Eukaryotic type I topoisomerase is enriched in the nucleolus and catalytically active on ribosomal DNA

EMBO J. 1985 May;4(5):1237-43.


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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Antibodies / immunology
  • Base Sequence
  • Cell Nucleolus / enzymology*
  • DNA Topoisomerases, Type I / analysis*
  • DNA Topoisomerases, Type I / immunology
  • DNA, Ribosomal / metabolism*
  • Microscopy, Electron
  • Rabbits
  • Transcription, Genetic


  • Antibodies
  • DNA, Ribosomal
  • DNA Topoisomerases, Type I