Regulation of chromosomal replication and transcription during early mammalian development

Replication-Coupled Chromatin Remodeling: An Overview of Disassembly and Assembly of Chromatin during Replication

The doubling of genomic DNA during the S-phase of the cell cycle involves the global remodeling of chromatin at replication forks. The present review focuses on the eviction of nucleosomes in front of the replication forks to facilitate the passage of replication machinery and the mechanism of replication-coupled chromatin assembly behind the replication forks. The recycling of parental histones as well as the nuclear import and the assembly of newly synthesized histones are also discussed with regard to the epigenetic inheritance.

Effect of DNA damage on stable transformation of mammalian cells with integrative and episomal plasmids

The efficiency of stable transformation of human cells by integrative (non-replicating) plasmids carrying a selectable gene has been shown to be markedly enhanced by the introduction into the plasmid DNA of bulky damage, such as cyclobutane pyrimidine dimers or psoralen photoadducts. Enhanced transformation (ET) occurs in all human cells tested, including DNA repair-deficient cells from the hereditary syndrome xeroderma pigmentosum, but significantly less, if at all, in rodent cells. ET has been observed with a variety of integrative plasmid constructs, suggesting the generality of the phenomenon; as expected, ET is due to an increase in the number of cells carrying integrated plasmid sequences. In contrast to integrative plasmids, stable transformation by episomal (autonomously replicating) plasmids derived from the Epstein-Barr virus is only depressed by the introduction of photoproducts; furthermore, pronounced inactivation of transformation mediated by episomal plasmids becomes apparent in xeroderma pigmentosum cells. Altogether, these results suggest that DNA damage increases the probability of stable insertion of heterologous non-replicating DNA into human chromosomes. Moreover, the differential sensitivity to DNA damage of human cell transformation mediated by integrative versus episomal plasmids suggests caution in using such assay to measure host cell reactivation capacity; processing of DNA damage in mammalian cells might differ significantly between intra- versus extra-chromosomal DNA. Since ET may be induced by damage outside the selectable gene carried on integrative plasmids, we propose a model that involves local disruption of chromatin structure by helix-distorting DNA lesions flanking actively transcribed sequences; alternatively, reorganization of such altered DNA structure might be favored by the presence of topoisomerase-like activities in the proximity of active genes. Because ET can also be induced by DNA damage to the recipient cells, it is speculated that similar mechanism(s) might be involved in the generation of other types of non-homologous DNA recombination in damaged human chromosomes, including oncogenic cell transformation mediated by integrative DNA viruses.