Febs Letters Volume 121, Number 1-2./Volume 122, Number 1-2. [antikvár]

Volume 121, number 1 FEBS LETTERS November 1980 Review Letter HISTONE SYNTHESIS DURING THE DEVELOPMENT OF XENOPUS Hugh R. WOODLAND Department of Biological Sciences, University of Warwick, Coventry, CV4 7AL, England Received 20 August 1980 1. Introduction The chromatin of animal cells contains approximately equal masses of DNA and histone. In the division cycle of a typical euicaryotic somatic cell this equivalence is achieved by close coupling of histone and DNA synthesis. The first aspect of the coupling is temporal: almost all histone is made during the S-phase. Accordingly, histone mRNA is present during the S-phase and most appears to be degraded at its end [1—5]. However, small amounts of histone, particularly HI, may be made at other stages, and the amount may be significant in non-dividing cells (e.g. [6—8]) and especially in late erythropoiesis [9]. Since chromatin contains equal masses of DNA and his-tones this represents turnover, particularly of the HI fraction. The second aspect of the coupling is quantitative; because very little free histone can be detected m S-phase cells, the S-phase cell must make the same mass of DNA and histone [10,11]. This tight coupling does not apply to the embryos of the anuran amphibian Xenopus laevis. At 23°C its zygote divides to form 30 000 cells in the 9 h period between fertilization and the gastrula stage. After the first cell cycle of 1.5 h, in which the S-phase lasts 20 min and there are recognisable G1 and G2 periods, the cell cycle may be as short as 12 min, with an S-phase of as httle as 10 min and no recognisable G1 or G2 [12,13]. Tight regulation of histone synthesis by transcriptional control is not possible over such a short time scale without enormous reiteration of the histone genes. The embryo solves this problem in a different way. 2. Rates of histone synthesis through early development Rapid histone synthesis during the cleavage of Elsevier/North-Holland Biomedical Press Xenopus embryos has been reported by a number of authors [14—17]. The first quantitative study was made by Adamson and Woodland [18]. When absolute rates of histone synthesis were computed, they showed that at early stages the rate of histone synthesis is far in excess of that of DNA (fig. 1 A). In the fertilized egg histones are made at ~2500 pg/h, whereas the cell replicates only 6 pg of nuclear DNA during the first 1.5 h cell cycle. There is also temporal uncoupling of DNA and histone synthesis, since histones are made at an approximately constant rate throughout this cell cycle, even during mitosis [19]. By the late blastula stage the rate of DNA synthesized per embryo is ~10 000-fold greater than at the single cell stage, but the rate of histone synthesis has only increased 2-3-fold (fig.l). The pattern described above applies to the nucleosomal histones. In contrast the HI group of histones are made at a low rate until the early blastula stage (500 cells), but then increase to normal levels by the gastrula stage (30 000 cells) [18-20]. Although the amount of histone synthesized after fertiUzation exceeds that of DNA before the 1000 cell stage, subsequently there is a deficit (fig.lB). To make good this deficiency the egg should contain a pool of at least 140 ng of histone, made previously during oogenesis. Direct measurements revealed a pool of ~135 ng [21], accumulated by synthesis of 50 pg/h in the oocyte [18]. This is sufficient to assemble over 20 000 nuclei, exactly the amount necessary to make good the calculated histone deficit at the beginning of gastrulation. Thus it would seem that, within the Umitations of the measurements, the oogenetic pool of histones should be exhausted by the gastrula stage. The existence of stored histones has been confirmed by an independent method. Laskey et al. [22] showed that egg extracts could assemble ~80 000 pg 1