Chloroplast DNA topoisomerase I from cauliflower.

An ATP-independent DNA topoisomerase has been isolated from chloroplasts of cauliflower leaves (Brassica oleracea var. botrytis) through DEAE-cellulose, AF-blue Toyopearl, and hydroxyapatite column chromatography. The sedimentation coefficient and Stokes radius of this enzyme are 3.6S and 3.6 nm, respectively, and the molecular weight of native enzyme is estimated to be 54,000. This enzyme changes the linking number in steps of one. The enzyme activity is stimulated by MgCl2, and this enzyme shows optimum activity at 30 degrees C in the range of 3 mM MgCl2 + 100 mM KCl-10 mM MgCl2 + 50 mM KCl. The enzyme activity was reduced remarkably by N-ethylmaleimide, indicating that a free sulfhydryl group is important for the activity; heparin and ellipticine also reduced the activity. Both cauliflower chloroplast topoisomerase and spinach chloroplast topoisomerase can relax positive supercoils as well as negative supercoils. From these properties, cauliflower chloroplast topoisomerase can be classified as a eukaryotic type I DNA topoisomerase.