Showing papers on "Nucleolus published in 1985"
01 Jan 1985
TL;DR: In this paper, the authors describe the structure of preribosomes and their role in the pre-rRNA precoding process, and discuss the role of RNA polymerases in the precoding.
Abstract: I. Introduction.- II. Ribosomal Genes.- II. 1. Definitions.- II.2. Ribosomal RNA Genes.- II.2.1. Multiplicity.- II.2.2. Chromosomal Location.- II.2.3. Extrachromosomal rRNA Genes.- II.2.4. Organization and Structure.- II.2.4.1. Saccharomyces cerevisiae.- II.2.4.2. Tetrahymena.- II.2.4.3. Drosophila.- II.2.4.4. Xenopus laevis.- II.2.4.5. Higher Plants.- II.2.4.6. Mammalia.- II.2.5. General Features.- II.3. 5 S rRNA Genes.- II.3.1. Number and Chromosomal Location.- II.3.2. Organization and Structure.- II.4. Ribosomal Protein Genes.- II.5. Synopsis.- III. Transcription of Ribosomal Genes.- III. 1. Components of the Transcription Complex.- III. 1.1. RNA Polymerases.- III. 1.2. Nucleolar rDNA and r-Chromatin.- III.2. The Transcription Process >.- III.2.1. Topology of Primary Pre-rRNA.- III.2.2. Morphology of Transcribed rRNA Genes.- III.2.3. Transcribed and Non-Transcribed r-Chromatin.- III.2.4. Primary Transcripts and Primary Pre-rRNA.- III.2.5. Transcription Initiation and Termination.- III.2.5.1. Initiation.- III.2.5.2. Termination.- III.2.6. Transcription in vitro.- III.3. Transcription of 5 S rRNA Genes.- III.4. Transcription of r-Protein Genes.- III.5. Synopsis.- IV. Maturation of Preribosomes.- IV. 1. Structure of Primary Pre-rRNA.- IV. 1.1. Size and Primary Structure.- IV. 1.2. Modifications.- IV. 1.3. Conformation.- IV.2. Pre-rRNA Maturation Pathways.- IV.2.1. General Considerations.- IV.2.2. Common Pattern of Pre-rRNA Maturation.- IV.2.3. Multiplicity of Maturation Pathways.- IV. 2.4. Enzyme Mechanisms.- IV. 3. Preribosomes: Structure and Maturation.- IV. 4. Synopsis.- V. Molecular Architecture of the Nucleolus.- V. 1. Introduction.- V.2. Nucleolus Organizer.- V. 2.1. Chromosomes.- V.2.2. Interphase Nuclei.- V.3. Fibrillar and Granular Components.- V.3.1. The Fibrillar Component.- V.3.2. The Granular Component.- V.4. The Nucleolus and Other Nuclear Structures.- V.4.1. Nucleolus-Associated Chromatin.- V.4.2. The Junction with the Nuclear Envelope.- V.5. The Nucleolar Matrix.- V.6. Macromolecular Constituents.- V.6.1. DNA and RNA.- V. 6.2. Nucleolar Proteins.- V.6.2.1. General.- V.6.2.2. Ag-NOR Protein(s).- V. 6.2.3. Nucleolar Antigens.- V. 7. Outline.- VI. Regulation.- VI. l. General Considerations.- VI.2. Transscriptional Control.- VI. 2.1. Transitions in the State of Expression of rRNA Genes.- VI. 2.1.1. Inactive r-Chromatin.- VI.2.1.2. Potentialy Active and Transcribed rRNA Genes.- VI.2.2. Control of Transcription Rate.- VI.2.2.1. Role of RNA Polymerase I.- VI.2.2.2. Supply of Nucleoside-5'-Triphosphates...- VI.2.2.3. Role of Protein Synthesis.- VI.3. Posttranscriptional Control.- VI.3.1. Synthesis and Supply of r-Proteins.- VI.3.2. The Role of Pre-rRNA Structure.- VI.3.3. The Role of 5 S rRNA.- VI.3.4. Critical Control Sites.- VI.3.4.1. Alternative Processing Pathways and Intranuclear Degradation of Preribosomes and Ribosomes.- VI.3.4.2. Release From the Nucleolus and Nucleo- Cytoplasmic Transport of Ribosomes.- VI.3.4.3. Turnover of Ribosomes.- VI.4. Autogeneous Regulation of Ribosome Biogenesis in Eukaryotes: A Model.- VI.5. Synopsis.- VII. Ribosome Biogenesis in the Life Cycle of Normal and Cancer Cells.- VII. 1. Nucleologenesis and Nucleololysis.- VII. 1.1. Nucleoli and Ribosome Biogenesis During the Mitotic Cycle.- VII. 1.2. Nucleologenesis.- VII. 1.3. Nuclyeololysis.- VII.2. Inhibition of Ribosome Biogenesis.- VII.2.1. Inhibitors Interacting With DNA and Chromatin.- VII.2.2. Inhibitors That Act on RNA Polymerases.- VII.2.3. Inhibitors of Nucleoside-5'-Triphosphate Formation.- VII.2.4. The Effects of Analogues Incorporated into Polyribonu- cleotide Chains.- VII.2.5. Inhibitors of Protein Synthesis.- VII.2.6. Interpretation of Nucleolar Alterations.- VII.2.6.1. Nucleolar Segregation.- VII.2.6.2. Nucleolar Spherical Bodies and Perichromatin Granules.- VII.2.6.3. Microspherules.- VII.2.6.4. Nucleolar Fragmentation.- VII.3. Growth Transitions.- VII.3.1. Modulation of Growth Rates in Yeasts.- VII.3.2. Activation of Lymphocytes.- VII.3.3. Growth Stimulation of Cultured Cells.- VII.3.4. Differentiation of Myoblasts in Culture.- VII.3.5. Regeneration of Rat Liver.- VII.4. Senescent and Cancer Cells.- VII.4.1. Senscent Cells and Tissues.- VII.4.2. Cancer Cells.- VII.5. Synopsis.- References.
514 citations
TL;DR: Fibrillarin was found to be an early marker for the site of formation of the newly forming nucleolus by indirect immunofluorescence and immunoelectron microscopy as discussed by the authors.
448 citations
TL;DR: Using both electron microscopy and immunological methods, a number of changes occurring in rat fibroblasts after heat-shock treatment are characterized, including changes in both the number and size of the granular ribonucleoprotein components and a reorganization of the nucleolar fibrillar reticulum.
Abstract: Using both electron microscopy and immunological methods, we have characterized a number of changes occurring in rat fibroblasts after heat-shock treatment. Incubation of the cells for 3 h at 42 degrees-43 degrees C resulted in a number of changes within the cytoplasm including: a disruption and fragmentation of the Golgi complex; a modest swelling of the mitochondria and subtle alterations in the packing of the cristae; and alterations in cytoskeletal elements, specifically a collapse and aggregation of the vimentin-containing intermediate filaments around the nucleus. A number of striking changes were also found within the nuclei of the heat-treated cells: (a) We observed the appearance of rod-shaped bodies consisting of densely packed filaments. Using biochemical and immunological methods, these nuclear inclusion bodies were shown to be comprised of actin filaments. (b) Considerable alterations in the integrity of the nucleoli were observed after the heat-shock treatment. Specifically, there appeared to be a general relaxation in the condensation state of the nucleoli, changes in both the number and size of the granular ribonucleoprotein components, and finally a reorganization of the nucleolar fibrillar reticulum. These morphological changes in the integrity of the nucleoli are of significant interest since previous work as well as studies presented here show that two of the mammalian stress proteins, the major stress-induced 72-kD protein and the 110-kD protein, localize within the nucleoli of the cells after heat-shock treatment. We discuss these morphological changes with regards to the known biological and biochemical events that occur in cells after induction of the stress response.
443 citations
TL;DR: The result indicates that, for at least the 72-kDa protein, the protein has been purified in apparently "native" form and its nucleolar distribution is stress dependent.
Abstract: A new and rapid purification procedure has been developed for the mammalian 70,000-dalton (70-kDa) heat-shock (or stress) proteins. Both the constitutive 73-kDa protein and the stress-induced 72-kDa protein have been purified by a two-step procedure employing DE52 ion-exchange chromatography followed by affinity chromatography on ATP-agarose. The two proteins, present in approximately equal amounts in either the 12,000 X g supernatant or pellet of hypotonically lysed heat-shock-treated HeLa cells, were found to copurify in relatively homogenous form. The purified proteins were covalently labeled with the fluorescent dye tetramethylrhodamine isothiocyanate, and the fluorescently labeled proteins were introduced back into living rat embryo fibroblasts via microinjection. The microinjected cells maintained at 37 degrees C showed only diffuse nuclear and cytoplasmic fluorescence. After heat-shock treatment of the cells, fluorescence was observed throughout the nucleus and more prominently within the nucleolus. This result is consistent with our earlier indirect immunofluorescence studies which showed a nuclear and nucleolar distribution of the endogenous 72-kDa stress protein in heat-shock-treated mammalian cells. The result also indicates that, for at least the 72-kDa protein, (i) the protein has been purified in apparently "native" form and (ii) its nucleolar distribution is stress dependent.
386 citations
TL;DR: A model in which ATP‐driven cycles of binding and release of hsp70 help to solubilize aggregates of proteins or RNPs that form after heat shock is proposed.
Abstract: The major heat shock protein, hsp70, is an ATP-binding protein which is synthesized in very large amounts in response to stress. In unstressed, or recovered, mammalian cells it is found in both nucleus and cytoplasm. Under these conditions, its interaction with nuclei is weak, and it is readily released from them upon lysis of cells in isotonic buffer. After heat shock, hsp70 binds tightly first to some nuclear component(s) and then to nucleoli. It can be released from these binding sites rapidly and specifically in vitro by as little as 1 microM ATP, but not by non-hydrolysable ATP analogues. Studies of hsp70 deletion mutations show that the ability of mutants to be released by ATP correlates with their ability to migrate to heat-shocked nucleoli and aid their repair in vivo. We propose a model in which ATP-driven cycles of binding and release of hsp70 help to solubilize aggregates of proteins or RNPs that form after heat shock. Cells also contain proteins related to hsp70 that are synthesized in the absence of stress. The most abundant of these shows the same behaviour as hsp70 after heat shock, and thus may perform a related function in both normal and stressed cells.
311 citations
TL;DR: This nucleolar 34-kDa protein resembles several nucleoplasmic proteins that are associated with heterogeneous nuclear RNA with respect to isoelectric point, Mr, presence of NG,NG-dimethylarginine, and its high glycine content.
241 citations
TL;DR: Ribosomal DNA sequences are at least 20‐fold enriched in topoisomerase/DNA complexes isolated directly from a chromosomal setting, relative to total DNA, which is the first direct evidence that topoisomersase I is catalytically active on ribosomalDNA in vivo.
Abstract: The distribution of eukaryotic DNA topoisomerase I in the cell has been analyzed at four levels: (i) at the level of the nuclear matrix; (ii) at the cytological level by immunofluorescence of whole cells; (iii) at the electron microscopic level using the protein A/colloidal gold technique; and (iv) at the level of DNA to identify in situ the sequence upon which topoisomerase I is catalytically active. Although topoisomerase I is clearly distributed non-randomly in the nucleus, the unique distribution of the enzyme is not related to the nuclear matrix. The data support the conclusion that topoisomerase I is heavily concentrated in the nucleolus of the cell; furthermore, particular regions within the nucleolus are depleted of topoisomerase. A technique has been developed which allows isolation and analysis of the cellular DNA sequences covalently attached to topoisomerase. Ribosomal DNA sequences are at least 20-fold enriched in topoisomerase/DNA complexes isolated directly from a chromosomal setting, relative to total DNA. This is the first direct evidence that topoisomerase I is catalytically active on ribosomal DNA in vivo.
199 citations
TL;DR: Silver staining is a simple, reliable cytological method for the demonstration of ribosomal gene activity and biochemical analyses of nucleolar proteins indicate that a limited array of specific proteins bind silver.
Abstract: Silver nitrate has been used as a cytological stain since the late 1800s. A modification of the Bielschowsky technique preferentially stains nucleoli and chromosomal nucleolus organizer regions (NORs). The specificity of staining is related to the method of preparation of the cytological specimens. The silver binds proteins and may be associated with the phosphate groups of certain phosphoproteins. Biochemical analyses of nucleolar proteins indicate that a limited array of specific proteins bind silver. A number of investigations have demonstrated that silver staining is indicative of active ribosomal RNA transcription, although a minor component may be associated with the fibrillar centers of cells in which ribosomal genes are inactive. Silver staining is a simple, reliable cytological method for the demonstration of ribosomal gene activity.
168 citations
TL;DR: To elucidate the possible role of nucleolar phosphoprotein B23 in ribosome synthesis, drugs which inhibit the processing of ribosomal RNA were employed and protein B23 was found co-localized with the pre-rRNPs as determined by ELISA assays which agrees with previous studies.
153 citations
TL;DR: Results demonstrate that the nucleolus is formed by fusion of prenucleolar structures whose biochemical composition resembles the matureucleolus, since PNBs contain at least two of the major nucleolar proteins.
Abstract: A time course study was conducted on nucleologenesis after release from a mitotic block in the presence and absence of actinomycin D to determine the composition and fate of prenucleolar bodies (PNBs). Prenucleolar bodies, whether naturally occurring or induced by actinomycin D treatment, stain with silver and contain phosphoproteins B23 and C23, two of the major proteins of the interphase nucleolus as determined by double label immunofluorescence with specific antibodies. The nucleolus is formed by fusion of PNBs, which subsequently “reorganize” and form internal fibrillar and peripheral granular regions. Actinomycin D prevents fusion of PNBs, which are then randomly dispersed throughout the nucleus but they still contain proteins B23 and C23. These results demonstrate that the nucleolus is formed by fusion of prenucleolar structures whose biochemical composition resembles the mature nucleolus, since PNBs contain at least two of the major nucleolar proteins.
125 citations
TL;DR: This chapter discusses the evolution of self-splicing RNA and highlights Tetrahymena pre-rRNA splicing is self-catalyzed, which lowers the activation energy for a chemical reaction in which that same molecule is a reactant.
Abstract: Publisher Summary This chapter discusses the evolution of self-splicing RNA Tetrahymena , a genus of ciliated protozoa, has some unique advantages for studies of rRNA genes and their expression The rRNA genes (rDNA) reside in the nucleoli of the macronucleus as linear, extrachromosomal molecules Unlike chromosomal rDNA in many higher eukaryotes, where the rDNA is heterogeneous because of variations in the nontranscribed spacer, the Tetrahymena rDNA molecules are essentially homogeneous in size and nucleotide sequence Tetrahymena , like the other ciliated protozoa, contains two types of nuclei within the same cell The transcriptionally active macronucleus is polyploid and divides mitotically during vegetative growth The germinal micronucleus is diploid and divides mitotically In addition, the chapter also highlights Tetrahymena pre-rRNA splicing is self-catalyzed Therefore, the molecule lowers the activation energy for a chemical reaction in which that same molecule is a reactant The lowering of activation energy results in a large acceleration in the rate of the reaction The reactions catalyzed by the RNA are described in the chapter
TL;DR: A remarkable correlation between cyclin antigen distribution and topographical patterns of DNA synthesis is shown and argues for a role of cyclin in some aspect of DNA replication.
TL;DR: Observations indicate that the nucleolus-derived particles containing ribosomal protein S1 are different from cytoplasmic ribosomes and, in the living cell, are selectively recollected after mitosis into the newly formed nuclei and translocated into a specific nucleolar subcompartment, i.e., the granular component.
Abstract: Using antibodies to various nucleolar and ribosomal proteins, we define, by immunolocalization in situ, the distribution of nucleolar proteins in the different morphological nucleolar subcompartments. In the present study we describe the nucleolar localization of a specific ribosomal protein (S1) by immunofluorescence and immunoelectron microscopy using a monoclonal antibody (RS1-105). In immunoblotting experiments, this antibody reacts specifically with the largest and most acidic protein of the small ribosomal subunit (S1) and shows wide interspecies cross-reactivity from amphibia to man. Beside its localization in cytoplasmic ribosomes, this protein is found to be specifically localized in the granular component of the nucleolus and in distinct granular aggregates scattered over the nucleoplasm. This indicates that ribosomal protein S1, in contrast to reports on other ribosomal proteins, is not bound to nascent pre-rRNA transcripts but attaches to preribosomes at later stages of rRNA processing and maturation. This protein is not detected in the residual nucleolar structures of cells inactive in rRNA synthesis such as amphibian and avian erythrocytes. During mitosis, the nucleolar material containing ribosomal protein S1 undergoes a remarkable transition and shows a distribution distinct from that of several other nucleolar proteins. In prophase, the nucleolus disintegrates and protein S1 appears in numerous small granules scattered throughout the prophase nucleus. During metaphase and anaphase, a considerable amount of this protein is found in association with the surfaces of all chromosomes and finely dispersed in the cell plasm. In telophase, protein S1-containing material reaccumulates in granular particles in the nucleoplasm of the newly formed nuclei and, finally, in the re-forming nucleoli. These observations indicate that the nucleolus-derived particles containing ribosomal protein S1 are different from cytoplasmic ribosomes and, in the living cell, are selectively recollected after mitosis into the newly formed nuclei and translocated into a specific nucleolar subcompartment, i.e., the granular component. The nucleolar location of ribosomal protein S1 and its rearrangement during mitosis is discussed in relation to the distribution of other nucleolar proteins.
TL;DR: It is suggested that ribocharin is a specific component of precursor particles of the large ribosomal subunit, which dissociates from the 65S particle before passage through the nuclear envelope, and is reutilized in ribosome biogenesis.
TL;DR: Although total nucleolar volume differs between cells at different stages of development, the genetic variation influencing volume appears to have similar effects in root tip, premeiotic and young pollen cells.
Abstract: The major nucleoli in the wheat variety Chinese Spring are formed at the nucleolus organisers (NORs) on chromosomes 1B and 6B. Minor nucleoli are formed from the NORs on chromosomes 5D and 1A. Nucleolar volume is poorly correlated with the number of ribosomal RNA genes in the NOR region. The nucleolus on chromosome 1B is twice the volume of that on chromosome 6B even though only half as many ribosomal RNA genes reside at the 1B locus compared with the number at the 6B locus. Nucleolus size is correlated with the size of the NOR constriction seen in metaphase chromosomes. When major NORs are deleted, the volumes of the remaining nucleoli increase to compensate for the deletions. Varying the dosage of many of the chromosomes in the total complement influences the volume of the minor nucleoli. Chromosomes 1D and 6B have been shown to carry genes regulating total nucleolar volume. Although total nucleolar volume differs between cells at different stages of development, the genetic variation influencing volume appears to have similar effects in root tip, premeiotic and young pollen cells.
TL;DR: The 100 K association with the preribosomal RNP components of the interphase nucleolus supports evidence for a role of this protein in pre‐rRNA transcription and pre ribosomal processing.
TL;DR: Taste buds in foliate papillae of the rhesus monkey were examined by electron microscopy and three distinct cell types were identified, consistent with the notion that cell renewal occurs in each of the three cell populations.
Abstract: Taste buds in foliate papillae of the rhesus monkey were examined by electron microscopy. Three distinct cell types were identified. Type I cells were narrow elongated cells containing an oval nucleus, bundles of intermediate filaments, several Golgi bodies, and characteristic apical membrane-bounded dense granules. These cells exhibited morphological variations: some had a moderately dense cytoplasm, perinuclear free ribosomes, and flattened sacs of rough endoplasmic reticulum; others had a more lucent cytoplasm, dilated irregular rough endoplasmic reticulum, lysosome-like dense bodies, and lipid droplets. Type II cells typically contained a spherical, pale nucleus, a prominent nucleolus, supranuclear and infranuclear Golgi bodies, mitochondria with tubular cristae, and one or two centrioles. This cell type, too, showed some variation in the relative amounts of ribosomes and smooth endoplasmic reticulum, which varied inversely with each other. Type III cells were characterized by a clear apical cytoplasm essentially devoid of ribosomes and containing microtubules. In a few type III cells, the peri- and infranuclear regions contained many ribosomes and some rough endoplasmic reticulum. In most Type III cells, there were large numbers of dense and clear vesicles in the peri- and infranuclear regions; some of the vesicles were grouped in synapse-like arrangements with adjacent nerves. The morphological variations exhibited by all three cell types could be accounted for by age differences in each of the cells. This would be consistent with the notion that cell renewal occurs in each of the three cell populations.
TL;DR: Bright nucleolar immunofluorescence was observed in HeLa S3 cells by immunostaining with a monoclonal antibody to the nucleolar phosphoprotein B23, and this change in localization of fluorescence indicated that protein B23 had migrated out of the nucleoli.
TL;DR: This chapter describes the structural and functional aspects of NORs of human chromosomes and discusses the clinical significance of these regions.
Abstract: Publisher Summary The nucleolar organizer regions (NORs) have been demonstrated to be the morphological sites around which the nucleoli develop at the end of mitosis. This chapter describes the structural and functional aspects of NORs of human chromosomes and discusses the clinical significance of these regions. The chromosomal associations resulted from NORs play an important role in at least three types of chromosomal disorders, and the most frequent of them is the meiotic nondisjunction causing trisomic condition. The degree of NOR activity is represented by different types of nucleoli, such as small ring-shaped nucleoli with low activity and compact nucleoli with full activity. The human acrocentric chromosomes because of the presence of NORs on short arms have a tendency to remain associated with each other during the cell division, which have deleterious consequences through meiotic nondisjunction. However, the biological and clinical implications of NOR size heteromorphisms of human acrocentrics are poorly understood.
TL;DR: These studies demonstrate that specific antisera against cAMP-dependent protein kinase subunits may be used in combination with immunogold electron microscopy to identify the ultrastructural location of the subunits and to provide a semi-quantitative estimate of their relative cellular density.
Abstract: An immunocolloidal gold electron microscopy method is described allowing the ultrastructural localization and quantitation of the regulatory subunits RI and RII and the catalytic subunit C of cAMP-dependent protein kinase. Using a postembedding indirect immunogold labeling procedure that employs specific antisera, the catalytic and regulatory subunits were localized in electron-dense regions of the nucleus and in cytoplasmic areas with a minimum of nonspecific staining. Antigenic domains were localized in regions of the heterochromatin, nucleolus, interchromatin granules, and in the endoplasmic reticulum of different cell types, such as rat hepatocytes, ovarian granulosa cells, and spermatogonia, as well as cultured H4IIE hepatoma cells. Morphometric quantitation of the relative staining density of nuclear antigens indicated a marked modulation of the number of subunits per unit area under various physiologic conditions. For instance, following partial hepatectomy in rats, the staining density of the nuclear RI and C subunits was markedly increased 16 h after surgery. Glucagon treatment of rats increased the staining density of only the nuclear catalytic subunit. Dibutyryl cAMP treatment of H4IIE hepatoma cells led to a marked increase in the nuclear staining density of all three subunits of cAMP-dependent protein kinase. These studies demonstrate that specific antisera against cAMP-dependent protein kinase subunits may be used in combination with immunogold electron microscopy to identify the ultrastructural location of the subunits and to provide a semi-quantitative estimate of their relative cellular density.
TL;DR: A major nucleolar protein with a molecular weight of 100 kD is directly implicated in the transcription of pre-ribosomal RNA (pre-rRNA) and appears to be cleaved into specific maturation products during pre- ribosome biogenesis.
TL;DR: Despite the fact that rRNA transcription occurs in the dense fibrillar component of Ehrlich tumour cell nucleoli, it is shown in the present paper that dense finema component containing rapidly labelled RNA can be visualized not only around but also inside the fibrillsar centres.
TL;DR: The early presence of the 100 kDa protein in the telophase nucleus suggests that it is essential for the reestablishment of nucleolar function after mitosis, an observation which supports the hypothesis that it plays a fundamental role in cell organization.
TL;DR: It is concluded that ribosome formation and turnover in neuronal and oligodendroglial cells are adapted to the protein synthetic levels in these two types of brain cells.
Abstract: The absolute amounts of precursor to ribosomal RNA (pre-rRNA) and ribosomal RNA (rRNA) in isolated rat brain neuronal and oligodendroglial nuclei were determined. The amount of the major pre-rRNA and rRNA species in neuronal nuclei was about twofold higher than in oligodendroglial nuclei. The relative rate of pre-rRNA synthesis in vivo was 2.3- to 2.7-fold higher in neuronal as compared with oligodendroglial nuclei. This corresponds to a 2.7-fold higher activity of the "template-bound" RNA polymerase I in isolated neuronal nuclei, whereas the activity of the "free" enzyme in both neuronal and glial nuclei was almost identical. The higher transcription rates of rRNA genes correlated with the markedly more prominent fibrillar component in neuronal nucleoli. The turnover times of the major pre-rRNA and rRNA species in neuronal and oligodendroglial nuclei were similar, except for 45S pre-rRNA, which turned over at an approximately 1.5-fold slower rate in neuronal nuclei. The relative rates of processing of pre-rRNA and of nucleocytoplasmic transport of rRNA in neuronal cells were approximately 2.7-fold higher than in oligodendroglial cells and corresponded to the differences in rRNA gene transcription rates. The established ribosome formation features correlated with an abundant (neurons) or exceedingly scarce (oligodendrocytes) nucleolar granular component. The turnover rate of cytoplasmic ribosomes in rat brain neurons was twofold slower than in oligodendrocytes, largely because of the about fivefold higher amount of ribosomes in the cytoplasm of neurons. We conclude that ribosome formation and turnover in neuronal and oligodendroglial cells are adapted to the protein synthetic levels in these two types of brain cells.
TL;DR: The distribution of tubulin and/or tubulin-containing structures was examined in separate classes of Xenopus laevis oocytes and in germinal vesicles isolated from them to reveal maximum tubulin concentration in the Balbiani body and in a ring-shaped formation around the nucleus in young oocytes.
Abstract: The distribution of tubulin and/or tubulin-containing structures was examined in separate classes of Xenopus laevis oocytes and in germinal vesicles isolated from them. Although a monoclonal antibody has been used, the technique applied on paraffin sections does not allow clear-cut definition of the state of tubulin present (monomeric, dimeric or polymerized form); however, the probable existence of assembled microtubules is indicated by supplementary techniques, i.e. histology and immunoperoxidase staining. Immunofluorescence reveals maximum tubulin concentration in the Balbiani body and in a ring-shaped formation around the nucleus in young oocytes. The Balbiani body disintegrates in the course of vitellogenesis, granules formed from its periphery migrate into the cytoplasm and gradually fill the entire cytoplasm as radial cords. In the ring-shaped formation around the nucleus strongly fluorescent cords and fibres are formed, particularly on the future vegetal-half-facing part of the nucleus. Reorganization of tubulin may be related to the establishment of a structure directing two-way shifts (1) of cytoplasmic organelles from the Balbiani body to the cytoplasm, and (2) of yolk proteins containing endosomes derived from the endocytically active oolemma to the yolk platelets. A distinct fluorescent fibrillar network is found inside the isolated germinal vesicles,- near the nucleus membrane. Peripheral nucleoli, often present in nuclear membrane protuberances, seem to be surrounded by this material, which is oriented along the surface, and as a basket towards the inside of the nucleus. It is assumed that the structures may participate in the transport of nucleoli from the nucleus to the cytoplasm via the nuclear envelope.
TL;DR: It is suggested that a possible function for the nucleolar vacuoles is the increase in the nucleolus-nucleoplasm exchange interface in response to a rapid increased in the output of nucleolar RNPs.
Abstract: The activation of the nucleolus of primary root cells of Sinapis alba embryos during the first 72 h of germination was monitored by autoradiographic, ultrastructural and microstereological methods. Autoradiographs showed that within 48 h, the nucleolus progressively resumed the capacity to synthesize pre-rRNA molecules at a high rate. In quiescent embryos the nucleolus was small, compact and composed of mixed granular and fibrillar components. Within the first 6 h of germination a strong nucleolar vacuolation occurred, accompanied by a decrease in the volume of the nucleolus and a concomitant high loss of its ribonucleoproteins (RNPs). From 6 to 24 h, nucleolar vacuolation decreased to reach a stable level. During this last period the volume of the nucleolus increased by the accumulation of the fibrillar component resulting from a slow pre-rRNA processing. At 24 h the nucleolus presented a predominantly fibrillar texture. After 24 h, nucleolus growth continued but was due to the accumulation of the granular component, indicating that pre-rRNA processing occurred at a higher rate than during the first day of germination. From 48 h the nucleolus was composed of well-delineated granular and fibrillar areas. Dense nucleolus-associated chromatin as well as fibrillar centres were always observed during the whole period of observation. In addition, previous studies on the nucleolus of radicle cells of Zea mays embryo during early germination were completed by studying changes in the nucleolar volume and in the density of pre-ribosomal subunits of the granular component. On the basis of the data obtained with both species we suggest that a possible function for the nucleolar vacuoles is the increase in the nucleolus-nucleoplasm exchange interface in response to a rapid increase in the output of nucleolar RNPs. The nucleolar growth pattern during early germination is also discussed.
TL;DR: The characteristics of the nucleolus and the NORs have been investigated in meiocytes of different grasshopper species and a marked correlation is observed between the number of nucleoli present in the spermatogonial cells and in the first meiotic prophase of a given species.
Abstract: By means of silver staining procedures of light microscopy the characteristics of the nucleolus and the NORs have been investigated in meiocytes of different grasshopper species. Our results show that: (1) Two is the most common number of chromosomes per haploid genome carrying active NORs although this number may vary from one up to five; (2) NOR activity is preferentially located on medium and short chromosomes but the X and the megameric chromosome are involved in nucleolar organization in a high proportion of the species studied; (3) The NOR location is normally restricted to one end in acro-telocentrics and to the short arm, near the centromere region, in metacentrics; (4) A marked correlation is observed between the number of nucleoli present in the spermatogonial cells and in the first meiotic prophase of a given species; (5) In some cases, the nucleoli are associated to chromosomes during spermatogonial premetaphases.
TL;DR: The statement that Down syndrome is caused by trisomy of chromosome 21 usually begs two questions: (1) Why do three chromosomes instead of two cause abnormal embryonic development and physiology?
Abstract: The statement that Down syndrome is caused by trisomy of chromosome 21 usually begs two questions: (1) Why do three chromosomes instead of two cause abnormal embryonic development and physiology?; and (2) How does a zygote get three chromosomes? This paper addresses the second question. Trisomy is usually the result of nondisjunction, which can be either maternal or paternal. Meiotic nondisjunction is the cause of trisomy 21 in 97-95070 of all cases of Down syndrome. A summary of studies of the parental origin of nondisjunction showed 118 to be maternal and 37 paternal.' Of the maternal cases of nondisjunction, 89 of 118 were in meiosis I and the remaining 29 were in meiosis 11. For the paternal nondisjunction, the 37 meiotic errors were equally divided between meiosis I and meiosis 11. The next largest cause of trisomy is Robertsonian translocation, which is present in 3-5% of all patients with Down syndrome.2 The 14/21 and 21/21 are the most frequent Robertsonian translocations. Considerably less than 1 percent are caused by reciprocal t ransl~cat ion.~ Since meiotic nondisjunction and Robertsonian translocation are the most common causes of Down syndrome, we are led to the obvious questions: What causes nondisjunction and what causes Robertsonian translocation?
TL;DR: The presence of the receptor-hormone complex in the nucleus is not solely responsible for the increased transcriptional activity of uterine nucleoli following in vivo hormone treatment, and this suggests that the longer term maintenance of the hormone-stimulated increase in nucleolar RNA synthesis is independent of continuous protein synthesis.
Abstract: Administration of estradiol to ovariectomized mature rats results in a biphasic early (4 h) and late (24 h) increase in transcriptional activity of isolated uterine nucleoli. The increased rate of nucleolar RNA synthesis is dependent upon the dose of estradiol over the range of 0.1 to 1 �ig/anima!, and exhibits hormone specificity. Administration of cycloheximide, an inhibitor of protein synthesis, prior to the administration of hormone or during the early phase of estrogen action ( 8 h) of estrogen action is without effect on the estrogen-induced increase in transcriptional activity of isolated uterine nucleoli. This suggests that the longer term maintenance of the hormone-stimulated increase in nucleolar RNA synthesis is independent of continuous protein synthesis. Results indicate that the estrogen-induced accumulation and subsequent decline in uterine nuclear estradiol receptor levels is unaffected by cycloheximide treatment. Together, these results indicate the presence of the receptor-hormone complex in the nucleus is not solely responsible for the increased transcriptional activity of uterine nucleoli following in vivo hormone treatment. The early activation of uterine nucleolar RNA synthesis by estrogen seems to result from the synthesis of a short-lived protein(s) that modified RNA polymerase I and/or the nucleolar chromatin template.
TL;DR: Mouse L-cell nucleoli were isolated from sonicated nuclei by centrifugation and extensively treated with pancreatic DNase or micrococcal nuclease to obtain "core nucleoli", infer that specific portions of rDNA are protected from DNase action by a tight association with nucleolar structural proteins.
Abstract: Mouse L-cell nucleoli were isolated from sonicated nuclei by centrifugation and extensively treated with pancreatic DNase or micrococcal nuclease to obtain "core nucleoli." Core nucleoli still contained the precursors to rRNA and about 1% of the total nuclear DNA, which remained tightly bound even after the removal of some chromatin proteins with 2 M NaCl. The core nucleolar DNA electrophoresed in a series of discrete bands, 20 to about 200 base pairs in length. Hybridization tests with specific DNA probes showed that the DNA was devoid of sequences complementary to mouse satellite, mouse Alu-like, and 5S RNA sequences. It also lacked sequences coding for cytoplasmic rRNA species, since it did not hybridize to the 18S to 28S portion of rDNA in Northern blot analyses and none of it was protected by hybridization to a 100-fold excess of total cytoplasmic RNA in S1 nuclease assays. However, the core nucleolar DNA did hybridize to nontranscribed and external transcribed spacer rDNA sequences. We infer that specific portions of rDNA are protected from DNase action by a tight association with nucleolar structural proteins.