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 III restriction endonuclease system resemble more closely the type I systems than the type II systems. Type III enzymes contain both nuclease and methylase activities in a two-subunit enzyme. They differ from type I enzymes in that they do not require ATP, they modify just one DNA strand, and the cleavage site is fairly close to the recognition site.