This family consists of a number of N terminal regions found in type I restriction enzyme R (HSDR) proteins. Restriction and modification (R/M) systems are found in a wide variety of prokaryotes and are thought to protect the host bacterium from the uptake of foreign DNA [1]. Type I restriction and modification systems are encoded by three genes: hsdR, hsdM, and hsdS. The three polypeptides, HsdR, HsdM, and HsdS, often assemble to give an enzyme (R2M2S1) that modifies hemimethylated DNA and restricts unmethylated DNA [2].

1: Analysis of type I restriction modification systems in the Neisseriaceae: genetic organization and properties of the gene products. Piekarowicz A, Klyz A, Kwiatek A, Stein DC; Mol Microbiol 2001;41:1199-1210. PMID:11555298

2: Regulation of endonuclease activity by proteolysis prevents breakage of unmodified bacterial chromosomes by type I restriction enzymes. Makovets S, Doronina VA, Murray NE; Proc Natl Acad Sci U S A 1999;96:9757-9762. PMID:10449767