DNA Topoisomerase I from Mitochondria of Xenopus laevis Oocytes

Activity of DNA topoisomerase I has been characterized in extracts of mitochondria purified from Xenopus laevis oocytes. Several lines of evidence have been obtained for the intramitochondrial localization of the enzyme. The mitochondria‐associated activity of DNA topoisomerase I represents 1 % of the activity recovered from a total ovary population of oocytes. The enzyme has been purified by DEAE‐cellulose, phosphocellulose and double‐stranded DNA cellulose chromatography and its properties compared to those of its nuclear‐cytosolic counterpart purified by the same purification steps. Both enzymes appear to possess very similar if not identical physico‐chemical and catalytic properties. Neither enzyme shows DNA gyrase activity and they are both equally sensitive towards trypanocide drugs such as ethidium bromide and berenil. The mitochondrial enzyme is able to remove positive superhelical turns and possibly acts as a swivelase during replication of mitochondrial DNA. These results confirm the pioneering work of Fairfield et al. (1979, J. Biol. Chem. 254, 9352–9354) on the presence of a DNA topoisomerase I in mitochondria. However our results support the conclusion that in Xenopus laevis oocytes the mitochondrial and nuclear cytosolic DNA topoisomerase I are in all likelihood one and the same enzyme.