ELABORATION OF THE MESSENGER TRANSPORT ORGANIZER PATHWAY FOR LOCALIZATION OF RNA TO THE VEGETAL CORTEX OF XENOPUS OOCYTES

Previous studies demonstrated that there were two pathways, the messen ger transport organizer (METRO) or early and the Vg1 or late, which fu nction during stages I to 3 of oogenesis for the localization of RNAs at the vegetal cortex of Xenopus oocytes. In the present study we anal yzed the properties of the METRO pathway, which localizes Xlsirt, Xcat 2, and Xwnt11 RNAs to a specific region of the vegetal cortex during s tage 1 of oogenesis. A combination of methodologies involving both fix ed material and living oocytes was used to analyze RNA localization. W e show that in early diplotene pre-stage 1 oocytes (25-50 mu m in diam eter) both endogenous and injected exogenous METRO RNAs translocated t o multiple mitochondrial aggregates (pre-mitochondrial clouds) that su rround the germinal vesicle (GV). However, by early stage I (diplotene oocytes, 50-200 mu m), all three of the RNAs discriminated between th e different clouds and translocated exclusively within the METRO of a single mitochondrial cloud. Therefore, in stage 1 diplotene oocytes th ere is a unique mechanism causing a change in the intrinsic property o f the mitochondrial clouds which designates one of them as the RNA tra nsport vehicle. During translocation through the cytoplasm Xlsirt and Xcat2 RNAs were detected associated with cytoplasmic particles of diff erent morphologies. Additionally, we also found that the translocation of RNAs through the early or METRO pathway, unlike that of the late p athway, occurred in the absence of intact microtubule and actin microf ilament cytoskeletal elements. This supports a cytoskeletal-independen t model for localization of RNAs through the METRO pathway. (C) 1996 A cademic Press, Inc.