Functional and molecular reorganization of the nucleolar apparatus in maturing mouse oocytes

In mammalian preovulatory oocytes, rRNA synthesis is down-regulated until e gg fertilization and zygotic genome reactivation, but the underlying regula tory mechanisms of this phenomenon are poorly characterized. We examined th e molecular organization of the rRNA synthesis and processing machineries i n fully grown mouse oocytes in relation to ongoing rDNA transcription and o ocyte progression throughout meiosis. We show that, at the germinal vesicle stage, the two RNA polymerase I(RNA pol I) subunits, RPA116 and PAF53/RPA5 3, and the nucleolar upstream binding factor (UBF) remain present irrespect ive of ongoing rDNA transcription and colocalize in stoichiometric amounts within discrete foci at the periphery of the nucleolus-like bodies. These f oci are spatially associated with the early pre-rRNA processing protein fib rillarin and in part with the pre-ribosome assembly factor B23/nucleophosmi n. After germinal vesicle breakdown, the RNA pol I complex disassembles in a step-wise manner from chromosomes, while UBF remains associated with chro mosomes until late prometaphase I. Dislodging of UBF, but not of RNA pol I, is impaired by the phosphatase inhibitor okadaic acid, thus strengthening the idea of a relationship between UBF dynamics and protein phosphorylation . Since neither RNA pol I, UBF, fibrillarin, nor B23 is detected at metapha se II, i.e., the normal stage of fertilization, we conclude that these nucl eolar proteins are not transported to fertilized eggs by maternal chromosom es. Together, these data demonstrate an essential difference in the dynamic s of the major nucleolar proteins during mitosis and meiosis. (C) 2000 Acad emic Press.