RESUMEN
Eukaryotes can have thousands of 45S ribosomal RNA (rRNA) genes, many of which are silenced during development. Using fluorescence-activated sorting techniques, we show that active rRNA genes in Arabidopsis thaliana are present within sorted nucleoli, whereas silenced rRNA genes are excluded. DNA methyltransferase (met1), histone deacetylase (hda6), or chromatin assembly (caf1) mutants that disrupt silencing abrogate this nucleoplasmic-nucleolar partitioning. Bisulfite sequencing data indicate that active nucleolar rRNA genes are nearly completely demethylated at promoter CGs, whereas silenced genes are nearly fully methylated. Collectively, the data reveal that rRNA genes occupy distinct but changeable nuclear territories according to their epigenetic state.
Asunto(s)
Proteínas de Arabidopsis/genética , Arabidopsis/genética , Nucléolo Celular/metabolismo , Núcleo Celular/metabolismo , Epigénesis Genética , ARN Ribosómico/genética , Metilación de ADN , Regulación de la Expresión Génica de las Plantas , Silenciador del Gen , Modelos Genéticos , MutaciónRESUMEN
The endocycle is a common developmental cell cycle variation wherein cells become polyploid through repeated genome duplication without mitosis. We previously showed that Drosophila endocycling cells repress the apoptotic cell death response to genotoxic stress. Here, we investigate whether it is differentiation or endocycle remodeling that promotes apoptotic repression. We find that when nurse and follicle cells switch into endocycles during oogenesis they repress the apoptotic response to DNA damage caused by ionizing radiation, and that this repression has been conserved in the genus Drosophila over 40 million years of evolution. Follicle cells defective for Notch signaling failed to switch into endocycles or differentiate and remained apoptotic competent. However, genetic ablation of mitosis by knockdown of Cyclin A or overexpression of fzr/Cdh1 induced follicle cell endocycles and repressed apoptosis independently of Notch signaling and differentiation. Cells recovering from these induced endocycles regained apoptotic competence, showing that repression is reversible. Recovery from fzr/Cdh1 overexpression also resulted in an error-prone mitosis with amplified centrosomes and high levels of chromosome loss and fragmentation. Our results reveal an unanticipated link between endocycles and the repression of apoptosis, with broader implications for how endocycles may contribute to genome instability and oncogenesis.
Asunto(s)
Apoptosis/genética , Proteínas Cdh1/metabolismo , Ciclina A/metabolismo , Proteínas de Drosophila/metabolismo , Drosophila melanogaster/citología , Inestabilidad Genómica , Mitosis/genética , Oogénesis/genética , Animales , Proteínas Cdh1/biosíntesis , Ciclo Celular/genética , Ciclina A/genética , Daño del ADN/genética , Daño del ADN/efectos de la radiación , Proteínas de Drosophila/biosíntesis , Drosophila melanogaster/genética , Drosophila melanogaster/metabolismo , Poliploidía , Interferencia de ARN , ARN Interferente Pequeño/genética , Receptores Notch/genética , Receptores Notch/metabolismo , Transducción de Señal/genéticaRESUMEN
Multiple pathways prevent DNA replication from occurring more than once per cell cycle. These pathways block re-replication by strictly controlling the activity of pre-replication complexes, which assemble at specific sites in the genome called origins. Here we show that mutations in the homologous histone 3 lysine 27 (H3K27) monomethyltransferases, ARABIDOPSIS TRITHORAX-RELATED PROTEIN5 (ATXR5) and ATXR6, lead to re-replication of specific genomic locations. Most of these locations correspond to transposons and other repetitive and silent elements of the Arabidopsis genome. These sites also correspond to high levels of H3K27 monomethylation, and mutation of the catalytic SET domain is sufficient to cause the re-replication defect. Mutation of ATXR5 and ATXR6 also causes upregulation of transposon expression and has pleiotropic effects on plant development. These results uncover a novel pathway that prevents over-replication of heterochromatin in Arabidopsis.
Asunto(s)
Proteínas de Arabidopsis/metabolismo , Arabidopsis , Replicación del ADN/fisiología , Heterocromatina/genética , N-Metiltransferasa de Histona-Lisina/metabolismo , Histonas/metabolismo , Metiltransferasas/metabolismo , Secuencias de Aminoácidos , Arabidopsis/citología , Arabidopsis/enzimología , Arabidopsis/genética , Proteínas de Arabidopsis/química , Proteínas de Arabidopsis/genética , Dominio Catalítico/genética , Metilación de ADN , Replicación del ADN/genética , Elementos Transponibles de ADN/genética , ADN de Plantas/análisis , ADN de Plantas/biosíntesis , Regulación de la Expresión Génica de las Plantas , Silenciador del Gen , Genoma de Planta/genética , Heterocromatina/metabolismo , Histonas/química , Lisina/metabolismo , Metilación , Metiltransferasas/química , Metiltransferasas/genética , Proteínas Mutantes/genética , Proteínas Mutantes/metabolismo , Mutación , Origen de RéplicaRESUMEN
The freshwater crustacean Daphnia is an emerging model system in the biology of aging. Diversity in aging patterns is thought to be caused by ecological variation in selection on age-specific performance. Previous work in Daphnia has shown a strong correspondence between selective differences and genetic variation in aging in the Daphnia pulex species complex. However, recent evidence suggests obligate asexuality could account for the more rapid aging found in pond genotypes compared with lake genotypes without invoking differences in selection. Evolutionary biologists have to date assumed equivalent operation of neutral processes when comparing aging across populations, but a shift in the breeding system changes the basic dynamics of neutral evolution. To test the hypothesis that the breeding system could explain the short lifespans of pond-dwelling Daphnia, we compared aging of sexual and asexual Daphnia clones from temporary ponds. Our data contradict the breeding system hypothesis. Differences in aging between the breeding systems were slight, and trended in the opposite direction from that predicted: asexual clones had longer lifespans and appeared to age more slowly than sexual clones. We conclude that divergent selection between habitats remains the best explanation for differences in aging between Daphnia species.