The specificity (S) subunit of a type I restriction enzyme; this subunit dictates DNA sequence specificity. This bacterium does not encode the associated endonuclease or methylase subunits. Contains two target DNA recognition domains (TRD), each specifying recognition of one of the two defined components of the target sequence (Probable). The TDRs show high structural similarity and each forms a globular structure. The two TDRs are separated by 2 long, conserved antiparallel helices that form a coiled coil structure (PubMed:16038930). Not essential, it can be deleted. Belongs to the type-I restriction system S methylase family.