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 egg. In Xenopus, the ability to replicate DNA is acquired during maturation at breakdown of the nuclear envelope by translation of a DNA synthesis inducer that is not present in the oocyte. 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.