Mitogenic factor secreted by Streptococcus pyogenes is a heat-stable nuclease requiring His122 for activity

1 Shionogi Institute for Medical Science, 2-5-1 Mishima, Settsu, Osaka 566, Japan 2 Discovery Research laboratory I, Shionogi & Co., Ltd, 3-1-1 Futaba-cho, Toyonaka-shi, Osaka 561, Japan *Author for correspondence: Takashi Yutsudo. Tel: +81 6 331 8081. Fax: +81 6 331 8612. e-mail: takashi.yutsudo@shionogi.co.jp ABSTRACT Summary: The gene encoding a mitogenic factor, termed MF, was cloned from Streptococcus pyogenes and the recombinant MF was overexpressed in Escherichia coli. 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 2+ and Ca 2+ , 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 122 was a residue essential to the nuclease activity. Keywords: mitogenic factor, heat-stable nuclease, DNA hydrolysis, Streptococcus pyogenes