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]. The DNA is embedded between the TRD and the MTase domain. The TRD makes contacts to the DNA bases primarily in the major groove, while the MTase domain makes several contacts to the DNA in the minor groove [1]. This domain corresponds to the DNA-methyltransferase. Structurally, it consists of a twisted beta-sheet flanked by alpha-helices on both sides.

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

5: Structural characterization of a modification subunit of a putative type I restriction enzyme from Vibrio vulnificus YJ016. Park SY, Lee HJ, Song JM, Sun J, Hwang HJ, Nishi K, Kim JS; Acta Crystallogr D Biol Crystallogr. 2012;68:1570-1577. PMID:23090406

6: Structural basis underlying complex assembly and conformational transition of the type I R-M system. Liu YP, Tang Q, Zhang JZ, Tian LF, Gao P, Yan XX; Proc Natl Acad Sci U S A. 2017;114:11151-11156. PMID:28973912