Perry C A, Annunziato A T
Department of Biology, Boston College, Massachusetts 02167.
Exp Cell Res. 1991 Oct;196(2):337-45. doi: 10.1016/0014-4827(91)90269-z.
During chromatin replication and nucleosome assembly, newly synthesized histone H4 is acetylated before it is deposited onto DNA, then deacetylated as assembly proceeds. In a previous study (Perry and Annunziato, Nucleic Acids Res. 17, 4275 [1989]) it was shown that when replication occurs in the presence of sodium butyrate (thereby inhibiting histone deacetylation), nascent chromatin fails to mature fully and instead remains preferentially sensitive to DNaseI, more soluble in magnesium, and depleted of histone H1 (relative to mature chromatin). In the following report the relationships between chromatin replication, histone acetylation, and H1-mediated nucleosome aggregation were further investigated. Chromatin was replicated in the presence or absence of sodium butyrate; isolated nucleosomes were stripped of linker histone, reconstituted with H1, and treated to produce Mg(2+)-soluble and Mg(2+)-insoluble chromatin fractions. Following the removal of H1, all solubility differences between chromatin replicated in sodium butyrate for 30 min (bu-chromatin) and control chromatin were lost. Reconstitution with H1 completely restored the preferential Mg(2+)-solubility of bu-chromatin, demonstrating that a reduced capacity for aggregation/condensation is an inherent feature of acetylated nascent nucleosomes; however, titration with excess H1 caused the solubility differences to be lost again. Moreover, when the core histone N-terminal "tails" (the sites of acetylation) were removed by trypsinization prior to reconstitution, H1 was unable to reestablish the altered solubility of chromatin replicated in butyrate. Thus, the core histone "tails," and the acetylation thereof, not only modulate H1-mediated nucleosome interactions in vitro, but also strongly influence the ability of H1 to differentiate between new and old nucleosomes. The data suggest a possible mechanism for the control of H1 deposition and/or chromatin folding during nucleosome assembly.
在染色质复制和核小体组装过程中，新合成的组蛋白H4在沉积到DNA上之前被乙酰化，然后随着组装过程进行而发生去乙酰化。在之前的一项研究中（Perry和Annunziato，《核酸研究》17，4275 [1989]）表明，当在丁酸钠存在的情况下进行复制时（从而抑制组蛋白去乙酰化），新生染色质无法完全成熟，而是仍然优先对DNaseI敏感，在镁中更易溶解，并且组蛋白H1含量减少（相对于成熟染色质）。在接下来的报告中，进一步研究了染色质复制、组蛋白乙酰化和H1介导的核小体聚集之间的关系。在有或没有丁酸钠的情况下进行染色质复制；分离的核小体去除连接组蛋白，用H1重新组装，并处理以产生Mg(2+)可溶和Mg(2+)不溶的染色质组分。去除H1后，在丁酸钠中复制30分钟的染色质（丁酸钠染色质）和对照染色质之间的所有溶解性差异都消失了。用H1重新组装完全恢复了丁酸钠染色质优先的Mg(2+)溶解性，表明聚集/凝聚能力降低是乙酰化新生核小体的固有特征；然而，用过量H1滴定导致溶解性差异再次消失。此外，当在重新组装之前通过胰蛋白酶消化去除核心组蛋白的N端“尾巴”（乙酰化位点）时，H1无法重新建立在丁酸盐中复制的染色质改变的溶解性。因此，核心组蛋白“尾巴”及其乙酰化不仅在体外调节H1介导的核小体相互作用，而且还强烈影响H1区分新旧核小体的能力。这些数据提示了一种在核小体组装过程中控制H1沉积和/或染色质折叠的可能机制。
