The hsd(host specificity defective) consists of flanking genes of hsdR, hsdM and hsdS, they have function of restriction, modification and recognition, respectively.

• Hetero complex enzyme consisting of 3 subunits
• Modification system have a methyltransferase activity that requires S-Adenosylmethionine as a donor of methyl group.
• Restriction system requires ATP and Mg²⁺.
• The site of recognition is distant from that of digestion.

Commercially available restriction enzymes belong to TypeII restriction-modification system. They can cut dsDNA without its associating methylase.

• Restriction subunit and modification subunit are acitive itself.
• Restriction system requires Mg²⁺.
• Restriction system cuts within its recognition site.

producing blunt ends

–> +

producing 5'-sticky ends

–> +

producing 3'-sticky ends

–> +

• Type I restriction systems: sophisticated molecular machines (a legacy of Bertani and Weigle).
  Murray NE
  Microbiol Mol Biol Rev64p412-34(2000 Jun)