The specificity (S) subunit of a type I restriction enzyme; this subunit dictates DNA sequence specificity. The M and S subunits together form a methyltransferase (MTase) that methylates A-3 on the top strand and A-4 on the bottom strand of the sequence 5'-TGAN(8)TGCT-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:4568607, PubMed:12654995). 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 (By similarity). The type I restriction/modification system is composed of three polypeptides R, M and S (PubMed:4568607). The restriction enzyme has stoichiometry R(2)M(2)S(1) while the methyltransferase is M(2)S(1) (By similarity). Contains two DNA recognition domains, each specifying recognition of one of the two defined components of the target sequence. 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 type-I restriction system S methylase family.