Nucleosomes package eukaryotic DNA and are composed of four different histone proteins, designated H3, H4, H2A, and H2B. Histone H3 has two main variants, H3.1 and H3.3, which show different genomic localization patterns in animals. We profiled H3.1 and H3.3 variants in the genome of the plant Arabidopsis thaliana and found that the localization of these variants shows broad similarity in plants and animals, along with some unique features. H3.1 was enriched in silent areas of the genome, including regions containing the repressive chromatin modifications H3 lysine 27 methylation, H3 lysine 9 methylation, and DNA methylation. In contrast, H3.3 was enriched in actively transcribed genes, especially peaking at the 3′ end of genes, and correlated with histone modifications associated with gene activation, such as histone H3 lysine 4 methylation and H2B ubiquitylation, as well as RNA Pol II occupancy. Surprisingly, both H3.1 and H3.3 were enriched on defined origins of replication, as was overall nucleosome density, suggesting a novel characteristic of plant origins. Our results are broadly consistent with the hypothesis that H3.1 acts as the canonical histone that is incorporated during DNA replication, whereas H3.3 acts as the replacement histone that can be incorporated outside of S-phase during chromatin-disrupting processes like transcription. N ucleosomes, the basic units of chromatin, consist of DNA wrapped around a core of eight histone proteins composed of two dimers of histones H2A and H2B and a tetramer of histones H3 and H4. In multicellular organisms, multiple variants of each histone protein coexist in the same nucleus. In the case of histone H3, three variants are found in both plants and animals: the canonical H3

Maintenance of genome integrity depends on histone chaperone-mediated chromatin reorganization. DNA replicationassociated nucleosome deposition relies on chromatin assembly factor-1 (CAF-1). Depletion of CAF-1 in human cells leads to cell death, whereas in Arabidopsis (Arabidopsis thaliana), where it is involved in heterochromatin compaction and homologous recombination, plants are viable. The mechanism that makes the lack of CAF-1 activity compatible with development is not known. Here, we show that the FASCIATA1 (FAS1) gene, which encodes the CAF-1 large subunit, is a target of E2F transcription factors. Mutational studies demonstrate that one of the two E2F binding sites in its promoter has an activator role, whereas the other has a repressor function. Loss of FAS1 results in reduced type A cyclin-dependent kinase activity, inhibits mitotic progression, and promotes a precocious and systemic switch to the endocycle program. Selective up-regulation of the expression of a subset of genes, including those involved in activation of the G2 DNA damage checkpoint, also occurs upon FAS1 loss. This activation is not the result of a global change in chromatin structure, but depends on selective epigenetic changes in histone acetylation and methylation within a small region in their promoters. This suggests that correct chromatin assembly during the S-phase is required to prevent unscheduled changes in the epigenetic marks of target genes. Interestingly, activation of the endocycle switch as well as introduction of activating histone marks in the same set of G2 checkpoint genes are detected upon treatment of wild-type plants with DNA-damaging treatments. Our results are consistent with a model in which defects in chromatin assembly during the S-phase and DNA damage signaling share part of a pathway, which ultimately leads to mitotic arrest and triggers the endocycle program. Together, this might be a bypass mechanism that makes development compatible with cell division arrest induced by DNA damage stress.