TY  - JOUR
A1  - Scheer, Ulrich
A1  - Sommerville, J.
T1  - Structural organization of nascent transcripts and hnRNA molecules in amphibian oocytes
N2  - Comparisons ofrelative lengths oflampbrush loops, nascent RNP transcripts and hnRNA molecules from oocytes of amphibia with different C-values show that there is an increasing trend in loop, and transcriptional unit, length with increase in genome size but no increasing trend with respect to RN A contour length.The formation of duplex regions and circles in RNP fibrils indicates that RNA processing may occur within the nascent fibrils. The hnRNA molecules from oocytes of the various amphibia readily form intermolecular duplex structures. These complementary sequences have a low kinetic complexity and are transcribed from highly repetitive sequences distributed throughout the genome. Their possible function is considered.
Y1  - 1981
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-39765
ER  - 
TY  - JOUR
A1  - Zentgraf, H.
A1  - Müller, U.
A1  - Scheer, Ulrich
A1  - Franke, W. W.
T1  - Evidence for the existence of globular units in the supranucleosomal organization of chromatin
N2  - No abstract available
Y1  - 1981
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-34123
ER  - 
TY  - JOUR
A1  - Bona, Marion
A1  - Scheer, Ulrich
A1  - Bautz, Ekkehard K. F.
T1  - Antibodies to RNA polymerase II (B) inhibit transcription in lampbrush chromosomes after microinjection into living amphibian oocytes
N2  - Antibodies directed against RNA polymerase II (B) from Drosophila melanogaster were obtained from rabbit sera and, as monoclonal immunoglobulins, from mouse hybridomas and shown to cross-react with the amphibian enzyme protein. Localization by indirect immunofluorescence microscopy revealed the association of this enzyme with chromatin of interphase nuclei of amphibian cells and its absence in nucleoli. Purified immunoglobulins were microinjected in to nuclei ofliving vitellogenic oocytes of Ple1lrodeles waltlii and X enopus laevis and their effects on transcriptional processes were monitored by biochemical and light and electron microscopic stud ies. RNA polymerase II antibodies from rabbit sera caused a rapid and almost complete release of nascent transcripts from the chromatin axis of the loops of lampbrush chromosomes, followed by collapse of the loops and their retraction on the main chromosome axis. Monoclonal murine antibodies to the Iarge RNA polymerase II subunits also inhibited transcription in chromosome Ioops but appeared to inhibit initiation rather than elongation events. Activities of class land III RNA polymerases were not significantly affected by injection of antibodies to polymerase II, indicating immunological differences between the three RNA polymerases. The potential value of the in vitro test system described , as a very sensitive assay for detecting proteins involved in transcription in living cells, is discussed. 1
Y1  - 1981
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-33128
ER  - 
TY  - JOUR
A1  - Franke, Werner W.
A1  - Scheer, Ulrich
A1  - Krohne, Georg
A1  - Jarasch, Ernst-Dieter
T1  - The nuclear envelope and the architecture of the nuclear periphery
N2  - No abstract available
Y1  - 1981
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-33108
ER  - 
TY  - JOUR
A1  - Franke, Werner W.
A1  - Kleinschmidt, Jürgen A.
A1  - Spring, Herbert
A1  - Krohne, Georg
A1  - Grund, Christine
A1  - Trendelenburg, Michael F.
A1  - Stöhr, Michael
A1  - Scheer, Ulrich
T1  - A nucleolar skeleton of protein filaments demonstrated in amplified nucleoli of Xenopus laevis
N2  - The amplified, extrachromosomal nucleoli of Xenopus oocytes contain a meshwork of -4-nm-thick filaments, which are densely coiled into higher-order fibrils of diameter 30-40 nm and are resistant to treatment with high- and low-salt concentrations, nucleases (DNase I, pancreatic RNase, micrococcal nuclease), sulfhydryl agents, and various nonionic detergents. This filamentous "skeleton" has been prepared from manually isolated nuclear contents and nucleoli as weil as from nucleoli isolated by fluorescence-activated particle sorting. The nucleolar skeletons are observed in light and electron microscopy and are characterized by ravels of filaments that are especially densely packed in the nucleolar cortex. DNA as weil as RNA are not constituents of this structure, and precursors to ribosomal RNAs are completely removed from the extraction-resistant filaments by treatment with high-salt buffer or RN ase. Fractions of isolated nucleolar skeletons show specific enrichment of an acidic major protein of 145,000 mol wt and an apparent pi value of -6.15, accompanied in some preparations by various amounts of minor proteins. The demonstration of this skeletal structure in "free" extrachromosomal nucleoli excludes the problem of contaminations by nonnucleolar material such as perinucleolar heterochromatin normally encountered in studies of nucleoli from somatic cells. It is suggested that this insoluble protein filament complex forms a skeleton specific to the nucleolus proper that is different from other extraction-resistant components of the nucleus such as matrix and lamina and is involved in the spatial organization of the nucleolar chromatin and its transcriptional products. In studies of the organization of the interphase nucleus, considerable progress has been made in the elucidation of the arrangement of chromatin components and transcriptional products. However, relatively little is known about the composition and function of another category of nuclear structures, the nonnucleoproteinaceous architectural components that are insoluble in solutions of low and high ionic strength, despite numerous studies dedicated to this problem. Such structures include (a) the nuclear envelope and its pore complexes (I, 15, 18, 23, 37, 41), (b) a peripheral layer of insoluble protein ("lamina"; I, 15, 22, 23, 59), (e) certain skeletal proteins related to the chromosome "scaffold" described by Laemmli and coworkers (see references 2 and 3), and (d) ill-defined tangles of fibrillar structures of the nuclear interior that are collectively described as residual "matrix" (6, 21 ; for reviews, see references THE JOURNAL OF CEll BrOlOGY . VOlUME 90 AUGUST 1981 289-299 © The RockefeIler University Press · 0021 -9525/ 81 / 08/ 0289/ 11 $1 .00 4 and 12). The latter, preparatively
Y1  - 1981
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-33130
ER  - 
TY  - JOUR
A1  - Scheer, Ulrich
T1  - Identification of a novel class of tandemly repeated genes transcribed on lampbrush chromosomes of Pleurodeles waltlii
N2  - Electron microscope preparations of lampbrush chromosomes from oocytes of Pleurodeles waltl;; have revealed a new class of tandemly repeated genes. These genes are highly active, as judged by the close spacing of nascent transcripts. They occur in clusters of >100 copies and are transcribed in units containing roughly 940 base pairs of DNA that are separated by nontranscribed spacers of an estimated DNA content of 2,410 base pairs. The size and the pattern of arrangement of these transcription units can not be correlated with any of the repetitious genes so far described.
Y1  - 1981
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-33153
ER  - 
TY  - JOUR
A1  - Scheer, Ulrich
A1  - Sommerville, John
A1  - Müller, Ulrike
T1  - DNA is assembled into globular supranucleosomal chromatin structures by nuclear contents of amphibian oocytes
N2  - The assembly of DNA into nucleosomal and supranucleosomal chromatin structures has been studied (i) by injection of circular DNA molecules (plasmids) into nuclei of Pleurodeles waltlii oocytes; and (ii) by in vitro incubation of plasmid molecules with the supernatant fraction from oocyte nuclei of Pleurodeles and Xenopus laevis, followed by purification of nucleoprotein structures formed with sucrose gradient centrifugation. [n both types of experiments , spread preparations of the newly assembled and transcriptionally inactive chromatin , examined by electron microscopy , show dense globular higher order (supranucleosomal) packing forms. Under partially relaxing (low salt) preparation conditions granular chromatin subunits of about 30 nm diameter can be seen either as widely spaced particles or in closely packed aggregates. The transcriptionally inactive endogenous chromatin of chromomeres of lampbrush chromosomes is arranged in similar higher order chromatin units. A correlation is found between the sizes of the DN A molecule probes used and the numbers of nucleosomes and higher order globules in the assembled chromatin structures. After prolonged dispersion in low salt buffers , these globular chromatin units unfold into chains of7-12 nucleosomes. The results support the concept that chromatin is arranged , under physiological ion concentrations as they are present in the nucleus , in supranucleosomal units of globular morphology.
Y1  - 1980
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-39671
ER  - 
TY  - CHAP
A1  - Franke, Werner W.
A1  - Scheer, Ulrich
A1  - Zentgraf, Hanswalter
A1  - Trendelenburg, Michael F.
A1  - Müller, U.
A1  - Krohne, G.
A1  - Spring, H.
T1  - Organization of transcribed and nontranscribed chromatin
N2  - No abstract available
KW  - Tumor / Zellteilung
Y1  - 1980
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-40656
ER  - 
TY  - JOUR
A1  - Scheer, Ulrich
T1  - Structural organization of spacer chromatin between transcribed ribosomal RNA genes in amphibian oocytes
N2  - Transcribed nucleolar chomatin, including the spacer regions interspersed between the rRNA genes, is different from the bulk of nontranscribed chromatin in that the DNA of these regions appears to be in an extended (B) conformation when examined by electron microscopy. The possibility that this may reflect artificial unfolding of nucleosomes during incubation in very low salt buffers as routinely used in such spread preparations has been examined by studying the influence of various ion concentrations on nucleolar chromatin structure. Amplified nucleolar chromatin of amphibian oocytes (Xenopus laevis, Pleurodeles waltlii, Triturus cristatus) was spread in various concentrations of NaCl (range 0 to 20 mM). Below 1 mM salt spacer chromatin frequently revealed a variable number of irregularly shaped beads, whereas above this concentration the chromatin axis appeared uniformly smooth. At all salt concentrations studied, however, the length distribution of spacer and gene regions was identical. Preparations fixed with glutaraldehyde instead of formaldehyde, or unftxed preparations, were indistinguishable in this respect. The observations indicate that (i) rDNA spacer regions are not compacted into nucleosomal particles and into supranucleosomal structures when visualized at chromatin stabilizing salt concentrations (e.g., 20 mM NaCl), and (ii) spacer DNA is covered by a uniform layer of proteins of unknown nature which, at very low salt concentrations (below 1 mM NaCl), can artificially give rise to the appearance of small granular particles of approximately nucleosome-like sizes. These particles, however, are different from nucleosomes in that they do not foreshorten the associated spacer DNA. The data support the concept of an altered nucleohistone conformation not only in transcribed chromatin but also in the vicinity of transcriptional events.
KW  - Cytologie
KW  - Chromatin structure
KW  - rDNA
KW  - amphibian oocytes
Y1  - 1980
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-41057
ER  - 
TY  - CHAP
A1  - Franke, Werner W.
A1  - Scheer, Ulrich
A1  - Spring, Herbert
A1  - Trendelenburg, Michael F.
A1  - Zentgraf, Hanswalter
T1  - Organization of nucleolar chromatin
N2  - No abstract available
Y1  - 1979
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-39410
ER  - 
TY  - CHAP
A1  - Scheer, Ulrich
A1  - Spring, Herbert
A1  - Trendelenburg, Michael F.
T1  - Organization of transcriptionally active chromatin in lampbrush chromosome loops
N2  - No abstract available
Y1  - 1979
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-39293
ER  - 
TY  - JOUR
A1  - Zentgraf, Hanswalter
A1  - Trendelenburg, Michael F.
A1  - Spring, Herbert
A1  - Scheer, Ulrich
A1  - Franke, Werner W.
A1  - Müller, Ulrike
A1  - Drury, Kenneth C.
A1  - Rungger, Duri
T1  - Mitochondrial DNA arranged into chromatin-like structures after injection into amphibian oocyte nuclei
N2  - Purified mitochondrial DNA (mitDNA) from ovaries ofXenopus lae vis was injected into the nuclei (germinal vesicles) of large viteUogenic oocytes of the same organism and examined by electron microscopy ofthe spread nuclear contents. Normally located nuclei of untreated oocytes as weil as peripherally translocated nuclei of centrifuged oocytes were used. In addition, oocyte nuclei isolated and incubated under liquid paraffin oil were injected with DNA. The integrity oftranscriptional structures of endogenous chromosomal (Iampbrush chromosomes) and extrachromosomal (nucleoli) genes of the injected nuclei was demonstrated. Microinjected mitDN A was identified as circles of chromatin exhibiting polynucleosome-like organization and a me an contour length of 2.6 J.Lm, corresponding to a compaction ratio of the mitDN A of about 2 : I. This DNA packing ratio is similar to that observed after preparation of various kinds of native chromatin in low salt buffers. The chromatin circles formed from injected mitDNA only very rarely exhibited lateral fibrils suggestive of transcriptional activity. These results suggest that purified mitDNA can be transformed to normally structured chromatin when exposed to oocyte nuclear contents but is rarely , if at all , transcribed in this form and in this environment.
Y1  - 1979
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-33174
ER  - 
TY  - JOUR
A1  - Scheer, Ulrich
A1  - Sommerville, John
A1  - Bustin, M.
T1  - Injected histone antibodies interfere with transcription of lampbrush chromosome loops in oocytes of Pleurodeles
N2  - Antibodies to calf thymus histone H2B were purified by chromatography on DEAE-cellulose and injected into oocyte nuclei of Pleurodeles waltlii. As shown by indirect immunofluorescence these antibodies cross-reacted strongly with corresponding histones associated with lampbrush chromosomes. Shortly after injection the lateral loops of the chromosomes retracted into the chromomeres and by 3 h postinjection the 'lampbrush' appearance was completely lost and the chromosomes appeared in light-microscopic preparations as rod-like structures consisting of 10ngitudina11y coalesced chromomeres. In control oocytes injected with non-immune immunoglobulins or antibodies against a ubiquitous transcript-associated protein no morphological alterations of the lampbrush chromosomes could be observed. Electron microscopic spreads of chromosomes prepared at various times after injection of anti-H2B revealed a progressive loss of transcriptional complexes from the loop axes. Finally, higher-order chromatin configurations, like supranuc1eosomal globules (' superbeads ') or cable-like chromatin strands 50- 60 nm thick predominated, indicating complete transcriptional inactivation of a11 chromosomal regions. The results indicate that H2B antibodies react specifically with his tones associated with the transcribed DNA of lateral loops in their native state. The resulting antigenantibody complexes seem to inhibit progression of the R A polymerases along the template, thus causing the premature release of transcripts, a process analogous to the stripping effect of actinomycin D. The demonstration of histones associated with heavily transcribed regions, which are not compacted into nucleosomes but largely extended, supports the current concept that unfolding of nucleosomes to a110w transcription of the DNA does not involve dissociation of histones. In contrast, amplified ribosomal RNA genes are unaffected by injected HzB antibodies. This does not necessarily indicate absence of his tones from nucleolar chromatin, since we do not know whether it is accessible in vivo to antibodies or whether the histone antigenie determinants are masked by the presence of other proteins. The technique of injecting specific antibodies should be widely applicable when analysing the in vivo distribution of chromosomal components at the electron-microscopic level and when studying complex metabolie processes, like the cleavage and modification of RNA, by selective inhibition of defined enzymic steps.
Y1  - 1979
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-33166
ER  - 
TY  - JOUR
A1  - Franke, Werner W.
A1  - Scheer, Ulrich
T1  - Morphology of transcriptional units at different states of activity
N2  - The morphology of two forms of transcription ally active chromatin, the nucleoli and the loops of lampbrush chromosomes, has been examined after fixation in situ or after isolation and dispersion of the material in media of low ionic strengths, using a variety of electron microscopic preparation techniques (e.g. spread preparations with positive or negative staining or without any staining at all, with bright and dark field illumination, with autoradiography, after pretreatment of the chromatin with specific detergents such as Sarkosyl NL-30; transmission and scanning transmission electron microscopy of ultrathin sections). Nucleolar chromatin and chromosomes from oocytes of various amphibia and insects as well as from green algae of the family of the Dasycladaceae were studied in particular detail. The morphology of transcriptional units that are densely packed with lateral ribonucleoprotein fibrils, indicative of great transcriptional activity, was compared with that of chromatin of reduced lateral fibril density, including stages of drug-induced inhibition. The micrographs showed that under conditions which preserve the nucleosomal organization in condensed chromatin studied in parallel, nucleosomes are not recognized in transcriptionally active chromatin. This holds for the transcribed regions as well as for apparently untranscribed (i.e. fibril-free) regions interspersed between ('spacer') and/or adjacent to transcribed genes and for the fibril-free regions within transcriptional units of reduced fibril density. In addition, comparison oflengths of repeating units of isolated rDNA with those observed in spread nucleolar chromatin indicated that this DNA is not foreshortened and packed into nucleosomal structures. Granular particles which were observed, at irregular frequencies and in variable patterns, in some spacer regions, did not result in a proportional shortening of the spacer axis, and were found to be resistant to detergent treatment effective in removing most of the chromatin associated proteins including histones. Thus, these particles behave like RNA polymerases rather than nucleosomes. It is suggested that structural changes from nucleosomal packing to an extended form of DNA are involved in the transcriptional activation of chromatin.
Y1  - 1978
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-41363
ER  - 
TY  - JOUR
A1  - Scheer, Ulrich
T1  - Changes of nucleosome frequency in nucleolar and non-nucleolar chromatin as a function of transcription: an electron microscopic study
N2  - The morphology of nucleolar and non-nucleolar (Iampbrush 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. I n either fUll-grown 00- cytes 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 wihout 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. I n 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.
Y1  - 1978
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-39750
ER  - 
TY  - CHAP
A1  - Franke, Werner W.
A1  - Zentgraf, Hanswalter
A1  - Scheer, Ulrich
T1  - Supranucleosomal and non-nucleosomal chromatin configurations
N2  - A significant contribution to the understanding of chromatin organization was the d iscovery of the nucleosome as a globular repeating unit of the package of DNA (Hewish and Burgoyne, 1973; Woodcock, 1973; Kornberg, 1974; Olins and Olins, 1974; for review see Oudet et al., 1978 a) . In accord with the original definition and in ag reement with most workers in this field of research we identify a nucleosome as a spheric alor slightly oblate gr anular particle 10-13 nm in diameter, containing about 200 base pairs of DNA and two of each of the four his tones H2a, H2b, H3 and H4. It is this structure in which the bulk of the nuclear chroma tin is organized in most eukaryotic cells, with the exception of the dinofl age llates (Rae and Steele, 1977; dinofl agellate DNA, however, c an be packed into nucleosoma l structures in vitro by addition of the appropriate amounts of histones;the same reference). Although it seems clear from the work reported that condensed and transcriptiona lly inactive chroma tin is contained in nucleosomes as the principle for first order p acking of DNA there are two important questions onto which we are focusing in the present study: ( i ) What is the higher order of p a cking present in - and perhaps typical-of - the condensed sta te of chromatin, and (ii) what is the specific form of arrangement of transcriptionally a ctive chromatin?
Y1  - 1978
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-39447
ER  - 
TY  - JOUR
A1  - Rungger, M.
A1  - Crippa, M.
A1  - Trendelenburg, M. F.
A1  - Scheer, Ulrich
A1  - Franke, Werner W.
T1  - Visualization of rDNA spacer transcription in Xenopus oocytes treated with fluorouridine
N2  - Under the intluence of 5-tluoro-uridine, the ultrastructure of the rDNA transcription units in Xenopus oocytes is altered. Whereas part of the matrix units maintains anormal aspect or shows various degrees of inhibition, in a strong proportion of the transcription units the alternating pattern of matrix units and fibril-free spacer regions is no longer recognized. Transcriptional complexes are found along the entire DNP axis, including the regions of the spacers. These observations support biochemical data on transcription in rDNA spacer region.
Y1  - 1978
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-33082
ER  - 
TY  - JOUR
A1  - Krohne, Georg
A1  - Franke, Werner W.
A1  - Scheer, Ulrich
T1  - The major polypeptides of the nuclear pore complex
N2  - Nuclear envelopes of maturing oocytes of various amphibia contain an unusually high number of pore complexes in very close packing. Consequently, nuclear envelopes , which can be manually isolated in great purity, provide a remarkable enrichment of nuclear pore complex material, relative to membranous and other interporous structures. When the polypeptides of nuclear envelopes isolated from oocytes of Xenopl/s la evis and Triturus alpestris are examined by gel electrophoresis, visualized either by staining with Coomassie blue or by radiotluorography after in vitro reaction with [3H]dansyl chloride , a characteristic pattern is obtained (10 major and 15 minor bands). This polypeptide pattern is radically different from that of the nuclear contents isolated from the same cell. Extraction of the nuclear envelope with high salt concentrations and moderateIy ac tive detergents such as Triton X- 100 results in the removal of membrane material but leaves most of the non-membranous structure of the pore complexes. The dry weight of the pore complex (about 0.2 femtograms) remains essentially unchanged during such extractions as measured by quantitative electron microscopy . The extracted preparations which are highly enriched in nuclear pore complex material contain only two major polypeptide components with apparent molecular weights of 150000 and 73000. Components of such an electrophoretic mobility are not present as major bands , if at all , in nuclear contents extracted in the same way. lt is concluded that these two polypeptides are the major constituent protein(s) of the oocyte nuclear pore complex and are specific for this structure. When nuclear envelopes are isolated from rat liver and extracted with high salt buffers and Triton X- 100 similar bands are predominant, but two additional major components of molecular weights of 78000 and 66000 are also recognized. When the rat liver nuclear membranes are further subfractionated material enriched in the 66000 molecular weight component can be separated from the membrane material, indicating that this is relatively loosely associated material , probably a part of the nuclear matrix . The results suggest that the nuclear pore complex is not only a characteristic ubiquitous structure but also contains similar, if not identical , skeletal proteins that are remarkably re sistant to drastic changes of ionic strength as weil as to treatments with detergents and thiol reagents.
Y1  - 1978
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-33078
ER  - 
TY  - JOUR
A1  - Scheer, Ulrich
A1  - Zentgraf, Hanswalter
T1  - Nucleosomal and supranucleosomal organization of transcriptionally inactive rDNA circles in Dytiscus oocytes
N2  - Oocytes of the water beetle, Dytiscus marginalis, contain large amounts of rDNA most of which is present in the form of rings containing one or several pre-rRNA genes. Electron microscopy of spread preparations of vitellogenic oocytes has shown that the rDNA is extended in chromatin rings with transcribed pre- rRNA genes and is not packed into nucleosomes (Trendelenburg eta!. , 1976). When similar preparations are made from previtellogenic ooytes in which a large proportion of the nuc1eolar chromatin is transcriptionally inactive, a different morphological form of this chromatin is recognized. In contrast to the transcribed chromatin rings the inactive nucleolar chromatin circles show the characteristic beaded configuration, indicative of nucleosomal packing. Nuc1eosomal packing is also indicated by the comparison of the lengths of these chromatin rings with both iso lated rDNA circ1es and transcribed chromatin rings. In addition, these inactive nuc1eofilaments often appear to be compacted into globular higher order structures of diameters from 21 to 34nm, each composed of an aggregate of 6-9 nuc1eosomes. While the estimated reduction of the overall length of rDNA, as seen in our preparations, is, on the average, only 2.2 - 2.4 fold in the nuc1eosomal state it is 10- 13 fold when supranuc1eosomal globules are present. These data show that the extrachromosomal rDNA of these oocytes goes through a cycle of condensation and extensio n, as a function of the specific transcriptional activity, and that the beaded state described here is exc1usively found in the non-transcribed state.
Y1  - 1978
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-33188
ER  - 
TY  - CHAP
A1  - Franke, Werner W.
A1  - Scheer, Ulrich
A1  - Trendelenburg, Michael F.
A1  - Zentgraf, H.
A1  - Spring, H.
T1  - Morphology of transcriptionally active chromatin
N2  - Some decades ago it was noted by cytologists that within the interphase nucleus large portions of the transcriptionally ("genetically," in their terms) inactive chromosomal material are contained in aggregates of condensed chromatin, the "chromocenters," whereas transcriptionally active regions of chromosomes appear in a more dispersed form and are less intensely stained with DNA-directed staining procedures (Heitz 1929, 1932, 1956; Bauer 1933). The hypothesis that condensed chromatin is usually characterized by very low or no transcriptional activity, and that transcription occurs in loosely packed forms of chromatin (including, in most cells, the nucleolar chromatin) has received support from studies of ultrathin sections in the electron microscope and from the numerous attempts to separate transcriptionally active from inactive chromatin biochemically (for references, see Anderson et al. 1975; Berkowitz and Doty 1975; Krieg and Wells 1976; Rickwood and Birnie 1976; Gottesfeld 1977). Electron microscopic autoradiography has revealed that sites of RNA synthesis are enriched in dispersed chromatin regions located at the margins of condensed chromatin (Fakan and Bernhard 1971, 1973; Bouteille et al. 1974; Bachellerie et al. 1975) and are characterized by the occurrence of distinct granular and fibrillar ribonucleoprotein (RNP) structures, such as perichromatin granules and fibrils. The discovery that, in most eukaryotic nuclei, major parts of the chromatin are organized in the form of nucleosomes (Olins and Olins 1974; Kornberg 1974; Baldwin et al. 1975) has raised the question whether the same nucleosomal packing of DNA is also present in transcriptionally active chromatin strands. Recent detailed examination of the morphology of active and inactive chromatin involving a diversity of electron microscopic methods, particularly the spreading technique by Miller and coworkers (Miller and Beatty 1969; Miller and Bakken 1972), has indicated that the DNA of some actively transcribed regions is not packed into nucleosomal particles but is present in a rather extended form within a relatively thin (4-7 nm) chromatin fiber.
Y1  - 1978
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-41097
ER  -