Koster J G, Destrée O H, Westerhoff H V
Department of Anatomy and Embryology, University of Amsterdam, The Netherlands.
J Theor Biol. 1988 Nov 21;135(2):139-67. doi: 10.1016/s0022-5193(88)80071-7.
Using literature data for transcriptional and translational rate constants, gene copy numbers, DNA concentrations, and stability constants, we have calculated the expected concentrations of histones and histone mRNA during embryogenesis of Xenopus laevis. The results led us to conclude that: (i) for X. laevis the gene copy number of the histone genes is too low to ensure the synthesis of sufficient histones during very early development, inheritance from the oocyte of either histone protein or histone mRNA (but not necessarily both) is necessary; (ii) from the known storage of histones in the oocyte and the rates of histone synthesis determined by Adamson & Woodland (1977), there would be sufficient histones to structure the newly synthesized DNA up to gastrulation but not thereafter (these empirical rates of histone synthesis may be underestimates); (iii) on the other hand, the amount of H3 mRNA recently observed during early embryogenesis (Koster, 1987, Koster et al., 1988) could direct a higher and sufficient synthesis of H3 protein, also after gastrulation. We present a quantitative model that accounts both for the observed H3 mRNA concentration as a function of time during embryogenesis and for the synthesis of sufficient histones to structure the DNA throughout early embryogenesis. The model suggests that X. laevis exhibits a major (i.e. some 14-fold) reduction in transcription of histone genes approximately 11 hours after fertilization. This reduction could be due to a decrease in the number of transcribed histone genes, a decreased rate constant of transcription with continued transcription of all the histone genes, and/or a reduction in the time during the cell cycle in which histone mRNA synthesis takes place. Alternatively, the histone mRNA stability might decrease approximately 16-fold 11 hours after fertilization.
利用转录和翻译速率常数、基因拷贝数、DNA浓度以及稳定性常数的文献数据，我们计算了非洲爪蟾胚胎发育过程中组蛋白和组蛋白mRNA的预期浓度。结果使我们得出以下结论：（i）对于非洲爪蟾而言，组蛋白基因的拷贝数过低，无法确保在极早期发育阶段合成足够的组蛋白，因此从卵母细胞继承组蛋白或组蛋白mRNA（但不一定两者都继承）是必要的；（ii）根据卵母细胞中组蛋白的已知储存量以及Adamson和Woodland（1977年）测定的组蛋白合成速率，在原肠胚形成之前会有足够的组蛋白来构建新合成的DNA，但之后则不然（这些组蛋白合成的经验速率可能被低估了）；（iii）另一方面，最近在早期胚胎发育过程中观察到的H3 mRNA量（Koster，1987年；Koster等人，1988年），即使在原肠胚形成之后，也能够指导更高水平且足够的H3蛋白合成。我们提出了一个定量模型，该模型既能解释胚胎发育过程中观察到的H3 mRNA浓度随时间的变化情况，又能解释在整个早期胚胎发育过程中合成足够的组蛋白来构建DNA的现象。该模型表明，非洲爪蟾在受精后约11小时，组蛋白基因的转录出现大幅（即约14倍）下降。这种下降可能是由于转录的组蛋白基因数量减少、所有组蛋白基因持续转录时转录速率常数降低，和/或细胞周期中组蛋白mRNA合成发生的时间减少。或者，受精后11小时，组蛋白mRNA的稳定性可能会下降约16倍。
