The nucleolus is the most obvious structure in the eukaryotic nucleus. It is known to be a ribosome-producing apparatus where ribosomal (r) DNA is transcribed and the primary rRNA transcripts are processed to produce three of the four rRNA species. Electron microscopy has shown that the nucleolus consists of three major components, a dense fibrillar component (DFC), a granular component (GC) and a fibrillar center (FC). The DFC and FCs are integrated into a fundamental nucleolar substructure called the nucleolonema. The DFC corresponds to the matrix of the nucleolonema, and the FC is an electron microscopic counterpart of argyrophobic lacunae localized in the nucleolonema. The spherical FCs are intermittently arranged along the length of the nucleolonema in actively growing cells but are fused with each other to form tubular FCs when rDNA transcription is hampered. The RNase-gold complex does not bind to the FC but to the DFC and the GC, suggesting that rDNA transcription does not occur in the FC although both fluorescence in situ hybridization (FISH) and electron microscopic in situ hybridization reveal that the rDNA is specifically localized in the FCs. Immunogold-labeling after bromo-UTP (BrUTP) incorporation shows that rDNA transcription takes place in the boundary region between the FC and the DFC, and primary rRNA transcripts are expected to be processed outward within the DFC. Data have accumulated suggesting that the nucleolonema is a fundamental substructure of the nucleolus, and its skeleton is the tandem arrangement of the FCs, which are resting harbors or storages of rDNA. This paper proposes that the transversal structural organization of the nucleolonema is centrifugally built up by several structural and functional domains: condensed and/or loosened rDNA, rDNA transcription zone, and transcript processing and ribosome assembly zones.
Fibrillarin is known to play an important role in precursor ribosomal RNA processing and ribosome assembly. The present study describes a fibrillarin homolog gene isolated from tobacco BY-2 cells and its expression during the cell cycle. The cDNA for a fibrillarin homolog, named NtFib1, was first cloned in Nicotiana tabacum with degenerate primers. It encodes 314 amino acids and the deduced amino acid sequence has some highly conserved functional domains, such as the glycine and arginine-rich (GAR) domain for nucleolar localization and the RNA-binding motif. The C-terminal region is highly conserved and has 7 beta-sheets and 7 alpha-helices which are peculiar to fibrillarin. Thus, it is suggested that the fibrillarin homolog of this plant species functions in the same way as the fibrillarin already known from human and yeast cells. Northern blot analysis of BY-2 cells synchronized with aphidicolin or a combination of aphidicolin and propyzamide showed that the histone H4 gene was specifically expressed in the S phase but NtFib1 mRNA remained at high levels during the cell cycle. Examination of the localization of NtFib1 protein tagged with green-fluorescent protein (GFP) suggested that some persisting in the mitotic apparatus was eventually incorporated into reconstructed nucleoli in late telophase. Newly synthesized GFP-tagged NtFib1 protein in the cytoplasm was added to the recycled protein in early mitosis. Highly concentrated actinomycin D completely inhibited the transcription of genes coding for rRNA (rDNA) but did not significantly suppress the amount of either NtFib1 mRNA or protein, although the NtFib1 protein was reversibly dislocated from nucleoli. Although hypoxic shock completely prohibited rDNA transcription, NtFib1 mRNA remained at the same level as in the control experiment, even after the 4 h treatment. These results indicate that the transcription of NtFib1 mRNA is not related to rDNA transcription and NtFib1 mRNA is resistant to disrupting factors during the cell cycle.
Liquid crystal displays LCD have become used widely recently. Color filter CF is obviously one of the key components of those. High contrast ratio of LCD is still demanding. Deterioration of contrast ratio has been pointed out to be caused by the leakage light of CF set between the crossed Nicols polarizers. Adding organic pigment in vehicle and dispersed with Zirconia bead of different diameter, photoresist of CF with different mechanical dispersion intensity level was made. CF was made by coating photoresist on glass substrate and dried. Optical properties of CF samples with different mechanical dispersion levels were measured and the degree of depolarization of those was investigated. We revealed that increasing mechanical dispersion level in manufacturing process of photoresist decreased the degree of depolarization, which increased the contrast ratio.
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