Defective S phase chromatin assembly causes DNA damage,activation of the S phase checkpoint,and S phase arrest

The S phase checkpoint protects the genome from spontaneous damage during DNA replication, although the cause of damage has been unknown. We used a dominant-negative mutant of a subunit of CAF-I, a complex that assembles newly synthesized DNA into nucleosomes, to inhibit S phase chromatin assembly and found that this induced S phase arrest. Arrest was accompanied by DNA damage and S phase checkpoint activation and required ATR or ATM kinase activity. These results show that in human cells CAF-I activity is required for completion of S phase and that a defect in chromatin assembly can itself induce DNA damage. We propose that errors in chromatin assembly, occurring spontaneously or caused by genetic mutations or environmental agents, contribute to genome instability.