The N-terminal section of a 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. 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.