Micrococcal nuclease (MNase; EC 3.1.3 1.1) was first described by Cunningham et al. who, in 1956, recovered a deoxyribonuclease from culture supematant of the bacterium Micrococcuspyogenes var. aureus (now named Staphylococcus aureus). This extracellular nuclease was unusually thermostable, required calcium as a cationic activator and hydrolysed the 5’-phosphodiester bond of deoxyribonucleotides to yield 3’-monoand oligonucleotides and a free 5’-hydroxyl group. The enzyme is comprised of 159 amino acid residues and weighs 16,800 dalton (Taniuchi et al., 1967) and optimal nucleolytic activity is obtained at pH 8.6 with calcium ions at 10mM (Cunningham, 1958). Cunningham initially called the enzyme “micrococcal DNase”, however, he renamed it “micrococcal nuclease” due to its ribonuclease activity (Cunningham, 1958). The two strains of S. aureus used as a source of MNase are Foggi and V8, with the higher yielding Foggi strain used for commercial enzyme production (Anfinsen et al., 1971). The requirement for calcium cannot be met by magnesium ions, however, weak activity for DNA may be obtained with strontium ions, whereas activity is completely lost in the presence of EDTA (Heins et al., 1967; Cotton et al., 1979). The ribonuclease and deoxyribonuclease activities of MNase are competitively inhibited by a number of 5’-monophosphonucleotide derivatives, deoxythymidine 3’,5’diphosphate (pdTp; the most tightly binding inhibitor) and some heavy metal cations (Cotton et al., 1979). Analysis of the crystal structure of the pdTp-MNase complex has shown the DNA binding site to be a groove which binds three phosphate groups to the 3’ side of the hydrolyic site of a single DNA strand (Anfinsen et al., 1971). Further chemical properties of the enzyme have been reviewed by Anfinsen et al. (1971). The specificity of MNase for cleavage of DNA in the linker region between nucleosomes (discussed in
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