title: Human topoisomerase I forms double cleavage complexes on natural DNA.
abstract: DNA topoisomerase I releases torsional stress generated in chromatin during transcription and replication. Usually topoisomerase I is recognized to work as a monomer, but previously we have shown that two molecules can form a dimer-like protein-protein complex on a 'suicide' DNA substrate resulting in a topoisomerase I double cleavage complex. Here we show that during the normal relaxation reaction a considerable fraction of human topoisomerase I formed transient dimers on plasmid DNA too. Recombinant as well as topoisomerase I purified from human cells formed double cleavage complexes within a distance of 12 or 14 nucleotides. When topoisomerase I was isolated from camptothecin-treated HeLa cells, a considerable fraction migrated to the same position as topoisomerase I bearing a covalently bound 12-to-14-mer oligonucleotide. Taken together our data suggest that human topoisomerase I double cleavage complexes are part of the normal catalytic cycle of this enzyme that occur in vitro and possibly also in vivo.
authors.affiliation: Leibniz Institute for Age Research, Fritz Lipmann Institute (FLI), Biochemical Group, Beutenbergstrasse 11, D-07745 Jena, Germany.
authors.forename: Kent
authors.initials: K
authors.lastname: Søe
authors.forename: Sabine
authors.initials: S
authors.lastname: Hartung
authors.forename: Frank
authors.initials: F
authors.lastname: Grosse
mesh_terms: D002166:Camptothecin / Q000494:pharmacology; D002384:Catalysis; D002843:Chromatin / Q000737:chemistry; D004247:DNA / Q000737:chemistry*; D004264:DNA Topoisomerases, Type I / Q000737:chemistry*; D019281:Dimerization; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006367:HeLa Cells; D006801:Humans; D009690:Nucleic Acid Conformation; D009841:Oligonucleotides / Q000737:chemistry; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry
chemical_list: D002843:Chromatin; D004791:Enzyme Inhibitors; D009841:Oligonucleotides; D011994:Recombinant Proteins; D004247:DNA; D004264:DNA Topoisomerases, Type I; D002166:Camptothecin