Showing papers on "Nucleolus published in 1970"

Specific Nucleolar and Nucleoplasmic RNA Polymerases

Abstract: The DNA-dependent RNA polymerase activity present in rat liver nuclei has been solubilized and purified from whole nuclei and from subnuclear fractions. As reported earlier (Roeder, R. G., and W. J. Rutter, Nature, 224, 234 (1969)), two major chromatographically distinct enzymatic species (I and II) are present in whole nuclei. Subfractionation of whole nuclei into nucleolar and nucleoplasmic fractions had little effect on the total recovery of activity. Purified nucleoli contain predominantly polymerase I, whereas the nucleoplasmic fraction is greatly enriched for polymerase II. A third minor peak of activity has also been resolved in the nucleoplasmic preparations. We conclude that the RNA polymerases are specifically localized within the nucleus and may, therefore, play specific roles in the regulation of genetic transcription.

The ribosome cycle, nucleoli, and cytoplasmic nucleoloids in the meiocytes of Lilium.

Abstract: During the meiotic prophase in pollen mother cells ofLilium species the ribosome population of the cytoplasm diminishes to a minimum reached during the diplotene-diakinesis interval. This fall is associated with a drop in the ribonuclease-extractable RNA present in whole, fixed cells. A rise in RNA and in number of observable ribosomes follows during the meiotic mitoses and in the very early life of the spores. Bodies with the characteristics of nucleoli—here termed “nucleoloids”—are released into the cytoplasm at anaphase I and anaphase II, and it is suggested that these may represent the main mechanism through which the ribosome population of the cytoplasm is restored after the prophase elimination.

Fine structural changes in theDrosophila oocyte nucleus during a short period of RNA synthesis : An autoradiographic and ultrastructural study of RNA synthesis in the oocyte nucleus ofDrosophila.

Abstract: H3-uridine was injected into the abdomen ofD. melanogaster andD. immigrans and after 10, 30, 60 and 120 min of incorporation, the ovaries were prepared for autoradiography. The oocyte nucleus was found to synthesize RNA during a short period of vitellogenesis (stage 10A). Ultrastructural studies of the oocyte nucleus were made at the stage active in RNA synthesis and many electron-dense structures were found to appear at this time. Since none of these structures resembled nucleoli in fine structure, it is suggested that the RNA synthesized is non-ribosomal. Other ultrastructural modifications of the oooyte nucleus are presented and discussed.

Aberrant intranucleolar maturation of ribosomal precursors in the absence of protein synthesis

Abstract: To help elucidate the role of protein in the maturation of ribosomal RNA in cultured L cells, we have studied the effects of cycloheximide upon the maturation process and upon the intranucleolar ribonucleoprotein particles containing the "preribosomal RNA's." Five parameters of these particles were analyzed: (a) extractability, (b) sedimentation characteristics in sucrose gradients, (c) RNA composition, (d) buoyant density in CsCl gradients, and (e) effects of increased ionic strength on the buoyant density. When protein synthesis is inhibited, the rate of conversion of the precursor 45S ribosomal RNA is rapidly diminished, falling to less than 30% of the control rate within 1 hr. Nevertheless, in terms of the first three parameters there is no difference between control and cycloheximide nucleolar particles. However, the cycloheximide particles have a lower and more heterogeneous buoyant density and a more variable response to increased ionic strength. The results imply that the protein composition of the cycloheximide particles is different from that of particles from control cells, and that the entire protein complement is not necessary for the first cleavages in the maturation process, although it is necessary for the normal rate of processing and for the eventual appearance of both 18S and 28S rRNA in mature ribosomes.

The development of the nucleolus of the ovarian nurse cell of Drosophila melanogaster

Abstract: A description is given of the development of the nucleoli of the ovarian nurse cells of Drosophila melanogaster during stages 7 through 10 of oogenesis. This developmental period lasts about a day, and during it the volumes of the nurse cell nucleolus, nucleus and cytoplasm all double once every 4–5 hours. The nucleolar bodies within the endopolyploid nurse cell nucleus grow until they form a thick network that is shaped like a shell whose outer boundary lies close to the inner surface of the nuclear envelope. RNA of nucleolar origin continually enters the cytoplasm. The nuclei of the nurse cells directly connected to the oocyte are most active in terms of DNA replication and RNA transcription. The nurse cells empty their cytoplasm into the oocyte which doubles its volume every 2 hours. The ribosomes stored in the ooplasm are derived almost exclusively from the nurse cell. The doubling time for the rDNA of the nurse cells is about 9 hours, and about 1,000 rRNA molecules are transcribed per rDNA cistron per hour during vitellogenesis.

Effect of Hyperthermia on Normal and Neoplastic Cells in Vitro

Abstract: The effects of hyperthermia on 3 lines of HeLa cells, 3 lines of human diploid cells, and BHK 21 cells were similar. Exposure of exponentially growing cells to 45–46° for periods of 15 to 60 min resulted in disappearance of nucleolar DNA and diffusion of nucleolinar material, as demonstrated by toluidine blue-molybdate, acid phosphatase, and lead staining throughout the nucleolus. Ultrastructural studies of the heated cells indicated that the fibrillar component of the nucleolus, like the nucleolini, became diffusely distributed throughout the nucleolus and that the granular component of the nucleolus almost completely disappeared. Hyperthermia led to the formation of cytoplasmic inclusions in 100% of cells with nucleoli. The inclusions were cytochemically similar to the body of the nucleolus observed by light microscopy and consisted of aggregations of ribosome-like granules in electron micrographs. Evidence is presented to indicate that the inclusions were derived from the nucleolus and possibly from the granular component. Hyperthermia also resulted in disaggregation of polysomes. The effects of heating to 46° for 15 min were completely reversible.

Time Course and Dose-Response Characteristics of Aflatoxin B1 Effects on Rat Liver RNA Polymerase and Ultrastructure

Abstract: Summary Treatment of 100-g male Fischer rats with an i.p. dose of 1 mg/kg of aflatoxin B1 causes a rapid and marked inhibition of the activity of liver DNA-dependent RNA polymerase. Over a period of 36 hr after dosing, ultrastructural alterations of the liver cell nucleolus generally correlate with the inhibition of enzyme activity. Mg++-activated and Mn++-(NH4)2SO4-activated RNA polymerase activities, 15 min after dosing, are maximally inhibited by about 60%; this inhibition continues to 12 hr. By 36 hr, enzyme activities have returned to pretreatment levels. Hepatocyte nucleoli 15 min after treatment show decreased prominence of nucleolonema, and nucleolar microsegregation is observed. Macrosegregation of granular and fibrillar nucleolar components (nucleolar capping) is demonstrated within 1 hr after treatment, and the segregation persists up to 12 hr. By 36 hr, reintegration of nucleolar components is observed. Inhibition of RNA polymerase activity is directly related to aflatoxin B1 dose, with a correlation coefficient of -0.89 for the Mg++-activated system and -0.97 for the Mn++-(NH4)2SO4-activated system. Nucleolar disorganization is observable at 0.2 mg/kg aflatoxin B1 but is not demonstrable at 0.1 mg/kg, a dose which is capable of a 5% inhibition of both types of enzyme activity. A decrease in nuclear RNA/DNA ratio is observed under conditions where RNA polymerase activity is decreased.

Alteration of nucleolar ultrastructure in iris epithelial cells during initiation of Wolffian lens regeneration.

Abstract: Morphological aspects of nucleoli of iris epithelial cells participating in Wolffian lens regeneration in the adult newt (Triturus viridescens) were studied with electron and phase-contrast microscopes at various times after lens removal. In normal condition, the nucleoli appear as small, compact bodies composed of fibrous elements. The granular element is often lacking, but when present, is sparse and located at the periphery of the organelle. Within two days after lens removal, the size of the nucleolus increases, and its shape becomes complex and diversified. The granular element increases in absolute as well as relative amount. Granular and fibrous components become intermingled and together form the fibrogranular region. Portions of this region often project from the nucleolus as coarse threadlike extensions that become a prominent feature. Morphogenesis of the organelle continues during the subsequent two days. In parallel with these structural changes, the number of nucleoli per nucleus and the frequency of nuclei containing nucleoli increases. All these changes start in the nondividing phase of the iris epithelium before induced cell division. These changes in nucleoli are the earliest so far observed in iris epithelial cells after lens removal, and coincide in time with the activation of ribosomal RNA synthesis previously observed in this system.

Mutations affecting the size of the nucleolus in Xenopus laevis.

Abstract: In Xenopus laevis, nucleolar mutants which form a partial nucleolus have been isolated. These provide a unique opportunity for the genetic analysis of the very reiterated ribosomal RNA genes.

RNA SYNTHESIS IN HELA CELLS Pattern in Hypertonic Medium and Its Similarity to Synthesis during G2-Prophase

Abstract: Interphase HeLa cells manifest a stepwise shutoff of RNA synthesis when the tonicity of the extracellular medium is gradually increased. Synthesis of heterogeneous nuclear RNA is most sensitive and is selectively inhibited at 1.5 times isotonicity (450 milliosmols/liter), while 45S ribosomal RNA synthesis is not affected significantly below 2.0 times isotonicity. Transfer RNA synthesis is least sensitive to increased osmolarity and is not completely inhibited until the electrolyte concentration of the medium is elevated to 2.8 times isotonicity. Although the transcription and methylation of 45S ribosomal precursor is unaffected at 1.5 times isotonicity, there is pronounced impairment of its processing into 32S and 18S RNA. Using a refined cell synchronization technique, we have been able to compare these effects of hypertonicity with the shutoff of RNA synthesis which occurs during the G2-prophase interval of the cell division cycle. In this case, as with random cells in hypertonic medium, a selective inhibition of heterogeneous nuclear RNA synthesis and slowed processing of 45S ribosomal RNA were found, whereas synthesis of 45S and transfer RNA continued unabated throughout G2-prophase. While it is known that RNA synthesis essentially ceases during metaphase, we have noted that transfer RNA synthesis continues in metaphase at 10–15% of the interphase rate, which is of particular interest in view of the relative resistance of this species to hypertonicity. The close correlation between the patterns of cessation of RNA synthesis at mitosis and during exposure to hypertonic medium supports our earlier contention that alteration of intracellular electrolyte levels provides a useful model for studying the mechanism of mitosis.

A cytochemical and radioautographic study of human tissue culture cell nucleoli

Abstract: Fine structural aspects of human tissue culture cell nucleoli were studied by cytochemical and radioautographic methods. Ribonuclease and pepsin digestions were carried out on glutaraldehyde-fixed cells that, in some instances, were labeled with thymidine-3H prior to digestion. Double digestion by ribonuclease and pepsin revealed a fine fibrillar reticulum that appears to be the supportive structure of nucleolonemal threads. The nature of the reticulum remains to be determined. The question of whether it may represent a dispersed form of chromatin was raised. Structural findings suggested such an hypothesis but the results of radioautographic studies do not support it. The reticulum showed a striking absence of radioactive labeling following a 3 hr incorporation of thymidine-3H. Only few silver grains were observed occasionally in the fibrillar nucleolonema that may or may not be significant. The radioautographic results are believed to be inconclusive for the various reasons discussed. The possibility that the reticulum is composed of proteins has to be considered. It appears that basic proteins can resist pepsin digestion in aldehyde-fixed cells. Individual chromatin fibrils were found to be associated with the nucleolar reticulum. It is possible that these alone represent the dispersed genetically active chromatin of nucleoli.

Sites of Synthesis and Processing of Ribosomal RNA Precursors within the Nucleolus of Urechis caupo Eggs

Abstract: Nucleoli from unfertilized Urechis eggs, labeled with tritiated RNA precursors, have been isolated for simultaneous autoradiographic localization and biochemical analysis of labeled RNA. The production of the ribosomal RNA precursor (38S) and its first cleavage occur at the fibrillar core region of the nucleolus. The products, predominantly 30S RNA, are then rapidly transported and stored in the granular cortex of the nucleolus. The formation of the nucleolar cortex, therefore, seems to result from an accumulation of partially processed ribosomal RNA with its associated proteins.

Comparative Studies on the Ultrastructure of Nucleoli in Human Lymphosarcoma Cells and Leukemic Lymphocytes

Abstract: Summary The ultrastructural morphology of nucleoli was investigated in lymphosarcoma cells and leukemic lymphocytes of patients who were not treated with antitumor or antileukemic therapy. The nucleoli in lymphosarcoma cells were compact, contained well-defined nucleolonemas, or were ring-shaped; occasionally, transitional forms between these types of nucleoli were observed. The ultrastructure of the ring-shaped nucleoli present in differentiated lymphosarcoma cells was similar to that of mature leukemic lymphocytes. These nucleoli were characterized by a peripheral ribonucleoprotein shell surrounding a central core, adjacent to which there were chromatin clusters. A decrease of the granular components was noted in the ring-shaped nucleoli of both differentiated lymphosarcoma cells and mature leukemic lymphocytes, as compared with the compact nucleoli or nucleoli with nucleolonemas of the less differentiated lymphosarcoma cells or immature leukemic lymphocytes. The decrease of granular components in the ring-shaped nucleoli may reflect a low rate of the formation of granular nucleolar ribosomal precursors. In addition, small microspherules were noted in nucleoli of some lymphosarcoma cells.

Vacuoles in the Nucleoli of Zea Mays Root Apices and their Possible Significance in Nucleolar Physiology

Abstract: SUMMARYNucleolar vacuoles in eight regions of the root apex of Zea mays have been studied. The volume of the vacuoles ranges from about 1 μm3 to 0.001 μm3 with a mode value between 0.075 and 0.01 μm3. The number of vacuoles within a nucleolus ranges from 0–20 and is correlated with nucleolar volume. In all regions studied the average volume of the nucleolus occupied by vacuoles is about 3 percent. It is suggested that the metabolic capacity of the nucleolus may be reflected in the rate of formation of the vacuoles and the discharge of their contents to the cell.