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Cdc6 synthesis regulates replication competence in Xenopus oocytes

Abstract

The early division cycles of an embryo rely on the oocyte's ability to replicate DNA. During meiosis, oocytes temporarily lose this ability. After a single round of pre-meiotic S-phase, oocytes enter meiosis and rapidly arrest at prophase of meiosis I (G2)1. Upon hormonal stimulation, arrested oocytes resume meiosis, re-establish DNA replication competence in meiosis I shortly after germinal vesicle breakdown (GVBD), but repress replication until fertilization2,3. How oocytes lose and regain replication competence during meiosis are important questions underlying the production of functional gametes. Here we show that the inability of immature Xenopus oocytes to replicate is linked to the absence of the Cdc6 protein and the cytoplasmic localization of other initiation proteins. Injection of Cdc6 protein into immature oocytes does not induce DNA replication. However, injection of Cdc6 into oocytes undergoing GVBD is sufficient to induce DNA replication in the absence of protein synthesis. Our results show that GVBD and Cdc6 synthesis are the only events that limit the establishment of the oocyte's replication competence during meiosis.

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Figure 1: Xenopus immature oocytes have a defect in pre-replication complex (RC) assembly.
Figure 2: Cdc6 synthesis occurs during meiosis I after germinal vesicle breakdown (GVBD).
Figure 3: Injection of Cdc6 into maturing oocytes treated with CHX induces DNA replication.

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Acknowledgements

We thank M. DePamphilis, M. Doree, W. Dunphy, T. Hunt, U. Strausfeld, H. Takisawa, I. Todorov and M. Yoshida for providing reagents and C. MacDonald, C. Pfarr, B. Schneider and S. Ravnik for critical reading of the manuscript. This work was supported by grants from the American Heart Association, South Plains Foundation and the NIH (to M.C.). E.W. was supported by a NIH training grant.

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Correspondence to Martine Coué.

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Whitmire, E., Khan, B. & Coué, M. Cdc6 synthesis regulates replication competence in Xenopus oocytes. Nature 419, 722–725 (2002). https://doi.org/10.1038/nature01032

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