SRA

The methylation state of lysine 20 on histone H4 (H4K20) has been linked to cell cycle progression, Origin Recognition Complex (ORC) binding, and replication origin regulation. Monomethylation of H4K20 (H4K20me1) is mediated by the cell cycle-regulated histone methyltransferase PR-Set7, which is essential for genome integrity and cell cycle progression. PR-Set7 depletion in mammalian cells results in defective S-phase progression and the accumulation of DNA damage, which could be partially attributed to a defect in pre-Replication Complex (pre-RC) formation and origin activity. However, these studies were limited to a handful of mammalian origins, and it remains unclear how PR-Set7 and H4K20 methylation impact the replication program on a genomic scale. Using Drosophila Kc167 cells, we employed genetic, cytological, and genomic approaches to better understand the role of PR-Set7 and H4K20 methylation in regulating DNA replication and governing genome stability. We find that depletion of Drosophila PR-Set7 and loss of H4K20me1 result in the accumulation of DNA damage and an ATR-dependent cell cycle arrest. The cell cycle arrest occurs during the second S-phase following loss of PR-Set7 activity, suggesting that accumulation of nascent H4K20 is recalcitrant to the DNA replication program. Deregulation of H4K20 methylation had no impact on origin activation throughout the genome; instead, we found that the DNA damage marker, phosphorylated H2A.v (?-H2A.v), accumulated specifically in late replicating domains in the absence of PR-Set7. This suggests that the molecular basis for the cell cycle arrest and accumulation of DNA damage resulting from loss of PR-Set7 is stochastic fork collapse within late replicating domains. Overall design: Examination of BrdU enrichment in the presence of hydroxyurea to map activation of early replication origin under control, PR-Set7, ATR, and ATR/PR-Set7 RNAi treatment conditions in duplicates using Illumina HiSeq and Miseq respectively; map early origin in 6 other conditions using Illumina HiSeq; examination of H4K20me1 in early or late S-phase with two different experimental methods, using Hiseq; examination of a histone variant with control or PR-Set7 RNAi treatment, in duplicates, using Hiseq.