Promotion of Hyperthermic-Induced rDNA Hypercondensation in Saccharomyces cerevisiae

Ribosome biogenesis is tightly regulated through stress-sensing pathways that impact genome stability, aging and senescence. In Saccharomyces cerevisiae, ribosomal RNAs are transcribed from rDNA located on the right arm of chromosome XII. Numerous studies reveal that rDNA decondenses into a puff-like structure during interphase, and condenses into a tight loop-like structure during mitosis. Intriguingly, a novel and additional mechanism of increased mitotic rDNA compaction (termed hypercondensation) was recently discovered that occurs in response to temperature stress (hyperthermic-induced) and is rapidly reversible. Here, we report that neither changes in condensin binding or release of DNA during mitosis, nor mutation of factors that regulate cohesin binding and release, appear to play a critical role in hyperthermic-induced rDNA hypercondensation. A candidate genetic approach revealed that deletion of either HSP82 or HSC82 (Hsp90 encoding heat shock paralogs) result in significantly reduced hyperthermic-induced rDNA hypercondensation. Intriguingly, Hsp inhibitors do not impact rDNA hypercondensation. In combination, these findings suggest that Hsp90 either stabilizes client proteins, which are sensitive to very transient thermic challenges, or directly promotes rDNA hypercondensation during preanaphase. Our findings further reveal that the high mobility group protein Hmo1 is a negative regulator of mitotic rDNA condensation, distinct from its role in promoting premature condensation of rDNA during interphase upon nutrient starvation.

Distribution of rDNA in the nucleus ofGiardia lamblia: detection by Ag-I silver stain

Previous investigations have proved that diplomonads have primitive cell nuclei and lack a nucleolus. We determined the distribution of ribosomal DNA (rDNA) in diplomonad nuclei that lacked a nucleolus. Giardia lamblia was used as the experimental organism with Euglena gracilis as the control. The distribution of rDNA was demonstrated indirectly by the modified Ag-I silver technique that can indicate specifically the nucleolus organizing region (NOR) by both light and electron microscopy. In ultrathin sections of silver stained Euglena cells, all silver grains were concentrated in the fibrosa of the nucleolus, while no silver grains were found in the cytoplasm, nucleoplasm, condensed chromosomes or pars granulosa of the nucleus. In the silver stained Giardia cells, no nucleolus was found, but a few silver grains were scattered in the nucleus. This suggests that the rDNA of Giardia does not form an NOR-like structure and that its nucleus is in a primitive state.

Regulation of rRNA synthesis by TATA-binding protein-associated factor Mot1

Mot1 is an essential, conserved, TATA-binding protein (TBP)-associated factor in Saccharomyces cerevisiae with well-established roles in the global control of RNA polymerase II (Pol II) transcription. Previous results have suggested that Mot1 functions exclusively in Pol II transcription, but here we report a novel role for Mot1 in regulating transcription by RNA polymerase I (Pol I). In vivo, Mot1 is associated with the ribosomal DNA, and loss of Mot1 results in decreased rRNA synthesis. Consistent with a direct role for Mot1 in Pol I transcription, Mot1 also associates with the Pol I promoter in vitro in a reaction that depends on components of the Pol I general transcription machinery. Remarkably, in addition to Mot1's role in initiation, rRNA processing is delayed in mot1 cells. Taken together, these results support a model in which Mot1 affects the rate and efficiency of rRNA synthesis by both direct and indirect mechanisms, with resulting effects on transcription activation and the coupling of rRNA synthesis to processing.

Yeast nuclear envelope subdomains with distinct abilities to resist membrane expansion

Little is known about what dictates the round shape of the yeast Saccharomyces cerevisiae nucleus. In spo7Delta mutants, the nucleus is misshapen, exhibiting a single protrusion. The Spo7 protein is part of a phosphatase complex that represses phospholipid biosynthesis. Here, we report that the nuclear protrusion of spo7Delta mutants colocalizes with the nucleolus, whereas the nuclear compartment containing the bulk of the DNA is unaffected. Using strains in which the nucleolus is not intimately associated with the nuclear envelope, we show that the single nuclear protrusion of spo7Delta mutants is not a result of nucleolar expansion, but rather a property of the nuclear membrane. We found that in spo7Delta mutants the peripheral endoplasmic reticulum (ER) membrane was also expanded. Because the nuclear membrane and the ER are contiguous, this finding indicates that in spo7Delta mutants all ER membranes, with the exception of the membrane surrounding the bulk of the DNA, undergo expansion. Our results suggest that the nuclear envelope has distinct domains that differ in their ability to resist membrane expansion in response to increased phospholipid biosynthesis. We further propose that in budding yeast there is a mechanism, or structure, that restricts nuclear membrane expansion around the bulk of the DNA.

Spatio-temporal dynamics at rDNA foci: Global switching between DNA replication and transcription

We have investigated the in situ organization of ribosomal gene (rDNA) transcription and replication in HeLa cells. Fluorescence in situ hybridization (FISH) revealed numerous rDNA foci in the nucleolus. Each rDNA focus corresponds to a higher order chromatin domain containing multiple ribosomal genes. Multi-channel labeling experiments indicated that, in the majority of cells, all the rDNA foci were active in transcription as demonstrated by co-localization with signals to transcription and fibrillarin, a protein involved in ribosomal RNA processing. In some cells, however, a small portion of the rDNA foci did not overlap with signals to transcription and fibrillarin. Labeling for DNA replication revealed that those rDNA foci inactive in transcription were restricted to the S-phase of the cell cycle and were replicated predominantly from mid to late S-phase. Electron microscopic analysis localized the nucleolar transcription, replication, and fibrillarin signals to the dense fibrillar components of the nucleolus and at the borders of the fibrillar centers. We propose that the rDNA foci are the functional units for coordinating replication and transcription of the rRNA genes in space and time. This involves a global switching mechanism, active from mid to late S-phase, for turning off transcription and turning on replication at individual rDNA foci. Once all the rRNA genes at individual foci are replicated, these higher order chromatin domains are reprogrammed for transcription.

Evolution of ribosomal DNA unit on the X chromosome independent of autosomal units in the liverwort Marchantia polymorpha

In the haploid dioecious liverwort, Marchantia polymorpha, the X chromosome, but not the Y, carries a cluster of ribosomal RNA genes (rDNAs). Here we show that sequences of 5S, 17S, 5.8S and 26S rDNAs are highly conserved (>99% identity) between the X chromosomal and autosomal rDNA repeat units, but the intergenic spacer sequences differ considerably. The most prominent difference is the presence of a 615-bp DNA fragment in the intergenic spacer, X615, which has accumulated predominantly in the rDNA cluster of the X chromosome. These observations suggest that the rDNA repeat unit on the X chromosome evolved independently of that on autosomes, incorporating sex chromosome-specific sequences.

Nucleolar activity of germ cells in polytrophic ovaries of crane flies (Diptera: Tipulidae). Ultrastructural and cytochemical studies

Ultrastructural and histochemical studies confirmed extrachromosomal amplification of rDNA in nuclei of the oocyte and one of its sibling nurse cells during early stages of oogenesis in Tipula spp. Decondensation of the extra DNA body in the oocyte nucleus coincides with the appearance of multiple nucleoli. In contrast, the amplified copies of ribosomal genes in the nurse cell nucleus remain condensed (i.e. transcriptionally inactive). Roughly of the same size, all nurse cells look almost identical. Their nuclei are spherical and contain single, prominent nucleoli, clumps of chromatin and accumulations of granular material. The cytoplasm is packed with free ribosomes, while in close vicinity of the nuclear envelope many islets of fine granular nuage material can be found. These data indicate that the nurse cells in crane fly ovaries are synthetically active, i.e. contribute to the overall production of ribosomes and their final accumulation in the oocyte. The invariable volume of the nurse cells throughout oogenesis may therefore result from the differences in the dynamics of transcriptional activity and transport of ribosomes, rather than indicate their low synthetic activity.

rDNA amplification in previtellogenic and vitellogenic oocytes of symphylans (Arthropoda, Myriapoda)

Tube-shaped ovaries of symphylans house numerous developing oocytes that are accompanied by somatic follicular cells. Oocyte nuclei (germinal vesicles) are relatively large and ovoid. During early previtellogenesis they contain compact spherical bodies and lampbrush chromosomes immersed in a translucent karyoplasm. Fluorescent labeling with DAPI and propidium iodide has revealed the presence of both DNA and RNA in the spherical bodies. As previtellogenesis advances, small RNA- and AgNOR-positive nucleoli bud off from these bodies. Full-grown nucleoli consist of coarse-granular material and comprise electron-transparent vacuoles. Our results suggest that in symphylan germinal vesicles amplification of rDNA genes takes place, and that the spherical bodies represent accumulations of extrachromosomal rDNA (rDNA bodies) after commencement of transcriptional activity.

[Visualization of ribosomal genes transcription in SPEV culture cells using bromouridine triphosphate]

We applied a sensitive and specific method for detection of run-on rDNA transcription in cultured mammalian cells. This technique is based on the capability of RNA polymerase I to maintain transcriptional activity following cell fixation with methanol, and on the use of BrUTP as a precursor of rRNA synthesis. The results obtained have shown that in cultured pig cells (PK cells) the ribosomal genes are transcribed during interphase to become repressed at the end of mitotic prophase. The rDNAs are not transcribed at the prometaphase, metaphase and anaphase stages. The ribosomal genes become derepressed at early telophase. At early telophase, the number of BrUTP-incorporated sites is equal to that of the nucleolus organizing regions (NORs), but it is augmented during telophase progression. A similar dynamics of ribosomal gene reactivation is also revealed following spatial separation of NO-chromosomes between individual micronuclei caused by hypotonic chock. This indicates that the spatial integration of chromosomal NORs is not a prerequisite for ribosomal gene reactivation at mitosis.

Comparative genomic in situ hybridization (cGISH) analysis on plant chromosomes revealed by labelled Arabidopsis DNA

A new approach for comparative cytogenetic banding analysis of plant chromosomes has been established. The comparative GISH (cGISH) technique is universally applicable to various complex genomes of Monocotyledonae (Triticum aestivum, Agropyron elongatum, Secale cereale, Hordeum vulgare, Allium cepa, Muscari armenaticum and Lilium longiflorum) and Dicotyledonae (Vicia faba, Beta vulgaris, Arabidopsis thaliana). Labelled total genomic DNA of A. thaliana generates signals at conserved chromosome regions. The nucleolus organizing regions (NORs) containing the majority of tandemly repeated rDNA sequences, N-band regions containing satellite DNA, conserved homologous sequences at telomeres and additional chromosome-characteristic markers were detected in heterologous FISH experiments. Multicolour FISH analysis with repetitive DNA probes simultaneously revealed the chromosome assignment of 56 cGISH signals in rye and 61 cGISH signals in barley. Further advantages of this technique are: (1) the fast and straightforward preparation of the probe; (2) the generation of signals with high intensity and reproducibility even without signal amplification; and (3) no requirement of species-specific sequences suitable for molecular karyotype analysis. Hybridization can be performed without competitive DNA. Signal detection without significant background is possible under low stringency conditions. The universal application of this fast and simple one-step fluorescence banding technique for plant cytogenetic and plant genome evolution is discussed.

Heterogeneity of rDNA distribution and genome size in Silene spp

Genus Silene L. (Caryophyllaceae) contains about 700 species divided into 44 sections. According to recent taxonomic classification this genus also includes taxa previously classified in genera Lychnis and Melandrium. In this work, four Silene species belonging to different sections were studied: S. latifolia (syn. Melandrium album, Section Elisanthe), S. vulgaris (Inflatae), S. pendula (Erectorefractae), and S. chalcedonica (syn. Lychnis chalcedonica, Lychnidiformes). Flow cytometric analysis revealed a genome size of 2.25 and 2.35 pg/2C for S. vulgaris and S. pendula and of 5.73 and 6.59 pg/2C for S. latifolia and S. chalcedonica. All four species have the same chromosome number including the pair of sex chromosomes of the dioecious S. latifolia (2n = 2x = 24). Double target fluorescence in-situ hybridization revealed the chromosomal locations of 25S rDNA and 5S rDNA. A marked variation in number and localization of rDNA loci but no correlation between the numbers of rDNA clusters and genome size was found. FISH and genome size data indicate that nuclear genomes of Silene species are highly diversified as a result of numerous DNA amplifications and translocations.

Combination of electron microscopic in situ hybridization and anti-DNA antibody labelling reveals a peculiar arrangement of ribosomal DNA in the fibrillar centres of the plant cell nucleolus

The fibrillar centres (FCs) in the nucleoli of Allium cepa usually contained compact dense chromatin, which was always surrounded with light fibrous material (LFM). Distribution of 18S ribosomal DNA (rDNA) in the FCs was examined by in situ hybridization at the light and electron microscopic levels and the results were compared with those obtained by immunogold labelling with anti-DNA antibodies. Anti-DNA antibodies heavily labelled the dense chromatin of the FCs but scarcely labelled the LFM. However, electron microscopic in situ hybridization using the 18S rDNA probe showed that the label in the dense chromatin was extremely weak compared with that obtained by the anti-DNA antibody labelling: the specific label with anti-DNA antibodies of the dense chromatin was about 15 times as much as that of the LFM, whereas the specific label with in situ hybridization in the dense chromatin was only about 1.7 times higher than in the LFM. These results suggest that the rDNA encoding rRNA is preferentially released from the dense chromatin and that non-transcribed intergenic spacers remain in the dense chromatin as the anchoring sites of rDNA.

Organization of 5S rDNA in species of the fish Leporinus: two different genomic locations are characterized by distinct nontranscribed spacers

To address understanding the organization of the 5S rRNA multigene family in the fish genome, the nucleotide sequence and organization array of 5S rDNA were investigated in the genus Leporinus, a representative freshwater fish group of South American fauna. PCR, subgenomic library screening, genomic blotting, fluorescence in situ hybridization, and DNA sequencing were employed in this study. Two arrays of 5S rDNA were identified for all species investigated, one consisting of monomeric repeat units of around 200 bp and another one with monomers of 900 bp. These 5S rDNA arrays were characterized by distinct NTS sequences (designated NTS-I and NTS-II for the 200- and 900-bp monomers, respectively); however, their coding sequences were nearly identical. The 5S rRNA genes were clustered in two chromosome loci, a major one corresponding to the NTS-I sites and a minor one corresponding to the NTS-II sites. The NTS-I sequence was variable among Leporinus spp., whereas the NTS-II was conserved among them and even in the related genus Schizodon. The distinct 5S rDNA arrays might characterize two 5S rRNA gene subfamilies that have been evolving independently in the genome.

Chromosome analysis and rDNA FISH in the stag beetle Dorcus parallelipipedus L. (Coleoptera: Scarabaeoidea: Lucanidae)

In the present work the chromosome complement (2n = 18; 8AA + XY) of the stag beetle Dorcus parallelipipedus L. (Scarabaeoidea: Lucanidae) is analyzed using conventional Giemsa staining, banding techniques and ribosomal fluorescent in situ hybridization (rDNA FISH). rDNA FISH remains the unique tool for providing a clear-cut identification of Nucleolar Organizer Regions (NORs) when conventional banding methods such as silver- and CMA3-staining proved to be inadequate. The dull, homogeneous CMA3 fluorescence of all chromosomes indicates the absence of markedly GC rich compartmentalized regions in D. parallelipipedus genome. Silver impregnation inadequacy in detecting NOR regions is to be sought in the unusual extensive silver stainability of heterochromatic material which, on the contrary of what stated for vertebrates, seems to be a common feature in Scarabaeoidea species.

Cell compartmentalisation in planctomycetes: novel types of structural organisation for the bacterial cell

The organisation of cells of the planctomycete species Pirellula marina, Isosphaera pallida, Gemmata obscuriglobus, Planctomyces maris and "Candidatus Brocadia anammoxidans" was investigated based on ultrastructure derived from thin-sections of cryosubstituted cells, freeze-fracture replicas, and in the case of Gemmata obscuriglobus and Pirellula marina, computer-aided 3-D reconstructions from serial sections of cryosubstituted cells. All planctomycete cells display a peripheral ribosome-free region, termed here the paryphoplasm, surrounding the perimeter of the cell, and an interior region including any nucleoid regions as well as ribosome-like particles, bounded by a single intracytoplasmic membrane (ICM), and termed the pirellulosome in Pirellula species. Immunogold labelling and RNase-gold cytochemistry indicates that in planctomycetes all the cell DNA is contained wholly within the interior region bounded by the ICM, and the paryphoplasm contains no DNA but at least some of the cell's RNA. The ICM in Isosphaera pallida and Planctomyces maris is invaginated such that the paryphoplasm forms a major portion of the cell interior in sections, but in other planctomycetes it remains as a peripheral zone. In the anaerobic ammonium-oxidising ("anammox" process) chemoautotroph "Candidatus Brocadia anammoxidans" the interior region bounded by ICM contains a further internal single-membrane-bounded region, the anammoxosome. In Gemmata obscuriglobus, the interior ICM-bounded region contains the nuclear body, a double-membrane-bounded region containing the cell's nucleoid and all genomic DNA in addition to some RNA. Shared features of cell compartmentalisation in different planctomycetes are consistent with the monophyletic nature of the planctomycetes as a distinct division of the Bacteria. The shared organisational plan for the planctomycete cell constitutes a new type not known in cells of other bacteria.

[Role of the nucleolus in plant cell response to environmental physical factors]

Contemporary data on ultrastructural reconstruction and changes in functional nucleolus activity in plant cells affected by physical environmental factors are presented. The main attention is paid to modifications of ribosomal gene expression under these conditions and induced changes in r-chromatin structure and ribosomal DNA localization. Recent data on fine nucleolus structure and molecular aspects of its organization as well as influence of microgravitation and clinostating on structural and functional organization of plant nucleoli are reported.

Physical mapping, expression patterns and interphase organisation of rDNA loci in Portuguese endemic Silene cintrana and Silene rothmaleri

Double target in situ hybridization to root tip metaphase and interphase cells of Silene cintrana and Silene rothmaleri was used to allocate the position of 18S-5.8S-25S and 5S rRNA genes. In both species, the 18S-5.8S-25S rDNA probe labelled four sites located on the short arms of two submetacentric chromosomes. Only one locus for 5S rDNA was mapped adjacent to 18S-5.8S-25S genes in a subterminal position on the centromere side: in S. rothmaleri the 5S rDNA locus was adjacent to the small 18S-5.8S-25S locus while in S. cintrana it was near the large one. The NOR activity analysed by Ag-staining in metaphase cells revealed proportionality between in situ labelling dimensions and Ag-NORs. In both species all rDNA loci were potentially active, although in S. rothmaleri a tendency for the expression of only one locus was observed. Interphase organisation analysis of rDNA showed some differences between both species that were correlated with NOR activity.

Modification of wheat rDNA loci by rye B chromosomes: a chromatin organization model

The rDNA loci, and their associated NORs, on chromosomes 1B and 6B of the hexaploid wheat cv. Lindström have been used as a chromatin marker to investigate the functional basis of the phenotype effects of introgressed supernumerary B chromosomes (Bs) of rye. The rye Bs themselves lack genes, other than those which determine their mitotic drive mechanism, and the way in which they can modulate characters determined by the A chromosome background has always been a puzzle. An isogenic line of Lindström plants carrying different numbers of Bs was used as the experimental system to see how different doses of Bs (from 0 to 6) affected the activity of the wheat NORs and the organization of their rDNA loci at interphase. Silver staining on metaphase chromosomes was used to evaluate the previous activity of the NORs, and to reveal variations in their size; and the pTa71 FISH probe from wheat was used to visualize structural modifications to the interphase rDNA loci. A single B had no measurable effect, but, as the B-number increased, there were significant changes in the physical dimensions of the metaphase NORs, reflecting reduced levels of their activity earlier in the cell cycle, and also in the condensation patterns of the interphase rDNA loci. In addition, the higher B-numbers caused a size heteromorphism between the homologous NORs. A model is discussed which interprets the phenotypic effects of Bs generically, in nucleotypic terms, based of their being 'genetically inert' but 'chromosomally active'.

Physical mapping of rDNA genes, (TTAGGG)n telomeric sequence and other karyological features in two earthworms of the family Lumbricidae (Annelida: Oligochaeta)

A cytogenetical study was carried out on the chromosomes and nuclear DNA amounts of the terrestrial earthworms Octodrilus complanatus and Eisenia foetida (Annelida: Oligochaeta: Lumbricidae). Chromosomes were studied using Giemsa staining, banding methods and fluorescent in situ hybridization (FISH) with two repetitive DNA probes [rDNA and (TTAGGG)n]. rDNA FISH and silver staining consistently identified one chromosome pair per spread in both species. The telomeric sequence (TTAGGG)n hybridized with termini of all the chromosomes in both earthworms. Flow cytometry DNA assays showed that O. complanatus and E. foetida had different nuclear DNA contents (2C value=1.72 and=1.40 pg, respectively) but very similar base composition in their genomes.

Extrachromosomal rDNA and polarity of pro-oocytes during ovary development in Creophilus maxillosus (Coleoptera, Staphylinidae)

In telotrophic ovary of Creophilus maxillosus, the differentiation of the oocyte and nurse cells takes place within the linear clusters of sister oogonial cells. The amplification of rDNA occurs in the nuclei of pro-oocytes which are the most posterior cells of the clusters. During the consecutive oogonial divisions extrachromosomal rDNA segregates preferentially to the pro-oocyte of the next generation. We analyzed the ultrastructure of pro-oocytes and pro-nurse cells in the early and late phase of rDNA amplification in pupal ovary of Creophilus maxillosus. We found that pro-oocytes of the same generation contain variable amounts of extrachromosomal rDNA and that the presence of extra DNA is not limited to the nuclei of pro-oocytes; extra DNA is also present in the nuclei of some pro-nurse cells. Pro-oocytes can experience partial loss of extrachromosomal DNA during early oogonial divisions which is caused by the imprecise segregation of this material to the posterior pole. We believe that this imperfect segregation is a source of extrachromosomal DNA present in the nuclei of pro-nurse cells. Ultrastructural analysis showed that multiple nucleoli do not disperse in oogonial mitoses but remain associated with extrachromosomal chromatin and segregate with it to the posterior pole of the pro-oocyte. We also analyzed the ultrastructure of the germ plasm--a cytoplasmic structure present at the posterior pole of pro-oocytes. We have found that this structure contains spectrin and at the ultrastructural level is strikingly similar to the spectrosome which is present in germline cells of Drosophila. We also found spectrin in the intercellular bridges which connect oogonial cells and are known to contain fusomes.

3-D organization of ribosomal transcription units after DRB inhibition of RNA polymerase II transcription

In each bead of the nucleolar necklace, using adenosine analog DRB-treated PtK1 cells, we investigated the three components of rDNA transcription, i.e. the gene, transcription factor UBF and transcripts. In situ hybridization revealed the unraveling and 3-D dispersion of most of the rDNA coding sequences within the nucleus. The signals were small, of similar intensity and tandemly organized in the necklace. This observation is compatible with the fact that they might correspond to single gene units. Active transcription was visualized in these units, demonstrating that they were active functional units. Transcript labeling was not similar for each unit, contrary to UBF labeling. UBF and rRNA transcripts were only partially colocalized, as demonstrated by 3-D image analysis and quantification. As visualized by electron microscopy, the necklace was composed of a small fibrillar center partially surrounded by a dense fibrillar component. The 3-D arrangement of this individual unit in the necklace, investigated both by confocal and electron microscopy in the same cells, showed that the individual beads were linked by a dense fibrillar component. The reversibility of this organization after removal of DRB indicated that the beads in the necklace are certainly the elementary functional domain of the nucleolus. In addition, these results lead us to suggest that the organization of a functional domain, presumably corresponding to a single gene, can be studied by in situ approaches.

When rDNA transcription is arrested during mitosis, UBF is still associated with non-condensed rDNA

The mechanisms that control inactivation of ribosomal gene (rDNA) transcription during mitosis is still an open question. To investigate this fundamental question, the precise timing of mitotic arrest was established. In PtK1 cells, rDNA transcription was still active in prophase, stopped in prometaphase until early anaphase, and activated in late anaphase. Because rDNA transcription can still occur in prophase and late anaphase chromosomes, the kinetics of rDNA condensation during mitosis was questioned. The conformation of the rDNA was analyzed by electron microscopy from the G2/M transition to late anaphase in the secondary constriction, the chromosome regions where the rDNAs are clustered. Whether at transcribing or non-transcribing stages, non-condensed rDNA was observed in addition to axial condensed rDNA. Thus, the persistence of this non-condensed rDNA during inactive transcription argues in favor of the fact that mitotic inactivation is not the consequence of rDNA condensation. Analysis of the three-dimensional distribution of the rDNA transcription factor, UBF, revealed that it was similar at each stage of mitosis in the secondary constriction. In addition, the colocalization of UBF with non-condensed rDNA was demonstrated. This is the first visual evidence of the association of UBF with non-condensed rDNA. As we previously reported that the rDNA transcription machinery remained assembled during mitosis, the colocalization of rDNA fibers with UBF argues in favor of the association of the transcription machinery with certain rDNA copies even in the absence of transcription. If this hypothesis is correct, it can be assumed that condensation of rDNA as well as dissociation of the transcription machinery from rDNA cannot explain the arrest of rDNA transcription during mitosis. It is proposed that modifications of the transcription machinery occurring in prometaphase could explain the arrest of transcription, while reverse modifications in late anaphase could explain activation.

Association of DNA with nuclear matrix in in vitro assembled nuclei induced by rDNA from Tetrahymena shanghaiensis in Xenopus egg extracts

Nuclei assembled in vitro from Xenopus egg extracts and DNA display many properties known from in vivo nuclei. However, the distribution pattern of DNA in such nuclei remains unknown. We introduced rDNA from Tetrahymena macronuclei into Xenopus egg extracts and examined the association of specific DNA sequences with the nuclear matrix (NM) of the nuclei formed. We found that the 5' non-transcribed spacers (5'-NTS) specifically bound to the NM as previously shown in normal Tetrahymena macronuclei.

Cytoplasmic DNA in Entamoeba histolytica: its biological significance

We report a study on the DNA organization in Entamoeba histolytica using a ribosomal DNA probe. The rDNA genes were found forming mers which were separated in a typical ladder pattern by pulse field electrophoresis. DNA rosette structures were visualized through electron microscopy in DNA eluted from bands recognized by the ribosomal probe. The in situ hybridization experiments using a DNA probe suggested that the rDNA genes are portioned between the nucleus and a cytoplasmic structure. These findings provide new data on DNA organization in E. histolytica and open the question concerning the presence of a novel organelle in this eukaryotic parasite.

Metazoan rDNA enhancer acts by making more genes transcriptionally active

Enhancers could, in principle, function by increasing the rate of reinitiation on individual adjacent active promoters or by increasing the probability that an adjacent promoter is activated for transcription. We have addressed this issue for the repetitive metazoan rDNA enhancer by microinjecting Xenopus oocytes with enhancer-less and enhancer-bearing genes and determining by EM the frequency that each gene type forms active transcription units and their transcript density. We use conditions where transcription requires the normal rDNA promoter and is stimulated 30-50-fold by the enhancer. (In contrast, at saturating template conditions as used in previous EM studies, an aberrant mode of transcription is activated that is not affected by the rDNA enhancer or by the generally recognized rDNA promoter). The active transcription units on enhancer-less genes are found to be as densely packed with nascent transcripts and polymerases as those on enhancer-bearing genes and on the endogenous rRNA genes. Significantly, the enhancer-bearing genes are approximately 30-50-fold more likely to form such active transcription units than enhancer-less genes, consistent with their amounts of transcript. Complementary studies confirm that the enhancer does not affect elongation rate, the stability of the transcription complex, or transcript half-life. These data demonstrate that the repetitive metazoan rDNA enhancer causes more genes to be actively transcribed and does not alter the reinitiation rate on individual active genes.

Replication initiates at multiple dispersed sites in the ribosomal DNA plasmid of the protozoan parasite Entamoeba histolytica

In the protozoan parasite Entamoeba histolytica (which causes amoebiasis in humans), the rRNA genes (rDNA) in the nucleus are carried on an extrachromosomal circular plasmid. For strain HM-1:IMSS, the size of the rDNA plasmid is 24.5 kb, and 200 copies per genome are present. Each circle contains two rRNA transcription units as inverted repeats separated by upstream and downstream spacers. We have studied the replication of this molecule by neutral/neutral two-dimensional gel electrophoresis and by electron microscopy. All restriction fragments analyzed by two-dimensional gel electrophoresis gave signals corresponding to simple Y's and bubbles. This showed that replication initiated in this plasmid at multiple, dispersed locations spread throughout the plasmid. On the basis of the intensity of the bubble arcs, initiations from the rRNA transcription units seemed to occur more frequently than those from intergenic spacers. Multiple, dispersed initiation sites were also seen in the rDNA plasmid of strain HK-9 when it was analyzed by two-dimensional gel electrophoresis. Electron microscopic visualization of replicating plasmid molecules in strain HM-1:IMISS showed multiple replication bubbles in the same molecule. The location of bubbles on the rDNA circle was mapped by digesting with PvuI or BsaHI, which linearize the molecule, and with SacII, which cuts the circle twice. The distance of the bubbles from one end of the molecule was measured by electron microscopy. The data corroborated those from two-dimensional gels and showed that replication bubbles were distributed throughout the molecule and that they appeared more frequently in rRNA transcription units. The same interpretation was drawn from electron microscopic analysis of the HK-9 plasmid. Direct demonstration of more than one bubble in the same molecule is clear evidence that replication of this plasmid initiates at multiple sites. Potential replication origins are distributed throughout the plasmid. Such a mechanism is not known to operate in any naturally occurring prokaryotic or eukaryotic plasmid.

Spatial distribution of sex chromosomes and ribosomal genes: a study on human lymphocytes and testicular cells

The location of the sex chromosomes in relation to the rRNA genes in the nuclei of human lymphocytes and testicular cells was examined. Sex chromosomes were found to be located closer to ribosomal genes than would be expected assuming a random arrangement of these chromosomes with respect to rRNA genes. This proximity could be observed irrespective of the transcriptional activity of ribosomal genes indicating that the chromosomal material and not transcriptional activity is responsible for the intranuclear order of these chromosomes.

Changing nucleosome positions through modification of the DNA rotational information

The effects of the rotational information of DNA in determining the in vitro localization of nucleosomal core particles (ncps) have been studied in the Saccharomyces cerevisiae 5S rRNA repeat gene. We have altered the distribution of the phased series of flexibility signals present on this DNA by inserting a 25-bp tract, and we have analyzed the effects of this mutation on the distribution and on the frequencies of ncps, as compared with the wild type and a reference 21-bp insertion mutant. The variation of the standard free energy of nucleosome reconstitution was determined. The results show that the DNA rotational information is a major determinant of ncps positioning, define how many rotationally phased signals are required for the formation of a stable particle, and teach how to modify their distribution through the alteration of the rotational signals.

Transcription in the yeast rRNA gene locus: distribution of the active gene copies and chromatin structure of their flanking regulatory sequences

In growing yeast cells, about half of the 150 tandemly repeated rRNA genes are transcriptionally active and devoid of nucleosomes. By using the intercalating drug psoralen as a tool to mark accessible sites along chromatin DNA in vivo, we found that the active rRNA gene copies are rather randomly distributed along the ribosomal rRNA gene locus. Moreover, results from the analysis of a single, tagged transcription unit in the tandem array are not consistent with the presence of a specific subset of active genes that is stably maintained throughout cell divisions. In the rRNA intergenic spacers of yeast cells, an enhancer is located at the 3' end of each transcription unit, 2 kb upstream of the next promoter. Analysis of the chromatin structure along the tandem array revealed a structural link between transcription units and adjacent, 3' flanking enhancer sequences: each transcriptionally active gene is flanked by a nonnucleosomal enhancer, whereas inactive, nucleosome-packed gene copies are followed by enhancers regularly packaged in nucleosomes. From the fact that nucleosome-free enhancers were also detected in an RNA polymerase I mutant strain, we interpret these open chromatin structures as being the result of specific protein-DNA interactions that can occur before the onset of transcription. In contrast, in this mutant strain, all of the rRNA coding sequences are packaged in nucleosomal arrays. This finding indicates that the establishment of the open chromatin conformation on the activated gene copies requires elongating RNA polymerase I molecules advancing through the template.

Extrachromosomal amplification of rDNA in oocytes of Hemerobius spp. (Insecta, Neuroptera)

In previtellogenic oocytes of the neuropteran, Hemerobius spp., two distinct, DNA-positive intranuclear structures have been observed. Chromosomes of meiotic prophase assemble in the center of the oocyte nucleus forming a highly polymorphic karyosphere, which persists in this position until the very late stages of vitellogenesis. The extrachromosomal DNA body, containing amplified ribosomal genes, undergoes fragmentation and dispersion in the nucleoplasm. At the onset of previtellogenic growth, transcription of extra rDNA starts, which is accompanied by the appearance of dense, granular material (multiple nucleoli). Arising nucleoli gradually fill the nucleoplasm. At the electron microscopic (EM) level two electron dense structural forms of the granular material have been described. Together with general histological and ultrastructural analysis the amplification of rDNA genes in Hemerobius spp. oocytes has been demonstrated by means of the spreading technique, which has shown that extra rDNA is organized in rings containing various numbers of active ribosomal genes. The transcription activity of amplified genes is manifested in the form of typical "Christmas tree" structures.

Replication of transcriptionally active chromatin

In eukaryotic cells, active genes and their regulatory sequences are organized into open chromatin conformations in which nucleosomes can be modified, disrupted or totally absent. It has been proposed that these characteristic chromatin structures and their associated factors might be directly inherited by the newly synthesized daughter strands during chromosome duplication. Here we show that in the yeast Saccharomyces cerevisiae, replication machinery entering upstream of a transcriptionally active ribosomal RNA gene generates two newly replicated coding regions regularly packaged into nucleosomes, indicating that the active chromatin structure cannot be directly inherited at the replication fork. Whereas the establishment of an exposed chromatin conformation at some newly replicated rRNA gene promoters can occur shortly after the passage of the replication fork, regeneration of the active chromatin structure along the coding region is always a post-replicative process involving disruption of preformed nucleosomes.

Nucleolar organizer regions in follicular tumors of the thyroid

BACKGROUND

Nucleolar organizer regions (NORs) are loops of ribosomal DNA that occur in nucleoli and that transcribe to ribosomal RNA. NORs have been identified by means of the Ag-NOR technique in routinely processed tissues, and were found to be of discriminative value between some types of benign and malignant lesions.

METHODS

Follicular lesions of the thyroid (17 adenomas and 25 carcinomas) were examined. Ten normal thyroids served as the control group. All slides were stained by the Ag-NOR technique and the number of Ag-NOR dots were counted in 50 randomly selected cells. The mean number of Ag-NORs was calculated for each case. Data were statistically analyzed by the Student's unpaired t test.

RESULTS

The mean Ag-NOR counts were statistically higher in follicular carcinomas as compared to either follicular adenomas or the normal thyroid. Higher Ag-NOR counts were found in the more aggressively behaving tumors.

CONCLUSIONS

It is suggested that the Ag-NOR technique could be of use as an adjunct to diagnostic histopathology and as an indicator of biologic behavior in follicular tumors of the thyroid.

Formation and stability of higher order chromatin structures. Contributions of the histone octamer

Unique roles have been identified for the histone octamer in the formation and stabilization of higher order chromatin structures. Histone octamers were assembled onto 12 tandem repeats of Lytechinus 5 S rDNA, at either saturating or subsaturating ratios. The extent of oligonucleosome folding and intermolecular association in divalent salts was monitored using analytical and differential sedimentation techniques. Saturated oligonucleosomes (12 nucleosomes/DNA) sedimented at 29 S in very low salt buffer. In 1.0-2.0 mM MgCl2, saturated oligonucleosomes formed a maximally folded 55 S structure whose extent of compaction was equivalent to that of classical higher order 30-nm diameter chromatin structures. These results are in marked contrast to those obtained previously in NaCl, where the maximally folded oligonucleosome species sedimented at only approximately 40 S (Hansen, J. C., Ausio, J., Stanik, V. H., and van Holde, K. E. (1989) Biochemistry 28, 9129-9136). Mg(2+)-dependent formation of the 55 S conformation was inhibited by histone octamer depletion; the maximum sedimentation coefficient observed for rDNA molecules containing 10-11 nucleosomes in 2.0 mM MgCl2 was only 40 S. Above 2.0 mM MgCl2, the equilibrium was progressively shifted toward formation of large associated oligonucleosome species. The implications of these results to the mechanism of chromatin folding and its relationship to the biological activity of the chromatin fiber are discussed.

Depletion of functional ribosomal RNA operons in Escherichia coli causes increased expression of the remaining intact copies

The synthesis of ribosomal RNA is a complex and highly regulated process. To study this process, we have used deletion-insertions to disrupt sequentially from one to four of the seven rRNA (rrn) operons on the Escherichia coli genome. Inactivation of four rrn operons caused a 2.3-fold increase in the expression of a chloramphenicol acetyl transferase reporter gene fused to the tandem promoters of rrnA and a similar increase in the expression of the trp tRNA gene at the end of rrnC. This reflected enhanced expression of the remaining operons to compensate for having only three intact copies. The elevated expression was caused by an increase in both transcription initiation and RNA polymerase elongation rates specifically on rrn operons and occurred in the absence of changes in the intracellular concentration of ppGpp, suggesting that ppGpp is not involved in the regulation of this phenomenon. We discuss these results in relation to the ribosome feedback inhibition model described by Nomura and coworkers.

Multiple nucleosome positioning with unique rotational setting for the Saccharomyces cerevisiae 5S rRNA gene in vitro and in vivo

A simple no-background assay was developed for high-resolution in vivo analysis of yeast chromatin. When applied to Saccharomyces cerevisiae 5S rRNA genes (5S rDNA), this analysis shows that nucleosomes completely cover this chromosomal region, occupying alternative positions characterized by a unique helical phase. This supports the notion that sequence-intrinsic rotational signals are the major determinant of nucleosome localization. Nucleosomal core particles reconstituted in vitro occupy the same positions and have the same helically phased distribution observed in vivo, as determined by mapping of exonuclease III-resistant borders, mapping by restriction cleavages, and by DNase I and hydroxyl-radical digestion patterns.

The terminal balls characteristic of eukaryotic rRNA transcription units in chromatin spreads are rRNA processing complexes

When spread chromatin is visualized by electron microscopy, active rRNA genes have a characteristic Christmas tree appearance: From a DNA "trunk" extend closely packed "branches" of nascent transcripts whose ends are decorated with terminal "balls." These terminal balls have been known for more than two decades, are shown in most biology textbooks, and are reported in hundreds of papers, yet their nature has remained elusive. Here, we show that a rRNA-processing signal in the 5'-external transcribed spacer (ETS) of the Xenopus laevis ribosomal primary transcript forms a large, processing-related complex with factors of the Xenopus oocyte, analogous to 5' ETS processing complexes found in other vertebrate cell types. Using mutant rRNA genes, we find that the same rRNA residues are required for this biochemically defined complex formation and for terminal ball formation, analyzed electron microscopically after injection of these cloned genes into Xenopus oocytes. This, plus other presented evidence, implies that rRNA terminal balls in Xenopus, and by inference, also in the multitude of other species where they have been observed, are the ultrastructural visualization of an evolutionarily conserved 5' ETS processing complex that forms on the nascent rRNA.

Electron microscopic localization of ribosomal DNA in rat liver nucleoli by nonisotopic in situ hybridization

We have used postembedding nonisotopic in situ hybridization, with biotinylated rat ribosomal DNA (rDNA) as a probe and streptavidin coupled to 10-nm colloidal gold particles as the detection system, to localize rDNA sequences in rat liver nucleoli at the electron microscopic level. For comparison purposes, immunoelectron microscopy was performed for the detection of DNA. Our results indicate that ribosomal DNA sequences are enriched in the dense fibrillar component of the rat liver nucleolus. These data are discussed in relation to the putative site(s) for transcription of ribosomal genes.

Molecular dissection of a specific nuclear domain: the chromatin region of the ribosomal gene cluster in Xenopus laevis

Molecular dissection of the nuclear domain corresponding to the ribosomal chromatin cluster was investigated. The experimental scheme was based on the ability of restriction enzymes to digest the whole genome without affecting this region (several megabases in length). Such a strategy involved the judicious choice of restriction enzymes, which is possible in Xenopus laevis, where the rDNA sequence is known and the repeated units are organized into one unique cluster. SalI, XhoI, and EcoRV digestion produced frequent cutting of the genome leaving the ribosomal cluster intact. Isolation of the rDNA cluster was confirmed by separation of the digested DNA by pulsed-field electrophoresis. When applied to purified nuclei, this approach allowed the isolation of the ribosomal chromatin cluster under very mild conditions: no cleavages (either enzymatic or mechanical) were detectable. Since the purification scheme depends only on the DNA sequence outside of the rDNA cluster, it permits the obtention of this domain in different functional states. Electron microscopic analysis demonstrated that the domain organization is substantially preserved and maintains its looped organization (the size and the full number of loops were preserved). This purification scheme provides a powerful tool for studying the structure-function relationships within the ribosomal nuclear domain.

Nucleolar morphology and rDNA in situ hybridisation in monocytes

The aim of this study was to correlate morphological changes of nucleoli of non-proliferating monocytes to their functional activity, since nucleolar morphology is currently considered as a diagnostic marker for cell proliferation. Monocytes from healthy donors were fractionated by current counterflow centrifugation and kept in culture for 6 days. Cells were stimulated by the addition of 200 units/ml interferon gamma (IFN gamma). Under this stimulus the monocytes show no proliferation but a strongly augmented expression of type I Fc IgG receptor, human leucocyte antigen DR, human leucocyte antigen DP and human leucocyte antigen DQ. Morphological changes after stimulation included the appearance of multinucleated cells, typical signs of the activation of rRNA synthesis indicated by an increase in nucleolar size, and changes in nucleolar structure such as the appearance of reticulate and compact nucleoli. The number of nucleolus organiser regions (NORs) visualised by in situ hybridisation was compared with the position and number of nucleoli visualised by silver staining in interphase cells. In comparison with control cultures, activated monocytes show a distinct increase in the number of those NORs that take part in the formation of nucleoli. Our results show that, in non-proliferating activated monocytes, the morphology of nucleoli and the increase of NOR activity are similar to those in proliferating cells. NOR activation is therefore an indicator for cellular activity, but is not necessarily correlated with proliferation.

Electron microscopic localization of replication origins in Oenothera chloroplast DNA

The origins of chloroplast DNA (cpDNA) replication were mapped in two plastome types of Oenothera in order to determine whether variation in the origin of cpDNA replication could account for the different transmission abilities associated with these plastomes. Two pairs of displacement loop (D-loop) initiation sites were observed on closed circular cpDNA molecules by electron microscopy. Each pair of D-loops was mapped to the inverted repeats of the Oenothera cpDNA by the analysis of restriction fragments. The starting points of the two adjacent D-loops are approximately 4 kb apart, bracketing the 16S rRNA gene. Although there are small DNA length variations near one of the D-loop initiation sites, no apparent differences in the number and the location of replication origins were observed between plastomes with the highest (type I) and lowest (type IV) transmission efficiencies.

Functional analysis of internal transcribed spacer 2 of Saccharomyces cerevisiae ribosomal DNA

Using the previously described "tagged ribosome" (pORCS) system for in vivo mutational analysis of yeast rDNA, we show that small deletions in the 5'-terminal portion of ITS2 completely block maturation of 26 S rRNA at the level of the 29 SB precursor (5.8 S rRNA-ITS2-26 S rRNA). Various deletions in the 3'-terminal part, although severely reducing the efficiency of processing, still allow some mature 26 S rRNA to be formed. On the other hand, none of the ITS2 deletions affect the production of mature 17 S rRNA. Since all of the deletions severely disturb the recently proposed secondary structure of ITS2, these findings suggest an important role for higher order structure of ITS2 in processing. Analysis of the effect of complete or partial replacement of S. cerevisiae ITS2 with its counterpart sequences from Saccharomyces rosei or Hansenula wingei, points to helix V of the secondary structure model as an important element for correct and efficient processing. Direct mutational analysis shows that disruption of base-pairing in the middle of helix V does not detectably affect 26 S rRNA formation. In contrast, introduction of clustered point mutations at the apical end of helix V that both disrupt base-pairing and change the sequence of the loop, severely reduces processing. Since a mutant containing only point mutations in the sequence of the loop produces normal amounts of mature 26 S rRNA, we conclude that the precise (secondary and/or primary) structure at the lower end of helix V, but excluding the loop, is of crucial importance for efficient removal of ITS2.

Faithful in vivo transcription termination of Xenopus laevis rDNA. Correlation of electron microscopic spread preparations with S1 transcript analysis

DNA sequencing and subsequent functional in vitro analysis of the Xenopus laevis rDNA transcription termination has led to the identification of three transcription termination sequence elements: T1, located at the 3' end of the 28S rDNA; T2, a putative processing site 235 bp downstream of T1; T3, the principal terminator positioned 215 bp upstream of the gene promoter. As demonstrated for nuclear run-off assays, T3 was found to be the main terminator for Xenopus rDNA transcription. These in vitro data are in obvious contradiction to results obtained by electron microscopic (EM) spread preparations from rapidly isolated amplified oocyte nucleoli, i.e., an rDNA chromatin probe thought to represent the in vivo situation, indicative of transcription termination at sites T1-2. However, most interestingly, T3 had--again by the EM method--been identified as the exclusive terminator for NTS spacer transcription units. In order to answer the question of whether read-through transcription of the complete rDNA spacer sequence is obligatory for 40S pre-rRNA in vivo transcription, we analyzed several hundreds of spread rRNA genes from Xenopus oocyte nucleoli in great detail, applying two different spreading procedures, e.g., dispersal of amplified oocyte nucleoli shortly in detergent-free or detergent containing low-salt media prior to the EM spreading technique. Quantitation of EM spreads resulted in the finding that read-through rDNA spacer transcription beyond T1-2 termination sites (i.e., indicative of T3 transcription termination) can be visualized for the in vivo situation at a frequency of less than 3% of rRNA genes analyzed.(ABSTRACT TRUNCATED AT 250 WORDS)

Histone contributions to the structure of DNA in the nucleosome

We describe the application of the hydroxyl radical footprinting technique to examine the contribution of the core histone tails and of histones H3 and H4 to the structure of DNA in the nucleosome. We first establish that, as was previously determined for a nucleosome containing a unique sequence of DNA, mixed-sequence nucleosomes contain two distinct regions of DNA structure. The central three turns of DNA in the nucleosome have a helical periodicity of approximately 10.7 base pairs per turn, while flanking regions have a periodicity of approximately 10.0 base pairs per turn. Removal of the histone tails does not change the hydroxyl radical cleavage pattern in either mixed- or unique-sequence nucleosome samples. A tetramer of histones H3 and H4, (H3/H4)2, organizes the central 120 base pairs of DNA identically to that found in the nucleosome. Moreover, "tailless" octamers and the (H3/H4)2 tetramer recognize the same nucleosome positioning signals as the intact octamer.

Location of rRNA gene transcription sites within Ehrlich tumor cell nucleoli

The nucleolus is the morphological visualization of ribosome biogenesis. The spatial distribution of nucleolar DNA has been investigated in Ehrlich tumor cells by various immunogold labelling techniques. The DNase 1-sensitive regions within the nucleolus have been identified by in situ nick-translation at the ultrastructural level. The precise nucleolar location of rDNA and rRNA has been further specified by in situ hybridization and electron microscopy. Our results indicate that the fibrillar centers are the sole nucleolar structures where DNase 1-hypersensitive sequences, rDNA and rRNA are located together. These findings strongly suggest that rRNA gene transcription takes place within the confines of the fibrillar center, probably close to the boundary regions to the surrounding dense fibrillar component.

TFIIIA induced DNA bending: effect of low ionic strength electrophoresis buffer conditions

We have used a circular permutation gel shift assay to show that the 5S gene transcription factor, TFIIIA, induces a bend at the internal promoter of the Xenopus oocyte-type 5S gene. The degree of bending is comparable to what we have previously observed for TFIIIA induced bending of the Xenopus somatic-type gene [Schroth, G.P. et al. (1989) Nature 340, 487-488]. In addition, we show that TFIIIA induced DNA bending is dramatically affected by the ionic conditions used during gel electrophoresis. By modifying the conditions of the electrophoresis, we can detect two distinct conformations for the TFIIIA/DNA complex. In very low ionic strength buffers, the degree of DNA bending in the complex is estimated to be about 25 to 30 degrees, whereas in higher ionic strength buffers it is about 60 to 65 degrees. These data explain the apparent discrepancy between our results and the results of another study in which it was claimed that TFIIIA did not 'substantially' bend DNA [Zweib, C. and Brown, R.S. (1990) Nucleic Acid Res. 18, 583-587]. These results also demonstrate that the TFIIIA/DNA complex has a large degree of conformational flexibility. Both DNA bending and conformational flexibility are structural features which may provide a key insight into the function of TFIIIA as a positive transcription factor.

Localization of ribosomal cistrons at the ultrastructural level by in situ hybridization technique

In situ hybridization with 3H 28 S ribosomal RNA at the ultrastructural level shows labelling exclusively over the nucleolus and specifically over the dense nucleolar component (DNC). These findings clearly reveal that the ribosomal cistrons are located in the DNC of the nucleolus.

Sequential changes in the nucleoli of human spermatogonia with special reference to rDNA location and transcription

The nucleoli of human spermatogonia were studied using electron microscopy, silver staining, radioautography and in situ hybridization. In all types of A spermatogonia, nucleoli were consistently located at the periphery of the nucleus and contained a single fibrillar center associated with the nuclear envelope. In B spermatogonia, nucleoli were centrally located in the nuclei and showed several fibrillar centers or were found to disintegrate. Nucleolar morphology was found to be a good, though not an unequivocal indicator of spermatogonial type. The observed changes in nucleolar morphology reflect the differentiation of spermatogonia: the nucleolar disintegration seen in B spermatogonia corresponds to a pre-leptotene cessation of rDNA transcription. In radioautographs following 3H-uridine uptake, the label was consistently found over the dense fibrillar component, except in the B spermatogonia with disintegrating nucleoli, where no uptake could be detected. In situ hybridization demonstrated that the distribution of rDNA did not correspond to the site of the fibrillar center but to the dense fibrillar component. Compared with radioautographs, this finding clearly established that transcribed units of rDNA were located in the dense fibrillar component. Silver staining was strongly positive in fibrillar centers and in the dense fibrillar component. In Ap spermatogonia the silver deposit was often localized at the edge of the fibrillar threads. The relationships between silver-stained proteins and transcribed and nontranscribed portions of ribosomal genes are reevaluated.

The nucleolonema of plant and animal cells: a comparison

Depending on the author and the animal or plant origin of the material under study, the term "nucleolonema" is used in different contexts and thus indicates nucleolar ultrastructures that are different. In this paper, we attempt to clarify this state of affairs and to propose a definition for the plant cell nucleolonema.