The specificity (S) subunit of a type I methyltransferase (MTase); this subunit dictates DNA sequence specificity. The single R subunit has multiple frameshifts and is probably not expressed. The methyltransferase is composed of M and S polypeptides. Highly expressed after 6 and 96 hours growth, there are fewer copies at 96 hours (at protein level). Contains two DNA recognition domains, each specifying recognition of one of the two defined components of the target sequence. Belongs to the type-I restriction system S methylase family.