Distinct H4-K20 methylation states time DNA replication initiation in mammalian cells. Homo sapiens

The role of histone modifications in DNA replication remains poorly understood. Here, we show that histone H4K20 monomethylation (me1) favors the assembly of the pre-replication complex on chromatin, an association that is increased when H4K20me1 is converted to trimethylation (H4K20me3). Remarkably, these histone methylations enhance origin activity, indicating a positive role in both origin licensing and activation. Genome-wide approaches revealed that H4K20me1 and H4K20me3 are enriched in distinct GC-rich regions, where their presence correlates with early-replicating and late-firing origins respectively. Accordingly, depletion of H4K20me1 at early origins delays the replication of active chromatin. Conversely, loss of H4K20me2/3 causes a drop in the licensing and activity of late-firing origins, slowing down the replication of heterochromatin. Altogether, these results suggest that the concerted activity of mammalian H4K20 methyltransferases and the resulting distribution of H4K20me states are essential to ensure the order and replication timing of different chromatin states. Overall design: H4K20me1 and H4K20me3 profile on the whole genome in U2OS cell line