Restriction-modification system enzyme activities

Type II restriction-modification systems differ from their type I and type III counterparts in that the endonuclease and DNA methylase activities are conducted by two separate enzymes (not a single multisubunit complex). The restriction endonuclease cleaves both strands of the DNA duplex within a defined recognition sequence, while the companion DNA methylase methylates a specific base within the same recognition sequence. In contrast... 

Restriction-modification is a term for bacterial enzyme systems that cleave DNA sequences. Each system consists of two distinct enzyme activities a DNA methylase and an endonuclease that catalyzes the double-strand DNA break. Type I restriction endonuclease systems have both methylase and nuclease activities in one protein molecule, which contains three subunits. One subunit contains the nuclease, one the methylase, and one a sequence recognition determinant. The recognition site is not symmetrical, and cleavage occurs some distance (up to 10 kbp) away from the recognition site, although methylation occurs within the recognition site. 

The archetypal type I system is that of E.coli K12, EcoKl. This enzyme comprises three different subunits, the specificity (S) subunit which recognises the DNA sequence 5 AAC-(N)6-GTGC3 , the modification mtase (M) subunit, and the restriction endonuclease (R) subunit. Two M subunits bind to one S subimit to form an active modification mtase which... 

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