Historically, these bacterially synthesized enzymes were called restriction endonucleases because they restricted bacteriophages from infecting, by digesting their DNA.
There are four types of REs:
- Type I : Can both methylate (SAM dependent) and digest the DNA (ATP dependent). Cut several nucleotides away from the restriction sites.
- Type II: Cleave at the restriction site. No methyl-transferase activity.
- Type III : Cleave 20-30 nt away from the restriction sites in ATP dependent way. Have methyl-transferase activity.
- Type IV : Cleave methylated DNA.