Chromatin replication through cellular proliferation in eukaryotic cells

新复制的DNA在复制叉通过后不久组装成核小体。这种核小体的快速形成在功能上与DNA复制有关，并对维持增殖细胞的基因组完整性起着关键作用(Shibahara和Stillman，Cell，1999)。两个组蛋白结合蛋白CAF-1和ASF1被认为参与了这种复制偶联的核小体组装反应的某些过程。最近，我们使用CAF-1缺失的鸡DT40细胞系，发现在体内没有CAF-1和/或ASF1功能的情况下，S期的进程被推迟，DNA复制过程中快速的核小体组装受到干扰。(Sanematsu等人，J.Biol.Chem.，2006)。这是CAF-1和ASF1参与体内DNA复制过程中核小体快速组装的第一个直接证据。此外，我们还获得了一些有趣的证据，证明CAF-1参与了CHK1依赖的检查点通路(Takami等人，…拟南芥中CAF-1(Fas)的功能缺失突变体显示出两个分生组织的细胞和功能组织严重紊乱(Kaya等人，Cell，2001)。我们还发现，内源CACTA转座子的转录基因沉默(TGS)在fas突变体中以随机方式罕见地释放，而不会减少DNA甲基化。位于不同染色体位置的其他内源沉默基因也在转录水平上相互非伴随激活。此外，沉默的转基因β-葡萄糖醛酸苷酶(GUS)的TGs在fas突变体中也被随机去抑制，与发育异常无关，并且GUS在生长过程中保持激活状态以产生表达GUS的细胞簇(Ono等人，gene Cells，2006)。综上所述，我们强烈认为CAF-1通过多轮细胞分裂确保表观遗传状态的稳定维持，CAF-1功能的缺陷解释了fas突变体中各种表型的随机出现。较少

A newly replicated DNA is assembled into nucleosome soon after the passage of replication fork. This rapid formation of nucleosome is functionally linked to DNA replication and plays a critical role for the maintenance of genome integrity in proliferating cells (Shibahara and Stillman, Cell, 1999). Two histone-binding proteins, CAF-1 and ASF1, are assumed to be involved in some process of this replication-coupled nucleosome assembly reaction.Using CAF-1-deficient chicken DT40 cell lines, we had recently shown that without CAF-1 and/or ASF1 function, S-phase progression was delayed and a rapid nucleosome assembly during DNA replication was disturbed in vivo. (Sanematsu, et al., J.Biol.Chem., 2006). This is the first direct evidence for the involvement of CAF-1 and ASF1 in a rapid nucleosome assembly during DNA replication in vivo. In addition, we obtained some interesting evidence for that CAF-1 is involved in Chk1-dependent checkpoint pathway after the treatment with HU (Takami et al., … More Mol.Biol.Cell, 2007).The loss-of-function mutants of caf-1 (fas) in Arabidopsis displayed severely disturbed cellular and functional organization of both meristems (Kaya et al., Cell, 2001). We also had shown that transcriptional gene silencing (TGS) of endogenous CACTA transposons was released infrequently in a stochastic manner in fas mutants without decreasing DNA methylation. Other endogenous silent genes at different chromosomal sites were also transcriptionally activated non-concomitantly with each other. Furthermore, TGS of the silent transgene β-glucuronidase (GUS) was also de-repressed randomly in fas mutants, irrespectively of developmental abnormalities, and the activated state of GUS was maintained during growth to produce clusters of cells expressing GUS (Ono et al., Genes Cells, 2006). Taken together, we strongly suggest that CAF-1 ensures stable maintenance of epigenetic states through multiple rounds of cell divisions and that defects in CAF-1 functions explain the stochastic occurrence of various phenotypes in the fas mutants. Less