LOCALIZATION OF XCAT-2 RNA, A PUTATIVE GERM PLASM COMPONENT, TO THE MITOCHONDRIAL CLOUD IN XENOPUS STAGE-I OOCYTES

The mitochondrial cloud is a unique cell structure found in stage I Xe nopus oocytes that plays a role in mitochondriogenesis and in the dist ribution of germ plasm to the vegetal pole, Xcat-2 RNA specifically lo calizes to the mitochondrial cloud and moves with it to the vegetal su bcortex in stage II oocytes, Later, in the 4-cell embryo, it is found in a pattern identical to the germ plasm, Following microinjection int o stage I oocytes, synthetic Xcat-2 RNAs localize to the mitochondrial cloud within 22 hours, Transcripts are stable over this time period w ith very little evidence of degradation. The Xcat-2 3'untranslated reg ion was found to be both required and sufficient for mitochondrial clo ud localization. Further deletion analysis narrowed this localization signal to a 250 nucleotide region at the proximal end of the 3'untrans lated region, This region is different from, but overlaps with, a doma in previously shown to be sufficient to direct Xcat-2 to the vegetal c ortex in stage IV oocytes, Examination of early stage I oocytes reveal s a time when Xcat-2 is uniformly distributed, arguing against vectori al nuclear export into the mitochondrial cloud. Analysis of localizati on at different time points does not suggest active transport to the m itochondrial cloud, We postulate that localization occurs by selective entrapment of Xcat-2 within the cloud by localized binding sites.