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

• Published in: The EMBO journal 1985-05, Vol.4 (5), p.1237-1243
• Main Authors: Muller, M.T., Pfund, W.P., Mehta, V.B., Trask, D.K.
• Format: Article
• Language: English
• Subjects: Analytical, structural and metabolic biochemistry; Animals; Antibodies - immunology; Base Sequence; Biological and medical sciences; Cell Nucleolus - enzymology; DNA Topoisomerases, Type I - analysis; DNA Topoisomerases, Type I - immunology; DNA, Ribosomal - metabolism; Enzymes and enzyme inhibitors; Fundamental and applied biological sciences. Psychology; Isomerases; Microscopy, Electron; Rabbits; Transcription, Genetic
• ISSN: 0261-4189; 1460-2075
• DOI: 10.1002/j.1460-2075.1985.tb03766.x

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.