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An endodeoxyribonuclease specific to apurinic sites in DNA was purified 12000-fold from vegetative cells of the anaerobic thermophilic bacterium, Desulfotomaculum nigrificans strain IFO 13698. The enzyme specifically hydrolyses phosphodiester bonds of apurinic double-stranded DNA, without action on normal, alkylated, or single-stranded depurinated DNA. The endodeoxyribonuclease has a molecular weight of about 18500 and a sedimentation value of 2·2S. The enzyme has an optimum pH of 7·5–8·0 and an optimum temperature of 55 °C. The half-life of purified enzyme is 55 min at 60 °C but it can be heated at 60 °C for at least 60 min without loss of activity in the presence of BSA. The purified enzyme has an absolute requirement for Mg2+ or Mn2+, and is inhibited by EDTA. Enzyme activity is completely inhibited by 0·5 m-NaCl or 1 mm-p-chloromercuribenzoate. All these properties differ from those of apurinic/apyrimidinic endodeoxyribonucleases isolated from Bacillus subtilis, Bacillus stearothermophilus. and Escherichia coli.
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