Nucleolar chromatin

About: Nucleolar chromatin is a research topic. Over the lifetime, 170 publications have been published within this topic receiving 6776 citations.

Pore-linked filaments in anura spermatocyte nuclei

Abstract: Pore-linked filaments were visualized in spreads of anuran spermatocyte nuclei using transmission electron microscope. We used Odontophrynus diplo and tetraploid species having the tetraploid frogs reduced metabolic activities. The filaments with 20-40 nm width are connected to a ring component of the nuclear pore complex with 90-120 nm and extend up to 1µm (or more) into the nucleus. The filaments are curved and connect single or neighboring pores. The intranuclear filaments are associated with chromatin fibers and related to RNP particles of 20-25 nm and spheroidal structures of 0.5µm, with variations. The aggregates of several neighboring pores with the filaments are more commonly observed in 4n nuclei. We concluded that the intranuclear filaments may correspond to the fibrillar network described in Xenopus oocyte nucleus being probably related to RNA transport. The molecular basis of this RNA remains elusive. Nevertheless, the morphological aspects of the spheroidal structures indicate they could correspond to nucleolar chromatin or to nucleolus-derived structures. We also speculate whether the complex aggregates of neighboring pores with intranuclear filaments may correspond to pore clustering previously described in these tetraploid animals using freeze-etching experiments.

Study of the positional relationship of nucleolar chromatin and nucleolar compartments in somatic nuclei OPF the ciliate Didinium nasutum

Abstract: We showed earlier that nucleoli in interphase ciliates Didinium nasutum, appearing on single ultrathin sections as individual structures, actually are parts of more complex network-like structures in which fibrillar component is located on periphery, and granular--in the central part of a nucleolus. It is known, that nucleolar organizers in D. nasutum are represented by chromatin bodies connected with nucleoli. In this work we used 3D reconstruction on the basis of serial ultrathin sections to study localization of chromatin bodies which by morphological criteria might correspond to nucleolar organizers. Our data showed, that all such chromatin bodies settled down outside of nucleoli, near the periphery of fibrillar component. Even those chromatin bodies which on single sections looked completely surrounded by fibrillar nucleolar component, actually settled down in fibrillar component cavities open to nucleoplasm. Analysis of distribution of nucleolar chromatin bodies allowed us to conclude that activity in different parts of interphase complex network-like nucleoli of D. nasutum is approximately the same.

Competition for a limited pool of cohesin between centromeres and the rDNA triggers chromosomal instability that shortens replicative lifespan

Abstract: Sir2 is a highly conserved NAD+-dependent histone deacetylase that functions in heterochromatin formation and promotes replicative lifespan (RLS) in the budding yeast, Saccharomyces cerevisiae. Within the yeast rDNA locus, Sir2 is required for efficient cohesin recruitment and maintaining stability of the tandem array. In addition to the rDNA, ChIP-seq of an epitope-tagged cohesin subunit (Mcd1-13xMyc) in a sir2? mutant revealed subtle reductions of cohesin binding at all 16 centromeres. Coupled with the previously reported chromosome instability in sir2? cells and depletion of Sir2 in aged cells, we hypothesized that mitotic chromosome instability (CIN) due to Sir2 depletion could be a driver of replicative aging. In addition to Sir2, we discovered that other subunits of the Sir2-containing SIR and RENT complexes were depleted in aged cells, as were subunits of the cohesin and monopolin/cohibin complexes, implying the possibility of CIN. ChIP assays of the residual Mcd1-13xMyc in aged cells showed strong depletion from the rDNA and possible redistribution to centromeres, most likely in an attempt to maintain chromosome stability. Despite the shift in cohesin distribution, sister chromatid cohesion was partially attenuated in old cells and the frequency of chromosome loss was increased. This age-induced CIN was exacerbated in strains lacking Sir2 and its paralog, Hst1, but suppressed in strains that stabilize the rDNA array due to the deletion of FOB1 or through caloric restriction (CR). Furthermore, ectopic expression of MCD1 from a doxycycline-inducible promoter was sufficient to suppress to rDNA instability in aged cells and to extend RLS. Taken together we conclude that age-induced depletion of cohesin and multiple other nucleolar chromatin factors destabilize the rDNA locus, which then results in general CIN and aneuploidy that shortens RLS.

Nucleolar Changes in Hypotonic Treatment and the Possibility of Isolation of Nucleolar Fibrillar Complexes

Abstract: Nucleoli regularly contain the following components: the dense fibrillar component, fibrillar centers the granular component and the nucleolar chromatin (Schwarzacher and Wachtler, 1983; Goessens, 1984; Fakan and Hernandez-Verdun, 1986). Morphological studies suggest a parallel occurrence of the first two components: the fibrillar centers were never found in the active nucleoli without adjacent dense fibrillar component, and the fibrillar centers together with the dense fibrillar component (i.e. fibrillar complexes) seem to correspond to individual nucleolar organizers (Mirre and Stahl, 1981; Mirre and Kniebiehler, 1982; Hozak et al., 1986; Cataldo et al., 1988; Hozak et al., 1989).

Endoreduplication and fruit growth in tomato:evidences in favour of the karyoplasmic ratio theory

Abstract: Endopolyploidy occurs in many plant species and supports the process of differentiation of cells and organs. The functional role of endopolyploidy in plant cells remains poorly understood, mainly because the analysis is hampered by the fact that complex polyploid tissues usually include cells with different ploidy levels. During the development of tomato fruit, cells from the (fleshy) pericarp tissue become highly polyploid reaching DNA content barely encountered in other plant species (between 2C and 512C). To investigate the spatial and temporal distribution of endopolyploidy, it is necessary to address the DNA content of individual nuclei in situ. Populations of nuclei with different ploidy levels were isolated to characterize at the cytological level the consequences of endopolyploidy on the ultrastructure of nuclear and nucleolar chromatin, the nuclear shape and the relationship with other cellular organelles such as mitochondria. We were able to develop a new method based on BAC-FISH to determine in situ the ploidy level of different nuclei and consequently establish a ploidy map of tomato fruit pericarp. Based on this map, we demonstrated a link between the ploidy level, the complexity of nuclear shape and the number of mitochondria in the vicinity of polyploid nuclei. We were able to provide the first direct evidence that endoreduplication plays a role in the increased transcription of rRNA and mRNA in plant cells. We thus provided quantitative data in favour of the ‘karyoplasmic ratio’ theory and showed that endoreduplication is associated with a complex cellular re-organization during development of tomato fruit.