Regulation of the Cell Cycle during Early Xenopus Development

非洲爪蟾蜍早期发育期间细胞周期调控

成熟促进因子是部分被纯化的M期中的特殊活性物质，能诱导蛙卵母细胞的减数分裂。同时，成熟促进因子在广泛系统发育起源的有丝分裂细胞溶解物中被检测到。在没有蛋白合成情况下，我发现MPF的添加和去除可以驱使蛙受精卵的有丝分裂，比如核去膜及再生、染色体凝集及去凝集和抑制和启动内源性DNA与注射质粒的复制。我们也通过注射未受精的卵细胞质来研究M期的停滞，发现M期的停滞阻止了内源性胞质细胞周期的发生而导致MPF活性的稳定。通过注射Ca++可以使得胞质细胞周期的重新开始，从而染色体去凝集、DNA复制、MPF活性丧失。我们已调研了许多MPF的水平是如何影响DNA复制，发现其影响是基于染色质模板上而不是复制机制上。这些结果表明非洲爪蟾蜍胚胎细胞核周期事件（DNA复制和有丝分裂）是由MPF活性水平控制的，该事件由一个自动细胞周期振子的部分或全部来驱使。从简单的胚胎细胞周期来研究复杂的体细胞周期的方式被讨论。

Maturation-promoting factor (MPF) is a partially purified M-phase-specific activity that induces meiosis in frog oocytes and is detectable in mitotic lysates from cells of wide phylogenetic origins. We show here that without protein synthesis, addition and removal of MPF can drive the mitotic cycle in frog eggs, including nuclear membrane breakdown and reformation, chromosome condensation and decondensation, and suppression and initiation of DNA replication on endogenous DNA and on injected plasmid templates. We have also studied M-phase arrest induced by injection of unfertilized egg cytoplasm and show that this arrest blocks an endogenous cytoplasmic cell-cycle oscillator and causes the stabilization of MPF activity. The oscillator can be restarted by injection of Ca++, which causes chromosome decondensation, reinitiation of DNA replication, and loss of MPF activity. We have looked in more detail at how DNA replication responds to the level of MPF and show that the effects are on the chromatin template and not the replication machinery. These results suggest that in Xenopus embryos cell-cycle events of the nucleus, including DNA replication and mitosis, are controlled by the level of MPF activity, which is driven by or may be part of an autonomous cell-cycle oscillator. The way in which a more complicated somatic cell cycle may arise from the simple embryonic cell cycle is discussed.