title: The expression and localization of Scl-70/DNA topoisomerase I vary throughout the cell cycle.
abstract: OBJECTIVE
  We investigated the expression and localization of topoisomerase I by Western blot and indirect fluorescent antibody assay, respectively, using anti-Scl-70/topo I from patients with diffuse scleroderma. The contribution of topoisomerase I to DNA replication was assessed using cells treated with the topoisomerase I inhibitor camptothecin.
  
  
  RESULTS
  Scl-topo I was detected at all cell cycle phases as a single immunoreactive band of 100 kDa. Extracts from cells in the S phase contained the largest amount of immunoreactive Scl-70/topo I. Variations in the subcellular distribution of Scl-70/topo I were seen throughout the cell cycle, with a speckled nucleoplasmic distribution during G1 contrasting with concentration within the nucleolus during S. Camptothecin exposure blocked topoisomerase I expression and caused a significant decrease in DNA production.
  
  
  CONCLUSIONS
  These data suggest (1) that topomerase I is active mainly during the S phase and contributes to DNA replication, and (2) that topoisomerase I may be involved in ribosomal gene transcription.
authors.affiliation: Immunotechnology Unit, Zacatecas Autonemous University, Guadalupe, Mexico.
authors.forename: R
authors.initials: R
authors.lastname: Ramirez-Santoyo
authors.forename: A
authors.initials: A
authors.lastname: Lopez-Swiderski
authors.forename: E
authors.initials: E
authors.lastname: Avalos-Diaz
authors.forename: R
authors.initials: R
authors.lastname: Herrera-Esparza
mesh_terms: D000972:Antineoplastic Agents, Phytogenic / Q000494:pharmacology; D001324:Autoantigens / Q000032:analysis / Q000235:genetics / Q000378:metabolism; D015153:Blotting, Western; D002166:Camptothecin / Q000494:pharmacology; D002453:Cell Cycle / Q000187:drug effects / Q000276:immunology; D002455:Cell Division / Q000187:drug effects; D004261:DNA Replication / Q000187:drug effects / Q000276:immunology; D004264:DNA Topoisomerases, Type I / Q000032:analysis / Q000378:metabolism*; D019084:Fluorescent Antibody Technique, Indirect; D006801:Humans; D006918:Hydroxyurea / Q000494:pharmacology; D008940:Mitotic Index; D009687:Nuclear Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D019384:Nucleic Acid Synthesis Inhibitors / Q000494:pharmacology; D016196:S Phase; D012595:Scleroderma, Systemic / Q000201:enzymology / Q000378:metabolism; D059004:Topoisomerase I Inhibitors; D014407:Tumor Cells, Cultured / Q000737:chemistry / Q000166:cytology / Q000201:enzymology
chemical_list: D000972:Antineoplastic Agents, Phytogenic; D001324:Autoantigens; D009687:Nuclear Proteins; D019384:Nucleic Acid Synthesis Inhibitors; C048100:Scl 70 antigen, human; D059004:Topoisomerase I Inhibitors; D004264:DNA Topoisomerases, Type I; D006918:Hydroxyurea; D002166:Camptothecin