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M F Trendelenburg

Publications and source records attributed to M F Trendelenburg.

At least 55 records · Page 3Linked to original sources
PubMed1980

Transcription units of chicken ovalbumin gene observed after injection of cloned complete genes into Xenopus oocyte nuclei.

The organization of transcription of a well-characterized protein encoding gene was studied by microinjection and electron microscopy. Circular recombinant DNA molecules containing the complete chicken ovalbumin sequences (7.7 kilobases, contained in 11.5 kilobases of chicken DNA) were microinjected into germinal vesicles of living oocytes of the clawed toad Xenopus laevis, and their transcription was studied in nuclear spread preparations. Evaluation of spread chromatin showed a limited number of observed molecules transcribed in "specific" patterns--i.e., circular chromatin molecules containing transcription units approximately 2.3 micrometer long, consisting of regular series of densely packed lateral ribonucleoprotein fibrils gradually increasing in length. The appearance of these fibril gradients was similar to that of actively transcribed endogenous protein encoding genes contained in lampbrush-chromosome loops of the same nuclei and to the putative Bombyx silk fibroin transcription units. In addition, less-regular arrays of transcript fibrils were seen in some circles, including fully fibril-covered molecules, indicative of the occurrence of irregular transcriptional events. The results of this heterologous transcription experiment indicate that the transcriptional machinery of the amphibian oocyte nucleus is capable of transcribing protein encoding genes from an avian species in typical regular arrays of transcription units.

Animals↗
PubMed1979

Transcriptionally active and inactive regions of nucleolar chromatin in amplified nucleoli of fully grown oocytes of hibernating frogs, Rana pipiens (Amphibia, Anura). A quantitative electron microscopic study.

Chromatin structures contained in nuclei of full-grown oocytes of hibernating northern leopard frogs, Rana pipiens, kept at 4 degrees C were examined by light and electron microscopy, with special emphasis on the morphology of spread nucleoli. Light microscopic inspection of the in situ arrangement revealed that at this stage of oogenesis the numerous amplified nucleoli, together with the chromosomes, were included in a centrally located aggregate. Characteristically, the nucleoli appeared spheroidal and showed large size differences; many of them revealed a caveolated interior and close association with large dense bodies of granular (GDB) or fibrillar (FDB) substructure. A spreading procedure involving limited dispersal of nucleolar chromatin allowed the analysis of a large number of individual nucleoli from a specific nucleus. Electron microscopy showed that different types of nucleoli which could be distinguished in terms of transcriptional activity occurred in the same nucleus: (i) Nucleoli in which nearly all pre-rRNA gene regions appeared as fully covered by lateral fibrils, indicative of active transcription ('active nucleoli'); (ii) nucleoli that contained exclusively or predominantly chromatin strands free of lateral fibrils and were characterized by a uniformly beaded appearance, interpreted as transcriptionally inactive, nucleosome-packed chromatin ('inactive nucleoli'); and (iii) nucleoli in which a number of typical arrays of lateral fibril gradients occurred besides other regions that were free of such fibrils and apparently non-transcribed ('partly inactive nucleoli'). Quantitative evaluations of the proportions of active and inactive nucleolar chromatin indicated that a minimum of 70% of nucleolar DNA is not transcribed at this stage of oogenesis. Details of the pattern and ultrastructural organization of active and inactive chromatin regions are presented. Lampbrush chromosome structures were observed in the same nuclei and also showed transcriptional structures, though often with reduced packing density of lateral fibrils. The observations show that a number of genes of both kinds, those coding for pre-rRNAs and those coding for proteins, are transcribed in full-grown oocytes of hibernating frogs at 4 degrees C. However, the data also indicate that the number of transcribed genes and the frequency of transcriptional events are greatly reduced at this stage of oogenesis, especially in the nucleolar chromatin.

Animals↗
PubMed1978

Transcription patterns of amplified Dytiscus genes coding for ribosomal RNA after injection into Xenopus oocyte nuclei.

Oocytes of the frog Xenopus were injected with purified circular DNA containing amplified rRNA genes of the water beetle Dytiscus. Nuclear contents of injected oocytes were spread and examined by electron microscopy. Most of the Dytiscus DNA seen in injected nuclei contained regions indensely packed with polymerases and nascent transcripts. Apparently normal, as well as abnormal, patterns of transcription were observed. By this type of experiment, it may become possible to recognize the transcribed regions and immediate transcripts of cloned DNA molecules whose activity cannot be seen by electron microscopy of normal nuclei.

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↗
PubMed1974

Morphology of nucleolar cistrons in a plant cell, Acetabularia mediterranea.

The structural organization of transcriptionally active DNA that contains cistrons for precursor molecules of ribosomal RNA is described in positively stained spread preparations from nuclei and nucleoli isolated from the green alga, Acetabularia mediterranea Lmx. These nuclei contain large aggregates of nucleolar subunits in which fibril-covered regions, the putative active cistrons for precursors of ribosomal RNA, alternate with fibril-free intercepts, the "spacers." The length distribution of the different intercepts of this DNA is given, and the pattern is compared with those shown in animal cell systems. The data are discussed in relation to problems of transcription and of amplification of ribosomal RNA genes.

Acetabularia↗