E2F RegulatesFASCIATA1, a Chromatin Assembly Gene Whose Loss Switches on the Endocycle and Activates Gene Expression by Changing the Epigenetic Status

Abstract

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.

Impact of nucleosome dynamics and histone modifications on cell proliferation during Arabidopsis development

“…In mammalian cells, H3K9me3 is a silencing mark when found in the promoter, but is activating when is located within the transcribed regions (Berger, 2007). On the contrary, in Arabidopsis H3K9me3 is found in Arabidopsis euchromatin (Fuchs et al, 2006) and active promoters , whereas H3K9me2 is associated with repressed promoters Ramirez-Parra and Gutierrez, 2007a). Genomic analysis has extended these observations and confirmed that the H3K9me3 distribution colocalizes with euchromatin, with a slight bias towards the promoter regions (Turck et al, 2007), whereas H3K9me2 is highly enriched in transposons, pseudogenes and repressed genes.…”

“…These plants show a constitutively increased amount of double-strand breaks and of the expression of G2 DNA damage checkpoint genes, such as RAD51, PARP1 and BRCA1, in association with an increase in the homologous recombination frequency (Endo et al, 2006;Kirik et al, 2006;Schonrock et al, 2006;Ramirez-Parra and Gutierrez, 2007a). These observations suggest that loss of CAF-1 activity produces defects in chromatin assembly that could lead to genomic instability (Figure 1).…”

ArabidopsisRETINOBLASTOMA-RELATED Is Required for Stem Cell Maintenance, Cell Differentiation, and Lateral Organ Production

“…Upregulation of G2/M-specific genes reflects the stomata lineage proliferation we described in young leaves of RBRi-induced plants (Figures 5E to 5H). Additionally, several genes encoding proteins required for DNA replication-associated chromatin assembly, many of which contain E2F binding sites (Ramirez-Parra and Gutierrez, 2007), were upregulated 12 and 24 HAI (see Supplemental Figure 15 online). The upregulation of these genes is consistent with the increased cell proliferation due to RBR downregulation.…”

ANTI-SILENCING FUNCTION1 Proteins Are Involved in Ultraviolet-Induced DNA Damage Repair and Are Cell Cycle Regulated by E2F Transcription Factors in Arabidopsis

“…To test this hypothesis, we used a set of transgenic Arabidopsis seedlings with altered E2F expression (Ramirez-Parra and Gutierrez, 2007) and analyzed ASF1A and ASF1B mRNA levels in these plants by quantitative reverse transcription-PCR (qRT-PCR; Supplemental Table S2). CELL DIVISION CYCLE 6a (CDC6a; At2g29680), a well-characterized E2F target gene (Castellano et al, 2001), was used as a positive control.…”