Competence to replicate in the unfertilized egg is conferred by Cdc6 during meiotic maturation

Abstract

Meiotic maturation, the final step of oogenesis, is a crucial stage of development in which an immature oocyte becomes a fertilizable egg1. In Xenopus, the ability to replicate DNA is acquired during maturation at breakdown of the nuclear envelope2 by translation of a DNA synthesis inducer that is not present in the oocyte2,3. Here we identify Cdc6, which is essential for recruiting the minichromosome maintenance (MCM) helicase to the pre-replication complex, as this inducer of DNA synthesis. We show that maternal cdc6 mRNA but not protein is stored in the oocyte. Cdc6 protein is synthesized during maturation, but this process can be blocked by degrading the maternal cdc6 mRNA by oligonucleotide antisense injections or by translation inhibition. Rescue experiments using recombinant Cdc6 protein show that Cdc6 is the only missing replication factor whose translation is necessary and sufficient to confer DNA replication competence to the egg before fertilization. The licence to replicate is given by Cdc6 at the end of meiosis I, but the cytostatic factor (CSF) pathway, which maintains large amounts of active Cdc2/Cyclin B2, prevents the entry into S phase until fertilization.

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Figure 1: Cdc6 is missing in Xenopus oocytes but accumulates during oocyte maturation soon after GVBD.
Figure 2: cdc6 mRNA stored in the oocyte is translated after GVBD.
Figure 3: Cdc6 is the DNA replication induced activity acquired in vivo during maturation.
Figure 4: Cdc6 protein is the missing factor acquired soon after GVBD that confers to the egg the competence to replicate.
Figure 5: Interplay between replication induced by cdc6 and its negative control by MPF.

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Acknowledgements

We thank P. Françon for help; N. Montel for technical assistance; and B. Honda, D. Maiorano, D. Fisher and C. Jaulin for critically reading the manuscript. This work has been supported by grants from the Association pour la Recherche sur le Cancer and the Human Frontier Science Program.

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Correspondence to Marcel Méchali.

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Lemaître, J., Bocquet, S. & Méchali, M. Competence to replicate in the unfertilized egg is conferred by Cdc6 during meiotic maturation. Nature 419, 718–722 (2002). https://doi.org/10.1038/nature01046

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