This domain is also known as the target recognition domain (TRD). Restriction-modification (R-M) systems protect a bacterial cell against invasion of foreign DNA by endonucleolytic cleavage of DNA that lacks a site specific modification. The host genome is protected from cleavage by methylation of specific nucleotides in the target sites. In type I systems, both restriction and modification activities are present in one heteromeric enzyme complex composed of one DNA specificity subunit (this family), two modification (M) subunits and two restriction (R) subunits [2].

1: Biology of DNA restriction. Bickle TA, Kruger DH; Microbiol Rev 1993;57:434-450. PMID:8336674

2: The DNA recognition subunit of the type IB restriction-modification enzyme EcoAI tolerates circular permutions of its polypeptide chain. Janscak P, Bickle TA; J Mol Biol 1998;284:937-948. PMID:9837717