Koster J G, Destrée O H, Raat N J, Westerhoff H V
Hubrecht Laboratorium, Netherlands Institute for Developmental Biology, Utrecht.
Biomed Biochim Acta. 1990;49(8-9):855-77.
Using cloned histone gene variants, the expression of three types of histone gene clusters was studied at the RNA level during early development in Xenopus laevis. For each histone class the number of mRNA molecules per embryo strongly decreased upon ovulation and steadily increased during early development, with a slight decrease at the neurula stage. Variation of the stringency of hybridization revealed that none of the histone genes probed is specifically and uniquely expressed at any time point in embryogenesis. The observed variation of histone mRNA content with time after fertilization is consistent with what is known about rate constants for RNA synthesis and degradation and about histone mRNA storage in the oocyte, provided that approximately 11 h after fertilization a regulatory transition is proposed (KOSTER, DESTREE and WESTERHOFF (1988) J. Theor. Biol. 135, 139-167). Similarly, the observed amounts of histone mRNA could well be sufficient to direct the synthesis of the required amount of histone protein. Control Analysis reveals that late in development, the histone to DNA ratio is controlled virtually equally strongly by the rate constants of DNA replication, transcription and translation. The control coefficient for RNA degradation is only a little smaller, whereas that for proteolysis is negligible. Indeed, the deceleration of DNA synthesis around the Mid Blastula Transition (some 8 h after fertilization in our studies) is a regulatory step that is essential in order to allow Xenopus to synthesize sufficient histones to structure its DNA; otherwise the embryo would run out of histone protein and histone mRNA soon thereafter. A model that assumes that the lengthening of the cell cycle around the Mid Blastula Transition it itself a response to the decrease in the concentration of histone protein not complexed to DNA, is shown to account for the dynamics of histone and DNA synthesis during the first 50 h after fertilization.
利用克隆的组蛋白基因变体，研究了非洲爪蟾早期发育过程中三种组蛋白基因簇在RNA水平上的表达。对于每一类组蛋白，每个胚胎中mRNA分子的数量在排卵后大幅减少，并在早期发育过程中稳步增加，在神经胚阶段略有下降。杂交严谨性的变化表明，所检测的组蛋白基因在胚胎发生的任何时间点都没有特异性和独特的表达。受精后组蛋白mRNA含量随时间的变化与已知的RNA合成和降解速率常数以及卵母细胞中组蛋白mRNA的储存情况一致，前提是受精后约11小时提出了一个调控转变（科斯特、德斯特雷和韦斯特霍夫（1988年）《理论生物学杂志》135卷，第139 - 167页）。同样，观察到的组蛋白mRNA量很可能足以指导所需量组蛋白的合成。控制分析表明，在发育后期，组蛋白与DNA的比例实际上同样受到DNA复制、转录和翻译速率常数的强烈控制。RNA降解的控制系数仅略小，而蛋白水解的控制系数可忽略不计。事实上，在囊胚中期转变前后（我们的研究中受精后约8小时）DNA合成的减速是一个调控步骤，对于非洲爪蟾合成足够的组蛋白来构建其DNA至关重要；否则胚胎很快就会耗尽组蛋白和组蛋白mRNA。一个假设囊胚中期转变前后细胞周期延长本身是对未与DNA结合的组蛋白浓度降低的反应的模型，被证明可以解释受精后前50小时内组蛋白和DNA合成的动态变化。
