SUPPRESSION OF DNA-REPLICATION VIA MOS FUNCTION DURING MEIOTIC DIVISIONS IN XENOPUS-OOCYTES

Meiosis is characterized by the absence of DNA replication between the two successive divisions. In Xenopus eggs, the ability to replicate D NA develops during meiotic maturation, but is normally suppressed unti l fertilization. Here we show that development of the DNA-replicating ability depends on new protein synthesis during meiosis I, and that me re ablation of the endogenous c-mos product Mos allows maturing oocyte s to enter interphase and replicate DNA just after meiosis I. Moreover , we demonstrate that during normal maturation cdc2 kinase undergoes p recocious inactivation in meiosis I and then premature reactivation be fore meiosis II; importantly, this premature cdc2 reactivation absolut ely requires Mos function and its direct inhibition by a dominant-nega tive cdc2 mutant also results in nuclear reformation and DNA replicati on immediately after meiosis I. These findings indicate that suppressi on of DNA replication during meiotic divisions in Xenopus oocytes is a ccomplished by the Mos-mediated premature reactivation of cdc2 kinase. We suggest that these mechanisms for suppressing DNA replication may be specific for meiosis in animal oocytes, and that the ultimate biolo gical function, including the well known cytostatic factor activity, o f Mos during meiotic maturation may be to prevent undesirable DNA repl ication or parthenogenetic activation before fertilization.