Practice question

Type III restriction-modification enzymes are hetero-oligomers comprising two modification subunits and one restriction subunit encoded by mod and res genes. They recognize asymmetric five to six base pair sequences and require two copies of recognition site in inversely oriented head-to-head configuration on same DNA molecule for activation. Upon ATP hydrolysis and binding of S-adenosylmethionine, they translocate and cleave 25 to 27 base pairs downstream of one recognition site. This requirement for two sites distinguishes Type III from Type I random distant cutting and Type II precise palindromic cleavage.