Showing papers on "Nucleolar chromatin published in 1981"

A nucleolar skeleton of protein filaments demonstrated in amplified nucleoli of Xenopus laevis.

Abstract: The amplified, extrachromosomal nucleoli of Xenopus oocytes contain a meshwork of approximately 4-nm-thick filaments, which are densely coiled into higher-order fibrils of diameter 30-40 nm and are resistant to treatment with high- and low-salt concentrations, nucleases (DNase I, pancreatic RNase, micrococcal nuclease), sulfhydryl agents, and various nonionic detergents. This filamentous "skeleton" has been prepared from manually isolated nuclear contents and nucleoli as well as from nucleoli isolated by fluorescence-activated particle sorting. The nucleolar skeletons are observed in light and electron microscopy and are characterized by ravels of filaments that are especially densely packed in the nucleolar cortex. DNA as well as RNA are not constituents of this structure, and precursors to ribosomal RNAs are completely removed from the extraction-resistant filaments by treatment with high-salt buffer or RNase. Fractions of isolated nucleolar skeletons show specific enrichment of an acidic major protein of 145,000 mol wt and an apparent pI value of approximately 6.15, accompanied in some preparations by various amounts of minor proteins. The demonstration of this skeletal structure in "free" extrachromosomal nucleoli excludes the problem of contaminations by nonnucleolar material such as perinucleolar heterochromatin normally encountered in studies of nucleoli from somatic cells. It is suggested that this insoluble protein filament complex forms a skeleton specific to the nucleolus proper that is different from other extraction-resistant components of the nucleus such as matrix and lamina and is involved in the spatial organization of the nucleolar chromatin and its transcriptional products.

Evidence for the Existence of Globular Units in the Supranucleosomal Organization of Chromatin

Abstract: One of the significant steps in the past decade of chromatin research has been the finding of the nucleosome as the common principle form of DNA-histone organization (Hewish and Burgoyne 1973; Kornberg 1974; Olins and Olins 1974; for review see Chambon 1978). In most eukaryotic nuclei the bulk of the chromatin is associated with histones in a regular pattern and is organized, in the non-transcribed chromatin regions, in nucleosomal particles, which can be visualized by electron microscopy of spread chromatin in extended nucleosomal filaments, i.e. 10–12 nm large beads-on-a-string (Fig. 1 a). This principle of packing in globular repeating units results in an overall foreshortening of the DNA by a factor of 5–6. However, some other forms of DNA arrangement in chromatin have also been reported: (i) the dinoflagellate chromosome (for references see Rae and Steele 1978); (ii) DNA newly replicated in the absence of protein synthesis (Riley and Weintraub 1979); (iii) actively transcribed chromatin in which the nucleohistone is organized in such a way that the contained DNA is in a predominantly extended configuration (Fig. 1b; Foe et al. 1976; Foe 1978; Franke et al. 1976, 1978; Reeder et al. 1978; Trendelenburg et al. 1976; for references see Franke et al. 1979); and (iv) chromatin present in cells infected with adenovirus (Kedinger et al. 1978; Matsuguchi et al. 1979; Miller and Hodge 1975) and herpes simplex virus (Muller et al. 1980): In the case of herpes-infected cells, a total of four different forms of chromatin are observed (Fig. 1c).

An Electron Microscopic Analysis of Transcription of Nucleolar Chromatin Isolated from Tetrahymena pyriformis

Abstract: Transcriptionally active nucleoli and solubilized nucleolar chromatin were visualized by electron microscopy. The palindromic structure of the chromatin was demonstrated by spreading the chromatin on glow-discharged grids. In the presence of single-strand binding Eco HDP protein the preribosomal RNA transcripts are seen attached to the RNA-polymerase molecules in the electron micrographs. Each palindrome contains two preribosomal RNA genes. The strict termination properties of the transcription are indicated by the absence of transcriptional complexes in the distal parts of the molecules. — Investigation of the process of transcription in nucleolar chromatin being depleted of a termination protein by ammonium sulfate-treatment showed in agreement with biochemical studies that only some of the RNA polymerases terminate properly while others transcribe into the distal spacer region. The elongation rate is estimated to be slightly lower than in the gene region. The results are discussed in relation to biochemical studies of the transcriptional properties of the chromatin.

Studies of Molecular Events in Isolated Nucleolar Chromatin in Response to Chemicals and Ionizing Radiation

Abstract: Chromatin is an important target for the effect of chemicals and ionizing radiation in living cells The structural alterations induced lead to disturbance of the genetic expression, ie of the biosynthesis of RNA and of protein One useful approach to explore the molecular background of various observed end-effects is to study the effect of damage on a specific gene which has been isolated in its chromatin form

Reproduction of Nucleolar Chromatins in Mouse Cells is not Associated with Disoganization of Nucleoli

Abstract: The cyclic change of nucleolar number per nucleus in synchronized L-929 cells was observed during the traverse of cell cycles. The reduction of the nucleolar number, however, is not due to the disappearance of nucleoli during the replication of their chromatins for the following reasons : 1) The average number of nucleoli of synchronized L-cells was 1.84 for S phase and 1.46 for G2 and/or G1 phase. 2) Incorporation of [3H]thymidine into nucleolar DNA of synchronized L-cells was observed mainly in the S phase. Therefore, the lowest average number of nucleoli per nucleus was not observed in the S phase, in which the bulk of nucleolar DNA was under replication. 3) In randomly growing mouse ascites tumor cells, the specific activities of DNAs of nucleolar and extranucleolar chromatins which incorporated [3H]thymidine in vivo were similar. Therefore, the newly synthesized extranucleolar chromatin was not the precursor of nucleolar chromatin. 4) Three known DNA polymerases α, β and γ were found to be present in the extracts of isolated nucleoli from Ehrlich ascites tumor cells. So nucleoli themselves appear to have the machinery for DNA replication. These results suggest that the reproduction of nucleolar chromatins in mouse cells is not associated with disorganization of the whole nucleolar structure.