The many faces of chromatin assembly factor 1 Elena Ramirez-Parra and Crisanto Gutierrez Centro de Biologia Molecular ‘Severo Ochoa’, Consejo Superior de Investigaciones Cientificas, Universidad Autonoma de Madrid, Cantoblanco, 28049 Madrid, Spain Chromatin organization requires that histones associate with DNA in the form of nucleosomes the position and composition of which is crucial for chromatin dynamics. Histone chaperones help to deliver specific histone proteins to the sites where chromatin is being newly formed or remodeled. Association of H3–H4 during DNA replication depends on the chromatin assembly factor 1. The study of Arabidopsis plants carrying loss-of-function alleles in each of the three chromatin assembly factor 1 subunits has highlighted the links between chromatin assembly in proliferating cells and other cellular pro- cesses. These are the G2 DNA damage checkpoint, hom- ologous recombination, endoreplication control and transcriptional regulation of specific gene sets, all contributing to the plasticity of plants in dealing with alterations in DNA replication-associated chromatin assembly. Introduction In eukaryotes, genomic DNA associates with histone and non-histone proteins, becoming a highly compact structure – chromatin. The basic structural subunit of chromatin is the nucleosome, which contains 150 base pairs of DNA wrapped around a histone octamer core of two molecules of each of the histones H2A, H2B, H3 and H4. Thus, genome duplication consists not only in DNA replication, but also in the reorganization of new histone octamers onto newly synthesized DNA, a process that is initiated by the incorp- oration of histones H3 and H4. However, histones do not associate with DNA directly to form mature nucleosomes. Instead, histone chaperones (Box 1) facilitate this process by recruiting histones to the chromosomal sites where chromatin is being reconstituted or reorganized [1]. Main- taining chromatin and genome integrity is crucial for the normal development of eukaryotic organisms. Therefore, the function of histone chaperones, particularly in associ- ation with DNA replication, is of primary importance. The chromatin assembly factor 1 (CAF-1) chaperone functions in association with the DNA replication machinery to deposit histone H3 and H4 onto DNA, which is the initial event in DNA replication-associated chromatin assembly (Figure 1). Following the identification of genes encoding CAF-1 subunits in plants, their function has been reviewed [2,3]. More recently, various reports have demonstrated that a loss of function of CAF-1 has a pleiotropic effect. Here, we discuss the various cellular processes for which CAF-1 function is required, with the aim of integrating them in the light of recent findings on chromatin organiz- ation of genes particularly affected by CAF-1 function. Plant CAF-1 Subunit organization of CAF-1 CAF-1 is a highly conserved heterotrimeric complex. In yeast, where it is known as chromatin assembly complex (CAC), it consists of Cac1, Cac2 and Cac3 subunits [1,4], whereas in mammalian cells, CAF-1 consists of the p150, p60 and p48 subunits [5–7]. In Arabidopsis, the three subunits are encoded by the FASCIATA1 (FAS1), FAS- CIATA2 (FAS2) and MULTICOPY SUPPRESSOR OF Review TRENDS in Plant Science Vol.12 No.12 Glossary Checkpoint: cell-cycle checkpoints are control mechanisms that ensure the fidelity of cell division in eukaryotic cells, controlling whether each phase of the cell cycle has been accurately completed before progression into the next phase. The main function of checkpoints is to assess DNA damage. Chromocenter: a condensed structure formed by the association of centro- meric regions of heterochromatin. Double-strand break (DSB): a DNA lesion that affects both DNA strands. DSBs are the consequence of endogenous or exogenous genotoxic agents (ionizing radiation or chemicals). This lesion is repaired mainly by two different pathways: homologous recombination (HR) and non-homologous end-joining (NHEJ). E2F: a family of eukaryotic transcription factors that are essential for the G1–S- phase transition but also for the regulation of other cellular and developmental processes. Arabidopsis thaliana contains six E2F proteins (namely, E2Fa–f). They are regulated by the plant retinoblastoma-related (RBR) protein and can function as transcriptional repressors or activators, depending on the E2F member and the cellular and genomic context. Endoreplication: the process of full-genome duplication without mitosis, resulting in an increase in the nuclear DNA content. Endoreplication is a common process in plants, where it has an essential association with developmental programs. Epigenetic: epigenetic regulation or chromatin modification refers to changes in gene expression that are not caused by DNA sequence. Molecular mechanisms that mediate epigenetic regulation include DNA methylation and modifications of histones, such as methylation, acetylation, ubiquitination, phosphorylation and incorporation of histone variants. Euchromatin: chromosomal regions with a high density of genes, which are often under active transcription. Heterochromatin: chromosomal regions, highly condensed throughout the cell cycle, that have been clearly linked to gene silencing. Constitutive hetero- chromatin is often associated with telomeres and pericentromeric regions of chromosomes and is rich in repetitive, permanently inactive chromosomal regions. However, facultative heterochromatin can be transcriptionally silenced in specific tissues or developmental stages, and remain active in others. Homologous recombination (HR): a DNA repair pathway that requires extensive DNA sequence homology between the interacting strands. Non-homologous end-joining (NHEJ): a DNA repair pathway that requires limited DNA sequence homology, or even no homology, at the ends of the interacting strands. Corresponding author: Gutierrez, C. (cgutierrez@cbm.uam.es). Available online 9 November 2007. www.sciencedirect.com 1360-1385/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2007.10.002