The subtype gamma methyltransferase (M) subunit of a type I restriction enzyme. The M and S subunits together form a methyltransferase (MTase) that methylates A-3 on the top and bottom strand of the sequence 5'-GAAN(7)RTCG-3'. In the presence of the R subunit the complex can also act as an endonuclease, binding to the same target sequence but cutting the DNA some distance from this site. Whether the DNA is cut or modified depends on the methylation state of the target sequence. When the target site is unmodified, the DNA is cut. When the target site is hemimethylated, the complex acts as a maintenance MTase modifying the DNA so that both strands become methylated (PubMed:12654995, PubMed:32483229). After locating a non-methylated recognition site, the enzyme complex serves as a molecular motor that translocates DNA in an ATP-dependent manner until a collision occurs that triggers cleavage (Probable). a 2'-deoxyadenosine in DNA + S-adenosyl-L-methionine = an N(6)-methyl-2'-deoxyadenosine in DNA + H(+) + S-adenosyl-L-homocysteine The type I restriction/modification system is composed of three polypeptides R, M and S; the restriction enzyme has stoichiometry R(2)M(2)S(1) while the methyltransferase is M(2)S(1). There is an equilibrium between R(2)M(2)S(1) and R(1)M(2)S(1); the latter is methylation and translocation proficient but restriction deficient. (Microbial infection) Holoenenzyme interacts with Escherichia phage T7 protein Ocr; this interaction leads to the inhibition of the restriction activity, but may still allow methylation and translocation. The C-terminus is required for assembly of the restriction enzyme complex. The individual domains assume different positions in the enzyme, allowing the holoenzyme to regulate the methylase, endonuclease and translocation states. Type I restriction and modification enzymes are complex, multifunctional systems which require ATP, S-adenosyl methionine and Mg(2+) as cofactors and, in addition to their endonucleolytic and methylase activities, are potent DNA-dependent ATPases. Belongs to the N(4)/N(6)-methyltransferase family.