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Fig. 1. Pathways of RNA localization in Xenopus oogenesis. (A) The early (METRO) localization pathway operates in early oogenesis (stages I and II) and uses the mitochondrial cloud (Balbiani body) to localize RNAs such as Xcat2 mRNA (red) and noncoding Xlsirts RNA (blue) to the vegetal pole of the oocyte. (1) In stage I oocytes, RNAs synthesized in the nucleus (yellow) enter (either via nuage or by the diffusion/entrapment mechanism) the mitochondrial cloud (mitochondria shown in green), which faces the vegetal pole of the oocyte. Xcat2 mRNA becomes localized to the germinal granules (red spheres) and Xlsirts RNA is localized between the germinal granules at the vegetal apex (METRO region) of the mitochondrial cloud. (2) In stage II oocytes, the mitochondrial cloud moves to the vegetal cortex and starts to disperse. (3) In stage III-VI oocytes, the mitochondrial cloud disperses, and germinal granules and localized RNAs form a disc at the apex of the vegetal cortex. (B) The late (Vg1) pathway localizes mRNAs such as Vg1 or VegT, using MTs, molecular motors and possibly the ER. (4) In stage I oocytes, these RNAs (purple) are uniformly distributed within the oocyte cytoplasm and are excluded from the mitochondrial cloud. (5) In late stage II oocytes, RNAs concentrate, in a wedge, around the moving mitochondrial cloud and a subdomain of ER, and translocate on MTs towards the vegetal pole. (6) Later in oogenesis (stages III-VI), late-pathway RNAs localize and anchor at the cortex of the vegetal half of the oocyte (for details, see Kloc and Etkin, 1995).





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