RESUMO
Previously, we have shown that loss of the histone 3 lysine 27 (H3K27) monomethyltransferases ARABIDOPSIS TRITHORAX-RELATED 5 (ATXR5) and ATXR6 (ATXR6) results in the overreplication of heterochromatin. Here we show that the overreplication results in DNA damage and extensive chromocenter remodeling into unique structures we have named "overreplication-associated centers" (RACs). RACs have a highly ordered structure with an outer layer of condensed heterochromatin, an inner layer enriched in the histone variant H2AX, and a low-density core containing foci of phosphorylated H2AX (a marker of double-strand breaks) and the DNA-repair enzyme RAD51. atxr5,6 mutants are strongly affected by mutations in DNA repair, such as ATM and ATR. Because of its dense packaging and repetitive DNA sequence, heterochromatin is a challenging environment in which to repair DNA damage. Previous work in animals has shown that heterochromatic breaks are translocated out of the heterochromatic domain for repair. Our results show that atxr5,6 mutants use a variation on this strategy for repairing heterochromatic DNA damage. Rather than being moved to adjacent euchromatic regions, as in animals, heterochromatin undergoes large-scale remodeling to create a compartment with low chromatin density.
Assuntos
Dano ao DNA/genética , Heterocromatina/genética , Arabidopsis/genética , Proteínas de Arabidopsis/genética , Cromatina/genética , Quebras de DNA de Cadeia Dupla , Reparo do DNA/genética , Regulação da Expressão Gênica de Plantas/genética , Histonas/genética , Metiltransferases/genética , Mutação/genética , Fosforilação/genéticaRESUMO
Histones package DNA in all eukaryotes and play key roles in regulating gene expression. Approximately 150 base pairs of DNA wraps around an octamer of core histones to form the nucleosome, the basic unit of chromatin. Linker histones compact chromatin further by binding to and neutralizing the charge of the DNA between nucleosomes. It is well established that chromatin packing is regulated by a complex pattern of posttranslational modifications (PTMs) to core histones, but linker histone function is less well understood. In this review, we describe the current understanding of the many roles that linker histones play in cellular processes, including gene regulation, cell division, and development, while putting the linker histone in the context of other nuclear proteins. Although intriguing roles for plant linker histones are beginning to emerge, much of our current understanding comes from work in animal systems. Many unanswered questions remain and additional work is required to fully elucidate the complex processes mediated by linker histones in plants.