Summary: The gene encoding a mitogenic factor, termed MF, was cloned from and the recombinant MF was overexpressed in Both the natural and recombinant MF had heat-resistant nuclease activity. The nuclease activity of MF was characterized using the recombinant protein. MF showed endonuclease activity, digesting ssDNA, dsDNA and tRNA. The optimal pH for the DNase activity of MF was 9.5. The DNase activity was enhanced approximately tenfold by the simultaneous presence of two divalent cations, Mg and Ca, compared to either alone and was inhibited by EDTA or NaCI. The heat stability of MF was biphasic; the DNase activity was heat-stable from 0 to 50 °C and over 80 °C but very unstable at around 60 °C. DNA digested by MF possessed 5′-phosphorylated and 3′-hydroxylated termini, identical to those obtained by digestion of DNA by pancreatic deoxyribonuclease I. A mutant clone revealed that His was a residue essential to the nuclease activity.


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