Fibrillar Component Of Nucleoli Research Articles

SR and SR-related proteins redistribute to segregated fibrillar components of nucleoli in a response to DNA damage

Pre-mRNA splicing factors are often redistributed to nucleoli in response to physiological conditions and cell stimuli. In telophase nuclei, serine-arginine rich (SR) proteins, which usually reside in nuclear speckles, localize transiently to active ribosomal DNA (rDNA) transcription sites called nucleolar organizing region-associated patches (NAPs). Here, we show that ultraviolet light and DNA damaging chemicals induce the redistribution of SR and SR-related proteins to areas around nucleolar fibrillar components in interphase nuclei that are similar to, but distinct from, NAPs, and these areas have been termed DNA damage-induced NAPs (d-NAPs). In vivo labeling of nascent RNA distinguished d-NAPs from NAPs in that d-NAPs were observed even after full rDNA transcriptional arrest as a result of DNA damage. Studies under a variety of conditions revealed that d-NAP formation requires both RNA polymerase II-dependent transcriptional arrest and nucleolar segregation, in particular, the disorganization of the granular nucleolar components. Despite the redistribution of SR proteins, splicing factor-enriched nuclear speckles were not disrupted because other nuclear speckle components, such as nuclear poly(A) RNA and the U5-116K protein, remained in DNA-damaged cells. These data suggest that the selective redistribution of splicing factors contributes to the regulation of specific genes via RNA metabolism. Finally, we demonstrate that a change in alternative splicing of apoptosis-related genes is coordinated with the occurrence of d-NAPs. Our results reveal a novel response to DNA damage that involves the dynamic redistribution of splicing factors to nucleoli.

Components of U3 snoRNA-containing complexes shuttle between nuclei and the cytoplasm and differentially localize in nucleoli: implications for assembly and function.

U3 small nucleolar RNA (snoRNA) and associated proteins are required for the processing of preribosomal RNA (pre-rRNA) and assembly of preribosomes. There are two major U3 snoRNA-containing complexes. The monoparticle contains U3 snoRNA and the core Box C/D snoRNA-associated proteins and an early preribosome-associated complex contains the monoparticle and additional factors that we refer to as preribosome-associated proteins. To address how and where the U3 snoRNA-containing preribosome assembles and how these processes are temporally and spatially regulated, we have examined the dynamics and distribution of human U3 complex-associated components in cells with active or inactive transcription of rDNA. We found that U3 complex-associated proteins shuttle between the nucleus and the cytoplasm independent of the synthesis and export of preribosomal particles, suggesting that the shuttling of these proteins may either provide opportunities for their regulation, or contribute to or modulate ribosome export. In addition, monoparticle and preribosome associated components predominantly localize to different nucleolar substructures, fibrillar components, and granular components, respectively, in active nucleoli, and partition separately into the two components during nucleolar segregation induced by inhibition of pol I transcription. Although the predominant localizations of these two sets of factors differ, there are significant areas of overlap that may represent the sites where they reside as a single complex. These results are consistent with a model in which U3 monoparticles associate with the fibrillar components of nucleoli and bind pre-rRNA during transcription, triggering recruitment of preribosome-associated proteins to assemble the complex necessary for pre-rRNA processing.

Effects of Fusarium oxysporum culture filtrates on carnation callus cell ultrastructure and cytoplasmic calcium distribution

Calli of Mediterranean carnation ( Dianthus caryophyllus L.) cvs. Corrida and Pulcino, susceptible and resistant respectively to Fusarium oxysporum f. sp. dianthi race 2, were exposed to fungus culture filtrates (FCF). The ultrastructure of cells from carnation callus was studied by transmission electron microscopy. Localization of intracellular Ca 2+ was achieved by a precipitation technique using potassium pyroantimonate. Cells from the resistant carnation cultivar exposed to 200 μl FCF for 15 min showed the presence of electron-dense granules in vacuolar and plasma membranes and marked ultrastructural changes. Treated or untreated cv. Corrida, as well as untreated cv. Pulcino callus cells, did not show any appreciable ultrastructural changes, and electron-dense precipitates were randomly distributed in the cytoplasm. The electron-dense granules, when analysed by electron probe microanalysis, appeared to be calcium precipitated with antimony. The involvement of calcium in the resistant carnation callus treated with FCF reported here, provides evidence of the possible involvement of this cation in the defence reaction. A fractal dimension study on the rough nature of the nuclei showed that nuclei of treated cv. Pulcino were highly invaginated and the arrangement of granular and fibrillar components of nucleoli were shown to be characteristic of high transcriptional activity. A quantitative ultrastructural study of the cytoplasmic organelles, performed on the micrographs, showed a significant ( P < 0·001) increase in dictyosome and endoplasmic cisternae in the resistant cultivar.

Isolation and initial characterization of nucleolar fibrillar remnants from the liver of rats treated with d-galactosamine

Administration of d-galactosamine (250 mg/kg body weight) to rats causes a rapid and complete block of transcription. Within 4 h the granular components of nucleoli disappear, while the fibrillar components are condensed and converted into fibrillar remnants. The amount of precursors to rRNA (pre-rRNA) and rRNA components (mostly 28S rRNA) is reduced to less than 10% of that in control nucleoli. Electron microscopic radioautography shows also that the nucleolar fibrillar remnants do not contain pre-rRNA in growth-arrested transcription complexes. The Ag-staining reaction, adapted for electron microscopy, is positive with the fibrillar components of nucleoli. After d-galactosamine, the positive Ag staining is associated exclusively with the nucleolar fibrillar remnants. The positive Ag staining and the presence of rRNA genes, as revealed by rRNA:DNA hybridization, show that the nucleolar fibrillar remnants contain nucleolus organizer components. The fraction of nucleolar fibrillar remnants, isolated from nuclei of d-galactosamine-treated rats, preserves its ultrastructural characteristics observed in situ. It contains mainly DNA and proteins, including histones and nucleolar matrix proteins. The implications of these results for the elucidation of structural-biochemical correlations in the nucleolus are discussed.

Localization of the pyroantimonate—osmium reaction product in plant cell nucleoli

Fixation using a pyroantimonate—osmium mixture has been carried out on plant tissues (Allium cepa roots and anthers). Excellent preservation of the cell ultrastructure was obtained, which made it possible for us to analyze the occurrence and distribution of pyroantimonate reactive sites within nuclei. A preferential location of antimonate deposits was on the fibrillar components of nucleoli, especially in the vicinity of the nucleolus organizing region. These observations have been compared with results obtained previously by other authors in animal cells. The functional significance of the antimonate deposit distribution within nucleoli is briefly discussed.

Evidence that the granular and fibrillar components of nucleoli contain 28 and 6S RNA, respectively

Electron microscopic studies of nucleoli were correlated with sucrose density gradient sedimentation analysis of nucleolar RNA in samples obtained from livers of control rats, rats treated with thioacetamide, rats treated with actinomycin D and rats treated with both compounds. By planimetric analysis the fibrillar elements comprised approximately 20 per cent of the nucleolar area in the controls; the percentages of the total nucleolar area containing fibrillar elements were markedly increased after administration of actinomycin D and markedly decreased after administration of thioacetamide. Corresponding changes were found in 6S RNA following administration of actinomycin D and thioacetamide. When both compounds were administered simultaneously, coalescence of the fibrillar elements was observed and the resulting spherules were apparently extruded from the nucleolus, leaving nucleoli largely composed of granular elements. In these nucleoli, the 6S RNA was almost absent. These data provide evidence that 6S RNA is present in the fibrillar element of the nucleolus and that the 28S RNA is present in granular elements.