Showing papers on "Nucleolus published in 1977"

Nuclear matrix: Isolation and characterization of a framework structure from rat liver nuclei

Abstract: A nuclear framework structure termed the nuclear matrix has been isolated and characterized. This matrix forms the major residual structure of isolated nuclei and consists largely of protein with smaller amounts of RNA, DNA, carbohydrate, and phospholipid. The nuclear matrix can be further resolved by combined treatment with DNase and RNase. The remaining nuclear protein structure, after extraction of 90 percent of the nuclear protein, 99.9 percent of the DNA, and 98 percent of the RNA and phospholipid, is termed the nuclear protein matrix. Electron microscopy of this final nuclear protein matrix reveals an interior framework structure composed of residual nucleolar structures associated with a granular and fibrous internal matrix structure. The internal matrix framework is derived from the interchromatinic structures of the nucleus, and is connected to a surrounding residual nuclear envelope layer containing residual nuclear pore complex structures. Sodium dodecyl sulfate-acrylamide gel electrophoresis of the nuclear matrix proteins demonstrates three major polypeptide fractions, P-1, P-2, and P-3, with average molecular weights of approximately 69,000, 66,000 and 62,000, as well as several minor polypeptides which migrate at approximately 50,000 and at higher molecular weights (>100,000). Polypeptides with molecular weights identical to those of P-1, P-2 and P-3 are also components of isolated nuclear envelopes and nucleoli, whereas isolated chromatin contains no detectable matrix polypeptides. This suggests that the major matrix polypeptides are localized in specific structural regions of the nucleus, i.e., nuclear envelope, nucleoli, and interchromatinic structures. The presence of cytochrome oxidase activity in the isolated nuclear matrix indicates that at least some integral proteins of the nuclear membrane are associated with the matrix.

Activation of mouse ribosomal RNA genes at the 2-cell stage.

Abstract: The Ag-AS method, developed by Goodpasture and Bloom (1975) stains transcriptionally active nucleolus organizer regions (NORs) on the chromosomes and in the interphase nuclei. Metaphases and interphase nuclei of early mouse embryos (unfertilized eggs, pronucleus stages, 2-, 4-, 8-cell stages, and morulae) were subjected to silver-staining. First staining of a single chromosome bearing an NOR was observed at the 2-cell stage. At the 4-cell stage 4–6 chromosomes, and at the 8-cell stage invariably all the 6 chromosomes known to bear NORs, respond positively to silver-staining. These results indicate that during mouse embryogenesis ribosomal RNA genes start to function at the 2-cell stage. The polar body does not respond to silver-staining, which supports the view that the polar body genome remains inactive.

The nucleolar cycle in man

Abstract: Tissue cultures of human embryonal kidney and ovary were examined. In the nuclei of both tissues, one to ten nucleoli have been found. The maximum number of nucleoli is connected with the gene expression of rDNA of the 10 nucleolus organizers of chromosome pairs Nos. 13, 14, 15, 21 and 22, which have secondary constrictions and are the satellite chromosomes in man. The small percentage of cells with 10, 9 and 8 nucleoli is attributed to the rapid association of 3 of the homologous acrocentrics (perhaps of group D). Two of the satellite (SAT) pairs probably associate later after mitosis. The process of fusion is dynamic, resulting in one interphase nucleous--a manifestation of the association of all SAT chromosomes. Dissociation of the nucleolus occurs upon entering prophase, due to the condensation of the chromosomes and retreat of rDNA to the respective secondary constrictions. As a result, the nucleolar number increases again. The pattern of the nucleolar kinetics within the course of one mitotic division is described.

Frequency of Ag-stained nucleolus organizer regions in the acrocentric chromosomes of man

Abstract: The Ag-stainability of the nucleolus organizer region (NOR) was studied in the acrocentric chromosomes identified by Q-banding of cultured lymphocytes in 51 karyotypically normal persons (31 males and 20 females). A consistent pattern of Ag-positive NORs was found in each individual. Ninety percent of the individuals have a modal number of 8–10 Ag-positive NORs per cell. The frequency of Ag-positive NORs is similar in all five acrocentrics. A statistically nonsignificant lower frequency is found in chromosome 22. Ag-negative NORs on both homologues were found in four cases. The observed frequency distribution of individuals with homozygous NOR-positive, heterozygous, and homozygous negative acrocentric chromosomes was in accordance with the Hardy-Weinberg law in all five pairs of the acrocentric chromosomes as well as in total. No sex difference was observed in our material.

Lengths and patterns of transcriptional units in the amplified nucleoli of oocytes of Xenopus laevis

Abstract: Transcriptionally active chromatin from peripheral amplified nucleoli of lampbrush-chromosome stage oocytes of Xenopus laevis was dispersed and spread in various solutions of low salt concentrations (including some with additions of detergents) and examined by electron microscopy. Nucleolar material from oocytes of animals with normal (2-nu) and mutant (1-nu) genetical constitution of nucleolus organizers was compared. Histograms showing the distributions of the lengths of matrix units, apparent spacer intercepts, and the total repeating units of the rDNA containing chromatin axes revealed a significant degree of heterogeneity, with indications of subclasses and predominant repeat unit size classes of 3.3 and 3.8 μm length. The correspondence of matrix unit length to the molecular weight of the first stable product of rDNA transcription was studied using gel electrophoresis of labelled pre-rRNA under non-denaturing and denaturing conditions. Evaluations of individual strands of nucleolar chromatin further demonstrated the existence of both (i) strands with obviously homogeneous repeating units and (ii) strands with intra-axial heterogeneity of rDNA subunits. “Preludecomplexes”, i.e. groups of transcriptional complexes in apparent spacer intercepts, were not infrequently noted. The data are compared with the measurements of lengths of repeating units in fragments of rDNA obtained by digestion with EcoRI endonuclease as described by Morrow et al. (1974) and Wellauer et al. (1974, 1976a, b). The results are discussed in relation to problems of variations in the modes of arrangement of the pre-rRNA genes, the state of packing of rDNA during transcription, and possible mechanisms of the amplification process.

Mechanism of the stimulation of RNA synthesis in rat liver nuclei by silybin.

Abstract: The influence of the flavonolignane derivatives silybin, silybinhemisuccinate and silybindihemisuccinate on the RNA synthesis in isolated nuclei from rat livers was studied. A stimulatory effect of silybin on the incorporation of [3H]UTP into nuclear RNA was found and the dose dependence could be demonstrated. The results support former findings that silybin stimulates RNA synthesis in rat livers in vivo and in hepatocytes. Experiments with purified nucleoli indicated that mainly the rate of rRNA synthesis is increased. The mechanism of this stimulation was elucidated by experiments with isolated RNA polymerases. It could be demonstrated that polymerase B is not affected by the flavonolignane, but the activity of polymerase A is increased, due to higher V at constant Km values. Thus silybin is shown to be an enzyme-stimulating effector.

Chromosome location of the ribosomal RNA genes in Triturus vulgaris meridionalis (Amphibia, Urodela). II. Intraspecific variability in number and position of the chromosome loci for 18S + 28S ribosomal RNA.

Abstract: Ribosomal genes have been localized on mitotic and lampbrush chromosomes of 20 specimens of Triturus vulgaris meridionalis by in situ hybridization with 3H 18S + 28S rRNA. The results may be summarized as follows: 1) each individual shows positive in situ hybridization at the nucleolus organizing region (NOR) on chromosome XI; 2) in addition, many specimens exhibit a positive reaction in chromosomal sites other than the NOR (additional ribosomal sites); 3) the chromosomal distribution of the additional sites appears to be identical in different tissues from the same specimen and to follow a specific individual pattern; 4) the additional ribosomal sites are preferentially found at the telomeric, centromeric or C-band regions of the chromosomes involved.

Evidence for postmeiotic expression of ribosomal RNA genes during male gametogenesis.

Abstract: Pre- and postmeiotic stages of male gametogenesis of 10 different vertebrate species belonging to mammals, birds, amphibians, and fishes were subjected to the Ag-AS staining technique (Goodpasture and Bloom, 1975). A uniform pattern of silver-staining is observable during spermatogenesis of the different vertebrate species. Silver-staining is present in spermatogonia and during the whole period of meiotic prophase, but totally absent during diakinesis and metaphase II of meiosis. In early spermatids silver-staining reappears and only disappears around the beginning of elongation of the spermatid nucleus. Since the Ag-AS technique is believed to stain only transcriptionally active nucleolus organizer regions, our findings indicate that ribosomal RNA genes become reactivated in the haploid spermatid.

Definition of subclasses of nucleoplasmic RNA.

Abstract: RNA synthesis in isolated HeLa cell nuclei prepared from cells pretreated with 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) is inhibited in a time-dependent manner. After 40-min pretreatment of cells with 60 muM DRB in the presence of actinomycin D (0.04 mug/ml), the rate of RNA synthesis in isolated nuclei, measured by [(3)H]UTP incorporation, is decreased by 63%. The DRB-resistant one-third of heterogeneous nuclear is distributed over the entire size range of heterogeneous nuclear RNA with some enrichment in the 18S range, as was observed earlier by pulse-labeling whole cells. A subclass of nucleoplasmic RNA molecules is defined in the approximate size range 110 to 250 x 10(3) daltons (330-740 nucleotides). By using heparin (2 mg/ml) to block the synthesis of smaller RNA, a peak in the chain-length range 330-740 nucleotides can be clearly resolved on 2.2% polyacrylamide/1% agarose gels in nuclei from control and DRB-treated cells. The synthesis of these molecules is largely ( approximately 90%) resistant to DRB but sensitive to alpha-amanitin at 1 mug/ml. The in vitro synthesis of molecules in the 140-330 residue range is also sensitive to alpha-amanitin at 1 mug/ml, and it is not at all affected by pretreatment of cells with DRB. Although the synthesis of the RNA in both the 330-740 and the 140-330 residue size ranges appears to be catalyzed by RNA polymerase II, the results with heparin suggest that there may be reinitiation of molecules in the 140-330 size range but not in the 330-740 range in vitro. The synthesis of 4.5S RNA ( approximately 100 nucleotides) and 5S RNA (120 nucleotides) is unaffected by pretreatment of cells with DRB and, as previously reported, is catalyzed by RNA polymerase III, with reinitiation occurring in vitro. Addition of DRB directly to isolated HeLa cell nuclei in vitro has no detectable effect on the overall rate of RNA synthesis.

Neuromelanin and RNA in cells of substantia nigra.

Abstract: We have found that with accumulation of neuromelanin granules within cell bodies of neurones of the human substantia nigra there is a reduction in cytoplasmic RNA and a decrease in nucleolar volume. These observations imply a gradual decrease in the functional capacity of the cell such that eventually, the cell is unable to produce sufficient protein to maintain its metabolic economy with atrophy and death ensuing. This reduction in protein synthesis may result from the mechanical displacement and disruption of the endoplasmic reticulum by the accumulated pigment granules.

Subcellular distribution of zinc in rat prostate studied by x-ray microanalysis: I. Normal prostate.

Abstract: Zinc is distributed subcellularly throughout the lateral prostate of the rat in both the stromal and epithelial elements. The connective tissue appears to be a major store of zinc. Within the epithelium, the highest concentrations of the element are found in the lysosomes, nucleoli, nuclear chromatin, secretory granules and luminal secretion. Histochemical studies indicate that the metal is bound relatively tightly within the nucleoli (associated with RNA) and in the secretory products of the cytoplasm. Changes in tissue zinc concentration, observed by other workers, following changes in various external stimuli, may not necessarily be reflected by proportionate changes in epithelial concentrations. The role of zinc in the epithelium is considered to be at least two-fold: firstly, for incorporation into vital cellular mechanisms necessary for cell maintenance and, secondly, for involvement in secretory products. It is also possible that the metal participates in the physiology of the sub-epithelial stroma.

Fidelity of ribosomal ribonucleic acid synthesis by nucleoli and nucleolar chromatin.

Abstract: Isolated nucleoli, nucleolar chromatin, and nucleolar DNA were used as templates for DNA synthesis in appropriately supplemented systems in which RNA polymerases other than RNA polymerase I were blocked by alpha-amanitin. With the aid of nucleotide analysis, DNA-RNA hybridization, and homochromatography fingerprinting, it was found that isolated nucleoli and nucleolar chromatin serve primarily as templates for synthesis of rRNA. However, the products formed with purified nucleolar DNA as a template do not contain the specific rRNA oligonucleotides nor are they appreciably hybridized to the rDNA region on cesium chloride gradients. These results indicate that whole nucleoli and nucleolar chromatin contain control mechanisms that restrict readouts by RNA polymerase I of nucleolar DNA to rDNA.

The behavior of nucleolus organizers in structurally changed karyotypes of barley

Abstract: Two standard karyotype barley lines and 18 lines with karyotypes reconstructed by means of induced reciprocal translocations have been studied with respect to nucleolus formation. The standard karyotype contains two pairs of satellite chromosomes (pairs 6 and 7). Five of the structurally changed karyotypes contain, as a result of reciprocal translocations between the standard satellite chromosomes, only one satellite chromosome pair, each chromosome with two satellites and two nucleolus organizing regions. Under these circumstances, only two of the four NORs are active in nucleolus formation while the other two — probably the transposed ones — remain inactive; hence the maximum number of primary nucleoli per nucleus is two. — When NORs are translocated to chromosomes with no NOR in the standard karyotyp, the normal pattern of nucleolus formation remains unchanged. The same is true after transposition of segments from other chromosomes to the satellites of the standard SAT-chromosome pairs 6 and 7. The results obtained are discussed with respect to effects of translocations on the activity and behaviour of nucleolus organizing regions.

Nucleolus formation in structurally reconstructed barley karyotypes with six satellite chromosomes

Abstract: Two structurally reconstructed karyotypes of Hordeum vulgare which, due to appropriate reciprocal translocations, contain three chromosome pairs with nucleolus organizing activity (chromosomes 57, 6 and 77 in translocation line T 21, chromosomes 36, 63, and 7 in translocation line T 627) have been studied with respect to the position and function of rDNA as inferrred from the pattern of nucleolus formation. The results obtained reveal quantitative relationships between the size of the secondary constriction (the amount of rRNA cistrons) and the number and size of nucleoli being formed. Quantitative and qualitative aspects of rDNA location and expression are being discussed.

Antigenic proteins of nucleolar chromatin of Novikoff hepatoma ascites cells.

Abstract: Nucleolar chromatin of Novikoff hepatoma ascites cells contains an antigen (no-Ag1) detected with antinucleolar antibodies by the immunodiffusion technique. This antigen was distinguished from the previously reported nuclear chromatin antigen NAg-1 (19) by the findings that tumor nucleolar antibodies which formed immunoprecipitin bands with no-Ag1 did not do so with NAg-1 and that tumor cytosol, which contains NAg-1, formed immunoprecipitin bands with tumor chromatin antibodies but not with antibodies to tumor nucleoli. Tumor nucleolar chromatin contains both NAg-1 and no-Ag1, but only no-Ag1 formed bands with tumor nucleolar antibodies, no-Ag1 is a component of tumor nucleolar chromatin that was not soluble in 0.075 M NaCl-0.025 M EDTA, pH 8, and only slightly soluble in 0.01 M Tris-HCl, pH 8. NO-Ag1 was not found in liver nucleoli. Antibodies to liver nucleoli formed immunoprecipitin bands with liver nucleolar antigens but none were confluent with those formed between tumor nucleolar antibodies and antigens of tumor nucleolar chromatin. Absorption of the tumor nucleolar antibodies with whole tumor cells or whole liver pressate did not alter band formation with no-Ag1. Three antigens in liver nucleoli were not found in tumor nucleoli.

Nuclear, nucleolar repair, or turnover of dna in adult rat brain

Abstract: Labeled thymidine administered to adult rats is incorporated at a low level into brain DNA as shown by biochemical and autoradiographic methods. This incorporation takes place in part into neuronal nuclei and nucleoli and also into glial nuclei. While incorporation into glial nuclei is interpreted to represent known glial cell proliferation, incorporation into neuronal nucleoli may be related to nucleolar DNA synthesis, which in turn is responsible for the regulation of nucleolar RNA synthesis. It could also be due to DNA repair synthesis. Assuming this latter phenomenon, our results suggest that nucleolar DNA is more sensitive than chromosomal DNA to ionizing radiations and other factors.

Fidelity of synthesis of preribosomal RNA in isolated nucleoli and nucleolar chromatin

Abstract: Comparisons were made of the T1 ribonuclease digests of 32P-labeled nucleolar 45S RNA of intact Novikoff hepatoma cells and the RNA synthesized in vitro by isolated nucleoli. Approximately 200 oligonucleotide spots were found in the two-dimensional chromatogram of 45S nucleolar RNA labeled in vivo, which includes fragments of 18S and 28S rRNA and nonconserved spacer regions; four spots containing 2'-O-methyl nucleotides were not found in the corresponding pattern of RNA labeled in vitro. This high degree of fidelity was retained in the patterns of spots from the RNA produced with nucleolar chromatin as template. This specific expression of rDNA was lost when the nucleolar chromatin was completely deproteinized. Specific spots found in the control patterns were absent and many nonspecific oligonucleotides were found to be labeled. A similar nonspecific chromatogram pattern was found when nucleolar chromatin was transcribed with RNA polymerase (nucleosidetriphosphate:RNA nucleotidyltransferase, EC 2.7.7.6) of Escherichia coli. These results show that specificity of genetic expression in vitro of isolated chromatin of eukaryotic systems is dependent on the chromatin-associated proteins and the type of RNA polymerase present.

Comparison of in vivo and in vitro ribosomal rna synthesis in nucleolar mutants of xenopus laevis

Abstract: Cells of embryos carrying a lethal nucleolar mutation have been maintained in vitro for extended periods of time. Normally these mutants live only 9 to 12 days after fertilization but their cells in culture will survive for more than 3 months. The extent of ribosomal RNA (rRNA) synthesis was determined in primary cultures prepared from normal embryos and nucleolar mutants having different numbers of ribosomal RNA genes. We found that the accumulation of radioactivity into rRNA for normal and mutant embryos was similar in vivo and in vitro. In primary cultures of normal embryos which have two nucleoli per cell and mutant embryos which have only one nucleolus per cell, the incorporation of radio-activity into rRNA was similar even though the normal cells have twice as many rRNA genes. Thus the mechanism which regulates dosage compensation of the rRNA genes operates both in vivo and in vitro.

Nucleolar chromatin in Chinese hamster ovary cells. Topographical distribution of ribosomal DNA sequences and isolation of ribosomal transcription complexes.

Abstract: A stepwise physico-chemical dissection of nucleoli isolated from Chinese hamster ovary cells has allowed the isolation of actively transcribed regions of chromatin, highly enriched in ribosomal genes. Nucleolar chromatin could be separated into its various topographical components. This separation should improve our knowledge of the structure/function relationships in chromatin as the fine structure of the nucleolus is very sensitive to changes in ribosomal RNA synthesis. The spatial distribution of ribosomal genes among the various chromatin areas of the nucleolus was assayed by hybridization with rRNA. In exponentially growing Chinese hamster ovary cells ribosomal genes are predominantly located in the intranucleolar stretches of nucleolus-associated chromatin. However, ribosomal sequences represent but a very minor part (less than 1 %) of intranucleolar DNA. In an attempt to improve the analysis of the actively transcribed chromatin areas, intranucleolar chromatin was submitted to additional fractionation steps involving, among others, isopycnic separation in metrizamide gradients. In these latter conditions a small subset of intranucleolar chromatin banded with preribosomal ribonucleoprotein. In this subfraction, about one-third of the DNA sequences corresponded to preribosomal RNA matrix units. The yield of recovery of this intranucleolar chromatin subfraction being closely related to the level of transcriptional activity of ribosomal genes, it appears likely that this separating procedure depends on the presence of ribosomal RNA transcription complexes and could, therefore, represent a general procedure for the isolation of actively transcribed areas from chromatin.

In vitro transcription of RNA in nuclei, nucleoli and chromatin from physarum polycephalum.

Abstract: Methods for isolating nuclei, nucleoli and chromatin from Physarum polycephalum which retain high levels of endogenous RNA polymerase activity are described. Under carefully controlled conditions with respect to mono- and divalent cation concentrations RNA synthesis in nuclei displayed linear kinetics for at least 30 min and the RNA products had a similar size distribution to nuclear RNA synthesis observed in vivo. Chromatin showed 60% of the nuclear transcriptional activity but no conditions were found where faithful transcription of the template occurred. Isolated nucleoli were 5-fold more active than nuclei and the endogenous RNA polymerase activity was insensitive to alpha-amanitin. Under carefully controlled conditions, the nucleoli appeared to support the accurate transcription, re-initiation and processing of rRNA chains in vitro.

Combined silver staining of the nucleolus organizing regions and Giemsa banding in human chromosomes.

Abstract: A combination of the silver-staining method of the nucleolus organizer regions (NORs) with a Giemsa-banding method is deccribed. This double staining allows a rapid identification of the NOR-bearing chromosomes.

A light- and electron-microscope study of nuclear structure throughout the cell cycle in the euglenoid Astasia longa (Jahn).

Abstract: The structure of nuclei of Astasia longa in synchronized cultures was examined at the light- and electron-microscope levels. Three types of nuclei, differing mainly in chromatin conformation, were observed during interphase and were tentatively classed in the G1, S and G2-periods. The fibrillar nucleolar regions exhibited a most complex organization and appeared to consist of convoluted, coarse filaments or nucleolonemata approximately 0.15 micrometer in diameter. Chromosome condensation was evidenced first by the longer, thicker profiles of chromatin observed in late prophase. Furthermore, the nucleolus, that persists throughout mitosis, began to elongate at late prophase. Furthermore, the nucleolus, that persists thorughout mitosis, began to elongate at this stage, simultaneously with the appearance of short, unoriented profiles of intranuclear microtubules. Chromosome condensation was complete by mid-metaphase and the nucleolus was elongated into a cylindrical shape with irregular extremities. Microtubule profiles were longer than in prophase; they were now oriented parallel to the nucleolus and frequently lay closely appressed to its sides. In anaphase, the chromosomes segregated into 2 groups, one towards each extremity of the dumb-bell-shaped nucleolus. The telophase chromosomes assumed a random orientation with respect to the still intact nucleolus. Throughout the division stages the persiting nucleolus maintained its ultrastructural organization and consisted partly of conspicuous nucleolonemal profiles which tended to be oriented along the major axis of this organelle. Nucleolar separation into 2 fragments occurred late in telophase and was followed by a reformation of daughter nuclei and initiation of cell fission during cytokinesis.

The localization of rDNA in small, nucleolus-like structures in human diplotene oocyte nuclei

Abstract: Small, nucleolus-like structures were demonstrated in the nuclei of human diplotene oocytes. At least some of these bodies were shown to be true micronucleoli by virtue of their ability to bind rRNA during RNA-DNA hybridization in situ.

Ultrastructural and radioautographic investigation of the nucleolar cycle in Physarum polycephalum. Characterization of DNA-containing subunits.

Abstract: The present study has been mainly focused on the nucleolar cycle in the slime mould Physarum polycephalum. The ultrastructural characteristics of the interphase nucleolus, in this species, are quite similar to those of nucleoli in other organisms: it is essentially constituted of large particulate zones surrounding denser regions which are predominantly fibrillar in texture. The latter nucleolar zones, following fixation with osmium tetroxide, are characterized by the presence of opaque granules approximately 25 nm in diameter. Contrary to the situation which generally prevails in other eukaryotes, the late prophase nucleolus fragments into numerous globular bodies which are recognizable by the presence of opaque particles. These fibrillogranular nucleolar fragments persist during mitosis and are observed to become incorporated in the newly formed nucleolus. High-resolution radioautographic observations reveal that these nucleolar remnants contain DNA. The present observations together with recent biochemical data from other authors on the characteristics and mode of duplication of nucleolar DNA in P. polycephalum have led us to the hypothesis that the nucleolus, in this organism, contains several distinct globular subunits each containing ribosomal DNA as a key component. The existence of such morphological subunits appears to account for the unusual behaviour of the nucleolus during the cell cycle.

RNA synthesis in isolated polytene nuclei from Chironomus tentans.

Abstract: An RNA synthesizing system with isolated polytene nuclei from Chironomus tentans is described. This system allows one to monitor the effect of salt concentration on chromosome structure and to assign in vitro RNA synthesis to structural modifications of the chromosome (i.e. nucleoli, Balbiani rings and puffs).-At a salt concentration of 0.15 M monovalent cations (standard salt medium=SSM) chromosomal structure appears to be best preserved during in vitro incubation. At low and high ionic strength the bands decondense and the microscopically visible chromosomal structure is lost completely. These three states of condensation and decondensation are distinguished with respect to RNA synthesis: (1) in low salt overall RNA synthesis is depressed, (2) in SSM ribosomal RNA synthesis predominates and continues for 30 min, (3) in high salt RNA synthesis is stimulated 3–4 fold again. This stimulation is due solely to chromosomal, non-ribosomal RNA synthesis, which proceeds in high salt for more than 10 h, though new initiation of RNA chains is prevented. Molecular weight determinations of the RNA synthesized demonstrate a time dependent increase in size of the newly synthesized molecules under these conditions. — Autoradiographs of nuclei incubated in SSM reveal prominent label in nucleoli, significant label in Balbiani rings and rather reduced activity at other sites. Addition of various exogenous RNA polymerases does not markedly alter this pattern. Autoradiographs of nuclei incubated in high salt exhibit extensive RNA synthesis spread over the chromosomes. Preparations of autoradiographs from isolated chromosomes show that the high salt induced label is localized in single bands. Though the majority of bands is still unlabelled, the actual number of bands exhibiting incorporation in high salt is higher than in any individual functional state in vivo. These results are discussed in terms of activated and preactivated genes.

Association of nucleolus organizer chromosomes in domestic sheep (Ovis aries) shown by silver staining.

Abstract: The nucleolus organizer regions of domestic sheep (Ovis aries), as shown by silver staining, are located terminally on chromosomes 1, 2, 3, 4, and 25. Significant differences between individuals in the number of Ag-NORs per cell were found. The frequency of involvement of individual chromosome pairs in nucleolar organization was found to be a characteristic of individual animals. Association frequencies of individual chromosomes were accounted for by their frequency of participation in nucleolar organization. No evidence for nonrandom association of chromosome pairs was found.