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N-band proteins of nucleolar organizers: chromosomal mapping, subnucleolar localization and rDNA binding


The ribosomal DNA(rDNA)-containing chromatin in eukaryotes forms a unique architecture called the “secondary constriction” or “nucleolus organiser region (NOR)” on mitotic chromosomes. To gain more insight into non-histone chromosomal proteins (NHCP), termed “N-band proteins”, that are specifically associated with the NOR in a wide variety of eukaryotes, we attempted to: (1) identify the NHCP responsible for N-band staining; (2) determine their stoichiometry; (3) map them on metaphase chromosomes; (4) determine their subnucleolar localization and (5) examine their possible ability to bind rDNA. Based on several criteria, including chromosomal localization, solubility, association with chromatin, and intranuclear localization, two of the nucleolus-rich NHCP, termed component B of mol.wt 55,000 and component C of mol.wt. 41,000, were tentatively identified as N-band proteins. Immunological studies using a polyclonal, monospecific antibody raised against component C show that this protein is in fact associated with the chromosomal telomeres where NORs are located. In nucleoli, N-band proteins appear to be compartmentalized into a structure that probably corresponds to fibrillar components. Both components B and C are among several NHCPs that showed, under in vitro conditions, a strong affinity for rDNA cloned in lambda phage but not for calf thymus genomic DNA or phage vector DNA. The antibody against component C effectively suppressed in vitro transcription by RNA polymerase I in nucleoli and nucleolar chromatin. Component C appears to exist in the nucleus at 3.75–5.13×103 copies per rDNA unit or 0.09–0.13 copy per nucleotide. These findings support the hypothesis that the NOR is a chromosomal site, architecturally not only unique but also different from other chromatin regions in that constituent DNA, i.e., rDNA, is organized in a specific manner by interacting with specific NHCP, i.e., N-band proteins.

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Correspondence to Sei-ichi Matsui.

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This study was supported in part by a grant from the National Science Foundation (PCM-8115600) awarded to S.M. Parts of this study were presented at The Annual Meetings of The American Society for Cell Biology in 1978, 1983 and 1984

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Matsui, S., Fuke, M., Chai, L. et al. N-band proteins of nucleolar organizers: chromosomal mapping, subnucleolar localization and rDNA binding. Chromosoma 93, 231–242 (1986). https://doi.org/10.1007/BF00292743

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  • Metaphase Chromosome
  • Mitotic Chromosome
  • Nucleolus Organiser Region
  • Chromosomal Mapping
  • Chromatin Region