title: Eukaryotic type I topoisomerase is enriched in the nucleolus and catalytically active on ribosomal DNA.
abstract: The distribution of eukaryotic DNA topoisomerase I in the cell has been analyzed at four levels: (i) at the level of the nuclear matrix; (ii) at the cytological level by immunofluorescence of whole cells; (iii) at the electron microscopic level using the protein A/colloidal gold technique; and (iv) at the level of DNA to identify in situ the sequence upon which topoisomerase I is catalytically active. Although topoisomerase I is clearly distributed non-randomly in the nucleus, the unique distribution of the enzyme is not related to the nuclear matrix. The data support the conclusion that topoisomerase I is heavily concentrated in the nucleolus of the cell; furthermore, particular regions within the nucleolus are depleted of topoisomerase. A technique has been developed which allows isolation and analysis of the cellular DNA sequences covalently attached to topoisomerase. Ribosomal DNA sequences are at least 20-fold enriched in topoisomerase/DNA complexes isolated directly from a chromosomal setting, relative to total DNA. This is the first direct evidence that topoisomerase I is catalytically active on ribosomal DNA in vivo.
authors.forename: M T
authors.initials: MT
authors.lastname: Muller
authors.forename: W P
authors.initials: WP
authors.lastname: Pfund
authors.forename: V B
authors.initials: VB
authors.lastname: Mehta
authors.forename: D K
authors.initials: DK
authors.lastname: Trask
mesh_terms: D000818:Animals; D000906:Antibodies / Q000276:immunology; D001483:Base Sequence; D002466:Cell Nucleolus / Q000201:enzymology*; D004264:DNA Topoisomerases, Type I / Q000032:analysis* / Q000276:immunology; D004275:DNA, Ribosomal / Q000378:metabolism*; D008854:Microscopy, Electron; D011817:Rabbits; D014158:Transcription, Genetic
chemical_list: D000906:Antibodies; D004275:DNA, Ribosomal; D004264:DNA Topoisomerases, Type I