A ROLE FOR CYCLIN E CDK2 IN THE TIMING OF THE MIDBLASTULA TRANSITION IN XENOPUS EMBRYOS/

During Xenopus development, the early cell cycles consist of rapid osc illations between DNA synthesis and mitosis until completion of the 12 th mitotic division. Then the cycle lengthens and becomes asynchronous , zygotic transcription begins, and G phases are established, a period known as the midblastula transition (MBT). Some aspects of the MBT, s uch as zygotic transcription, depend on acquisition of a threshold nuc lear to cytoplasmic (N/C) ratio, whereas others, such as maternal cycl in E degradation, are independent of nuclear events and appear to be c ontrolled by an autonomous maternal timer. To investigate the function of cyclin E during the early cycles, cyclin E/Cdk2 kinase activity wa s specifically inhibited in fertilized eggs by a truncated form of the Xenopus Cdk inhibitor, Xic1 (Delta 34Xic1). Delta 34Xic1 caused lengt hening of the embryonic cell cycles that correlated with increased lev els of mitotic cyclins. However, DNA synthesis was not inhibited. Seve ral hallmarks of the MBT were delayed for several hours in Delta 34Xic 1-injected embryos, including the disappearance of cyclins E and A, th e initiation of zygotic transcription, and the reappearance of phospho tyrosine on Cdc2. In both control and Delta 34Xic1-injected embryos, c yclin E was degraded after the 12th mitotic division as zygotic transc ription began, but experiments with alpha-amanitin show that cyclin E degradation is not dependent on zygotic transcription. Thus, the lengt h of the early cycles and the timing of maternal cyclin degradation de pend upon cyclin E/Cdk2 activity. Neither oscillations in cyclin E/Cdk 2 activity during the early cycles nor the disappearance of cyclin E a t the MBT were dependent on protein synthesis. These data suggest that cyclin E/Cdk2 is directly linked to an autonomous maternal timer that drives the early embryonic cell cycles until the MBT. (C) 1997 Academ ic Press.