EcoR124 and EcoR124/3: the first members of a new family of type I restriction and modification systems.

We have purified the EcoR124 and EcoR124/3 restriction enzymes and shown that they are type I enzymes by several criteria: subunit composition, DNA and S-adenosylmethionine-dependent ATPase activity, and site-specific DNA methylase activity. By immunochemical criteria these enzymes are related to each other but are unrelated to the two previously investigated families of type I restriction enzymes. They form therefore a new family which we call type IC. The arrangement of the structural genes coding for these enzymes and their transcriptional organisation have been determined. These are different from the common arrangement found for the other two families of type I enzymes.