Type IIL Restriction-Modification Enzyme MmeI is a large enzyme that integrates DNA recognition and methyltransferase and endonuclease activities within the same polypeptide [2, 3, 4]. MmeI is composed of five domains. An N-terminal endonuclease domain (residues 1-155), connects to a DNA-methyltransferase domain (MTase, residues 301-320) via a multi-helical spacer (residues 156-300). These are followed by the target recognition domain (TRD, residues 621-825), and a final C-terminal helical bundle (residues 826-919) [1]. This domain corresponds to the N-terminal endonuclease, it cuts the two DNA strands at one site simultaneously with enzyme bound at two sites interacting to accomplish the cleavage [2].

1: Structure of Type IIL Restriction-Modification Enzyme MmeI in Complex with DNA Has Implications for Engineering New Specificities. Callahan SJ, Luyten YA, Gupta YK, Wilson GG, Roberts RJ, Morgan RD, Aggarwal AK; PLoS Biol. 2016;14:e1002442. PMID:27082731

2: MmeI: a minimal Type II restriction-modification system that only modifies one DNA strand for host protection. Morgan RD, Bhatia TK, Lovasco L, Davis TB; Nucleic Acids Res. 2008;36:6558-6570. PMID:18931376

3: Isolation and computer-aided characterization of MmeI, a type II restriction endonuclease from Methylophilus methylotrophus. Boyd AC, Charles IG, Keyte JW, Brammar WJ; Nucleic Acids Res. 1986;14:5255-5274. PMID:3016643

4: Two intertwined methylation activities of the MmeI restriction-modification class-IIS system from Methylophilus methylotrophus. Tucholski J, Zmijewski JW, Podhajska AJ; Gene. 1998;223:293-302. PMID:9858752