Micrococcal nuclease (EC3.1.31.1, S7 Nuclease, MNase, spleen endonuclease, thermonuclease, nuclease T, micrococcal endonuclease, nuclease T', staphylococcal nuclease, spleen phosphodiesterase, Staphylococcus aureus nuclease, Staphylococcus aureus nuclease B, ribonucleate (deoxynucleate) 3'-nucleotidohydrolase) is an endo-exonuclease that preferentially digests single-strandednucleic acids. The rate of cleavage is 30 times greater at the 5' side of A or T than at G or C and results in the production of mononucleotides and oligonucleotides with terminal 3'-phosphates.[1] The enzyme is also active against double-stranded DNA and RNA and all sequences will be ultimately cleaved.
The enzyme has a molecular weight of 16.9kDa. The pH optimum is reported as 9.2. The enzyme activity is strictly dependent on Ca2+ and the pH optimum varies according to Ca2+ concentration.[2] The enzyme is therefore easily inactivated by EGTA.
This enzyme is the extracellular nuclease of Staphylococcus aureus. Two strains, V8 and Foggi, yield almost identical enzymes.[3] A common source is E.coli cells carrying a cloned nuc gene encoding Staphylococcus aureus extracellular nuclease (micrococcal nuclease).
The 3-dimensional structure of micrococcal nuclease (then called Staphyloccal nuclease) was solved very early in the history of protein crystallography, in 1969.[4] Higher-resolution, more recent crystal structures are available for the apo form[5] and for the thymidine-diphosphate-inhibited form.[6][7] As seen in the ribbon diagram above, the nuclease molecule has 3 long alpha helices and a 5-stranded, barrel-shaped beta sheet, in an arrangement known as the OB-fold (for oligonucleotide-binding fold) as classified in the SCOP database.
^Colin Dingwall, George P. Lomonossoff, Ronald A. Laskey, High sequence specificity of micrococcal nuclease, Nucleic Acids Research, Volume 9, Issue 12, 25 June 1981, Pages 2659–2674, https://doi.org/10.1093/nar/9.12.2659
