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Structure and Function of Biopolymers
Nuclear DNA topoisomerase from the Zea mays embryos
- N. G. Kholodny Institute of Botany, Academy of Sciences of the Ukrainian SSR
ATP-independent presumably type I DNA topoisomerase was partially purified from the nuclear extract of Zea mays mature embryos by separation in the polyethylene glycol-ammonium sulphate system with 2 M NaCl and by salt gradient elution from CM-Sepha-dex. The ability of enzyme to relax negative supercoils of plasmid pBR322 DNA was detected in the presence of EDTA, being however markedly stimulated by the magnesium ions and polyamines (spermidine and cadaverine). The reaction proceeded via either processive or distributive mechanism depending on the ionic strength of the enzyme assay mixture. Novobiocin and nalidixic acid (both at 100 u.g/ml) did not inhibit the enzyme activity. Modification with p-hydroxymercuribenzoate or iodoacetamide completely abolished the DNA topoisomerase activity, thus indicating the presence of essential cysteine residues in the enzyme molecules. The DNA topoisomerase activity was found to be completely depressed by the preparation of potent inhibitor of polynucleotide-binding proteins – aurintricarboxylic acid in partially polymerized form. Magnesium ions partially restored the DNA topoisomerase activity in the presence of aurintricarboxylic acid.
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