Among the first restriction enzymes to be purified were EcoRI (NEB #R0101) and EcoRII from Escherichia coli, and HindII and HindIII (NEB #R0104) from Haemophilus influenzae. These enzymes were found to cleave DNA at specific sites, generating discrete, gene-size fragments that could be re-joined in the laboratory. Researchers were quick to recognize that restriction enzymes provided them with a remarkable new tool for investigating gene organization, function and expression.
As the use of restriction enzymes spread among molecular biologists in the late 1970’s, companies such as New England Biolabs began to search for more. Except for certain viruses, restriction enzymes were found only within prokaryotes. Many thousands of bacteria and archaea have now been screened for their presence. Analysis of sequenced prokaryotic genomes indicates that they are common - all free-living bacteria and archaea appear to code for them.