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The Chromatin Assembly Factor Subunit FASCIATA1 Is Involved in Homologous Recombination in Plants

DNA replication in cycling eukaryotic cells necessitates the reestablishment of chromatin after nucleosome redistribution from the parental to the two daughter DNA strands. Chromatin assembly factor 1 (CAF-1), a heterotrimeric complex consisting of three subunits (p150/p60/p48), is one of the replication-coupled assembly factors involved in the reconstitution of S-phase chromatin. CAF-1 is required in vitro for nucleosome assembly onto newly replicated chromatin in human cells and Arabidopsis thaliana, and defects in yeast (Saccharomyces cerevisiae) affect DNA damage repair processes, predominantly those involved in genome stability. However, in vivo chromatin defects of caf-1 mutants in higher eukaryotes are poorly characterized. Here, we show that fasciata1-4 (fas1-4), a new allele of the Arabidopsis fas1 mutant defective in the p150 subunit of CAF-1, has a severe developmental phenotype, reduced heterochromatin content, and a more open conformation of euchromatin. Most importantly, homologous recombination (HR), a process involved in maintaining genome stability, is increased dramatically in fas1-4, as indicated by a 96-fold stimulation of intrachromosomal HR. Together with the open conformation of chromatin and the nearly normal expression levels of HR genes in the mutant, this result suggests that chromatin is a major factor restricting HR in plants.