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 probably methylates A-2 on the top strand and A-3 on the bottom strand of the sequence 5'-GAN(7)TAY-3'. As the bacterial DNA is methylated on this sequence and this is the only type I methylase in the genome, it is probably responsible for all of the methylation on this site in the genome. The R subunit has multiple frameshifts and is probably not expressed in this bacteria. The methyltransferase is composed of M and S polypeptides. Detected at low levels after 6 but not 96 hours growth (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.