Here we describe studies of double-stranded RNA (dsRNA) adenosine deaminase in Xenopus laevis, in particular during meiotic maturation, the period during which a stage VI oocyte matures to an egg. We show that dsRNA adenosine deaminase is in the nuclei of stage VI oocytes. Most importantly, we demonstrate that the cytoplasm of stage VI oocytes contains a factor that protects microinjected dsRNA from deamination when dsRNA adenosine deaminase is released from the nucleus during meiotic maturation. Our data suggest that the protection factor is a cytoplasmic dsRNA-binding protein or proteins that bind to dsRNA in a sequence-independent manner to occlude dsRNA from binding to dsRNA adenosine deaminase. The cytoplasmic double-stranded RNA-binding protein(s) does not bind to other nucleic acids and can be titrated at high concentrations of dsRNA. These studies raise the question of whether all dsRNA-binding proteins share endogenous substrates and also suggest potential means of regulating dsRNA adenosine deaminase in vivo.The double-stranded RNA (dsRNA) adenosine deaminase (dsRAD), initially called the unwinding/modifying activity, was detected first in Xenopus laevis (4, 25) and subsequently in organisms throughout the animal kingdom (34; for reviews, see references 1, 2, and 17). This enzyme deaminates adenosines within dsRNA to produce inosines (24). Recently dsRAD has been purified from Xenopus eggs; this protein is a single subunit that migrates at 120 kDa by sodium dodecyl sulfatepolyacrylamide gel electrophoresis (14). In vitro studies demonstrate that dsRAD can deaminate adenosines within intermolecular and intramolecular RNA duplexes (21, 29) but cannot act on adenosines within DNA or single-stranded RNA (ssRNA) (4, 33). Strong evidence indicating that dsRAD is responsible for RNA editing of mRNAs encoding the B subunit of a specific type of L-glutamate-activated ion channel has been presented (13). In addition to RNA editing, other biological roles for dsRAD have been proposed (for a review, see reference 2), and it is possible that dsRAD plays multiple roles in vivo.Initial experiments in X laevis showed that dsRAD could easily be detected by assaying the fate of dsRNA microinjected into the cytoplasm of eggs and early embryos (.8 cells; 4, 25).However, the activity could not be observed in similar microinjections into the cytoplasm of oocytes or embryos at later stages of development, specifically, subsequent to the midblastula transition (post-MBT). Further experiments revealed that dsRAD was present in post-MBT embryos but was localized to the nucleus and thus not detected in early experiments involving cytoplasmic microinjections (5). dsRAD has also been shown to be nuclear in the somatic cells of mammals (34). Taken together, these results raised the possibility that dsRAD is in the nuclei of stage VI oocytes and exists in the cytoplasm of eggs and early embryos because it is released from the nucleus during meiotic maturation, the process by which a stage VI oocyte becomes an egg (for a review, see ...