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.

For cleavage, the enzyme remains bound to the recognition site, and DNA is looped out around it, with concomitant supercoiling. About 10⁵ ATP molecules are hydrolyzed per cleavage event. Energy is probably needed for both translocation of the enzyme and supercoiling of the DNA. Both ATP and AdoMet are required for the cleavage activity. AdoMet may be an allosteric activator, because it is not broken down during the reaction.