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U Scheer

Publications and source records attributed to U Scheer.

At least 91 records · Page 5Linked to original sources
PubMed1978

Changes of nucleosome frequency in nucleolar and non-nucleolar chromatin as a function of transcription: an electron microscopic study.

The morphology of nucleolar and non-nucleolar (lampbrush chromosome loops) chromatin was studied in the electron microscope during states of reduced transcriptional activity in amphibian oocytes (Xenopus laevis, Triturus alpestris, T. cristatus). Reduced transcriptional activity was observed in maturing stages of oocyte development and after treatment with an inhibitor, actinomycin D. Strands of nucleolar chromatin appear smooth and thin, and contain only few, if any, nucleosomal particles in the transcribed units. This is true whether they are densely or only sparsely covered with lateral ribonucleoprotein fibrils. This smooth and non-nucleosomal character is also predominant in the interspersed, apparently nontranscribed rDNA spacer regions. During inactivation, however, nucleolar chromatin frequently and progressively assumes a beaded appearance in extended fibril-free--that is, apparently nontranscribed--regions. In either full-grown oocytes or late after drug treatment, most of the nucleolar chromatin is no longer smooth and thin, but rather shows a beaded configuration indistinguishable from inactive non-nucleolar chromatin. In many chromatin strands, transitions of fibril-associated regions of smooth character into beaded regions without lateral fibrils are seen. Similarly, in the non-nucleolar chromatin of the retracting lampbrush chromosome loops, reduced transcriptional activity is correlated with a change from smooth to beaded morphology. Here, however, beaded regions are also commonly found interspersed between the more or less distant bases of the lateral fibrils, the putative transcriptional complexes. In both sorts of chromatin, detergents (in particular Sarkosyl) that remove most of the chromatin proteins including histones from the DNA axis but leave the RNA polymerases of the transcriptional complexes attached were used to discriminate between polymerases and nucleosomal particles. The results suggest that nucleosomes are absent in heavily transcribed chromatin regions but are reformed after inactivation. In contrast to the findings with inactivated nucleolar genes, in lampbrush chromosome loops the beaded nucleosomal configuration appears to be assumed also in regions within transcriptional units that, perhaps temporarily, are not involved in transcription.

Animals↗
PubMed1977

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

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 length of matrix units, apparent spacer intercepts, and the total repeating units of the rDNA containing chromatin axes revealed a significant degree of heterogeity, with indications of subclasses and predominant repeat unit size classes of 3.3 amd 3.8 micron 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 of and (ii) strands with intra-axial heterogeneity of rDNA subunits. "Prelude complexes", 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.

Animals↗
PubMed1977

Frequencies of circular units of nucleolar DNA in oocytes of two insects, Acheta domesticus and dytiscus marginalis, and changes of nucleolar morphology during oogenesis.

The organization of the extrachromosomal nucleolar material in oocytes of two insect species with different ovary types, the house cricket Acheta domesticus (panoistic ovary) and the water beetle Dytiscus marginalis (meroistic ovary), was studied with light and electron microscopic techniques. Stages early in oogenesis were compared with fully vitellogenic stages (mid-to-late diplotene). The arangement of the nucleolar material undergoes a marked change from a densely aggregated to a dispersed state. The latter was characterized by high transcriptional activity. In spread and positively stained preparations of isolated nucleolar material, a high frequency of small circular units of transcribed rDNA was observed and rings with small numbers (1--5) of pre-rRNA genes were predominant. The observations suggest that the 'extra DNA body' observed in early oogenic stages of both species represents a dense aggregate ofnumerous short circular units of nucleolar chromatin, with morphological subcomponents identifiable in ultrathin sections. These apparently remain in close association with the chromosomal nucleolar organizer(s). The observations further indicate that the individual small nucleolar subunit circles dissociate and are dispersed as actively transcribed rDNA units later in diplotene. The results are discussed in relation to principles of the ultrastructural organization of nucleoli in other cell types as well as in relation to possible mechanisms of gene amplification.

Animals↗
PubMed1976

Experimental disintegration of the nuclear envelope. Evidence for pore-connecting fibrils.

The disintegration of the nuclear envelope has been examined in nuclei and nuclear envelopes isolated from amphibian oocytes from amphibian oocytes and rat liver tissue, using different electron microscope techniques (ultrathin sections and negatively or positively stained spread preparations). Various treatments were studied, including disruption by surface tension forces, very low salt concentrations, and nonionic detergents such as Triton C-100 and Nonidet P-40. The highest local stability of the cylinders of nonmembranous pore complex material is emphasized. As progressive disintegration occurred in the membrane regions, a network of fibrils became apparent which interconnects the pore complexes and is distinguished from the pore complex-associated about 15-20 nm thick, located at the level of the inner nuclear membrane, which is recognized in thin sections to bridge the interpore distances. With all disintegraiton treatments a somewhat higher susceptibility of the outer nuclear membrane is notable, but a selective removal does not take place. Final stages of disintegration are generally characterized by the absence of identifiable, membrane-like structures. Analysis of detergent-treated nuclei and nuclear membrane fractions shows almost complete absence of lipid components but retention bo significant amount of glycoproteins with a typical endomembrane-type carbohydrate pattern. Various alternative interpretations of these observations are discussed. From the present observations and those of Aaronson and Blobel (1,2), we favor the notion that threadlike intrinsic membrane components are stabilized by their attachment to the pore complexes, and perhaps also to peripheral nuclear structures,and constitute a detergent-resistant, interpore skeleton meshwork.

Animals↗
PubMed1976

Regulation of transcription of genes of ribosomal rna during amphibian oogenesis. A biochemical and morphological study.

Natural changes in the transcription of rRNA genes were studied in nucleoli from three oogenic stages of the newt Triturus alpestris with electron microscope, auto-radiographic, and biochemical techniques. From determinations of the uridine triphosphate pool sizes and [3H]uridine uptake, phosphorylation, and incorporation into 28S and 18S rRNAs in vivo it was estimated that the rate of rRNA synthesis was about 0.01% in previtellogenic oocytes and 13% in mature oocytes when compared to midvitellogenesis. Spread preparations of nucleoli showed significant morphological changes in the transcriptional complexes. The total number of lateral fibrils, i.e., ribonucleoproteins containing the nascent rRNA precursor, were drastically decreased in stages of reduced synthetic activity. This indicates that rRNA synthesis is regulated primarily at the level of transcription. The resulting patterns of fibril coverage of the nucleolar chromatin axes revealed a marked heterogeneity. On the same nucleolar axis occurred matrix units that were completely devoid of lateral fibrils, matrix units that were almost fully covered with lateral fibrils, and various forms of matrix units with a range of lateral fibril densities intermediate between the two extremes. Granular particles that were tentatively identified as RNA polymerase molecules were not restricted to the transciptional complexes. They were observed, although less regularly and separated by greater distances, in untranscribed spacer regions as well as in untranscribed gene intercepts. The results show that the pattern of transcriptional control of rRNA genes differs widely in different genes, even in the same genetic unit.

Animals↗
PubMed1976

Classification of loops of lampbrush chromosomes according to the arrangement of transcriptional complexes.

The arrangement of transcriptional units in the loops of lampbrush chromosomes from oocyte nuclei of urodele amphibia and from primary nuclei of the green alga Acetabularia have been studied in the electron microscope using spread preparations. Loops with different patterns of arrangement of matrix units (i.e. to a first approximation, transcriptional units) can be distinguished: (i) loops consisting of one active transcriptional unit; (ii) loops containing one active transcriptional unit plus additional fibril-free, i.e. apparently untranscribed, intercepts that may include 'spacer' regions; (iii) loops containing two or more transcriptional units arranged in identical or changing polarities, with or without interspersed apparent spacer regions. Morphological details of the transcriptional complexes are described. The observations are not compatible with the concept that one loop reflects one and only one transcriptional unit but, rather, lead to a classification of loop types according to the arrangement of their transcriptional units. We propose that the lampbrush chromosome loop can represent a unit for the coordinate transcription of either one gene or a set of several (different) genes.

Acetabularia↗
PubMed1975

Lampbrush-type chromosomes in the primary nucleus of the green alga Acetabularia mediterranea.

Structures with a lampbruch-chromosome-like morphology are described in the nucleoplasm of primary nuclei of the green alga, Acetabularia mediterranea, by light and electron microscopy in sections of cells fixed in situ and in spread preparations of isolated nuclear components. These chromosomes reveal typical loops (up to 20 micronm long), chromomere-like nodules (1-2 micronm in diameter), and 2-4 micronm large axial globules. Associations of some of these chromosomes with nucleolar structures and with the nuclear envelope are also recognized. The light microscopically identified loops are correlated with distinct fibrillogranular structures observed in the thin sections and with the very long matrix units seen in the spread preparations. The similarity of these structures to the lampbrush chromosomes of various animal cell types, all exclusively stages of meiotic prophase, is discussed as well as the possible relation of the appearance of lampbrush chromosomes to a defined phase of the vegatative growth of this alga.

Acetabularia↗
PubMed1975

Effects of actinomycin D on the association of newly formed ribonucleoproteins with the cistrons of ribosomal RNA in Triturus oocytes.

The effect of actinomycin D(AMD) on the association of the nascent ribonucleo-protein (RNP) fibrils containing the precursors of ribosomal RNA (pre-rRNA) with their template deoxyribonucleoprotein (rDNP) strands has been studied in lampbrush stage oocytes from Triturus alpestris. Ovary pieces were incubated in vitro either in media containing radioactive ribonucleosides and then, for various times, in solutions containing 25 mug/ml AMD, or were directly exposed to the drug. The ultrastructure of the nucleoli and the nuclear periphery was studied by electron microscopy of thin sections and positively stained spread preparations of isolated nuclear contents, and by light and electron microscope autoradiography. The fate of the labeled pre-rRNA was followed by gel electrophoresis of RNA extracted from manually isolated nuclei. Our results show that the growing fibrils which contain the nascent pre-rRNA progressively detach from the DNP strands, the majority being released between 45 and 180 min after application of the drug. The release pattern seems to be random and does not show preference for regions close to the initiator or terminator sites of the transcribed rDNP units. There is a pronounced tendency to removal of groups of adjacent mascent fibrils. The effect of the drug is very heterogeneous. Even after 3 h of treatment with AMD the nucleoli exhibit several individual transcriptional units which appear almost completely covered with lateral fibrils. Autoradiography revealed that most of this released RNP remains within the confinements of the nucleoli which show some foci of aggregation and condensation of fibrillar components but no clear "segregation" phenomenon. In the gel-electrophoretic analysis, a significant but moderate decrease of labeled pre-rRNA was noted only in the first stable pre-rRNA component, whereas pre-rRNA classes of lower molecular weight are very stable under these conditions. The results are discussed in relation to the stability of rDNA transcription complexes and as a basis for an explanation of the ultrastructural changes which are generally observed in nucleoli of AMD-treated cells. It is postulated that inhibition of transcription results in a slow but progressive release of the arrested incomplete RNP fibrils from the template.

Animals↗
PubMed1975

Growth of the nuclear envelope in the vegetative phase of the green alga Acetabularia. Evidence for assembly from membrane components synthesized in the cytoplasm.

The primary nucleus of the green alga Acetabularia grows about 25,000-fold in volume while it is separated from the endoplasmic reticulum and the whole cytoplasm by a special paranuclear cisterna of a vacuolar labyrinthum system which shows only very few (two to six per square micrometer) and small (ca. 40-120 nm in diamter) fenestrations. The nuclear envelope does not bear polyribosomes, nor do they occur in the entire zone intermediate between the nuclear envelope and the paranuclear cisterna. It is suggested that this special form of nuclear envelope growth takes place by assembly from cytoplasmically synthesized proteins that are translocated across the paranuclear cisterna in a nonmembrane-structured form.

Acetabularia↗