The subtype A restriction (R) subunit of a type I restriction enzyme that recognizes 5'-AACN(6)GTGC-3' and cleaves a random distance away. The R subunit is required for both endonuclease and ATPase activities but not for modification (PubMed:4868368, PubMed:6255295, PubMed:12654995, PubMed:9033396). Has endonucleolytic activity that requires Mg(2+), ATP and S-adenosyl-L-methionine (SAM); ATP can be replaced by dATP, no tested molecule could substitute for SAM. Generates double-stranded DNA with no nicks, by cutting one strand then the other within a few seconds. Cleaves only non-methylated DNA, hemi-methylated and fully methylated DNA are not substrates (PubMed:4868368, PubMed:9033396). 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 (PubMed:6248234). Endonucleolytic cleavage of DNA to give random double-stranded fragments with terminal 5'-phosphates, ATP is simultaneously hydrolyzed. Optimum pH is 7.5-8.0 for DNA restriction. The type I restriction/modification system is composed of three polypeptides R, M and S (PubMed:6255295). The restriction enzyme has stoichiometry R(2)M(2)S(1) (PubMed:9033396, PubMed:32483229). The methyltransferase is composed of M(2)S(1) (PubMed:9033396) (Probable). (Microbial infection) Interacts with Escherichia phage T7 protein Ocr; this interaction leads to the inhibition of the type I bifunctional endonuclease and methyltransferase restriction enzyme R.EcoKI composed of R(2)M(2)S(1). Encoded in the hsd locus, in the order hsdR-hsdM-hsdS. There is a promoter upstream of hsdR and another between hsdR and hsdM (PubMed:6255295). This probably allows expression of the methylase enzyme before the restriction-specific subunit (Probable). The C-terminus (approximately residues 1003-1170) is required for interaction with M(2)S(1) and with Escherichia phage T7 Ocr protein. Upon purification after overexpression about one-third has the initiating methionine removed. Type I restriction and modification enzymes are complex, multifunctional systems which require ATP, S-adenosyl methionine and magnesium as cofactors and, in addition to their endonucleolytic and methylase activities, are potent DNA-dependent ATPases. Belongs to the HsdR family. Extended N-terminus. Extended N-terminus. Extended N-terminus. Extended N-terminus.