xenopus, localized RNAs may function in mesoderm i nduction as well as in dorso-ventral axis specification. Localized RNA s also program asymmetric cell fates during Drosophila neurogenesis an d yeast budding.