Restriction-modificaton system

TypeI R-M system

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.

TypeII R-M system

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

SmaI substrate –> SmaI fragment 1 + SmaI fragment 2

producing 5'-sticky ends

EcoRI substrate –> EcoRI fragment 1 + EcoRI fragment 2

producing 3'-sticky ends

PstI substrate –> PstI fragment 1 + PstI fragment 2