Restriction Endonucleases: An Overview  Restriction enzymes were discovered about 30 years ago during investigations into the phenomenon of host-specific restriction and modification of bacterial viruses.  Bacteria initially resist infections by new viruses, and this "restriction" of viral growth stemmed from endonucleases within the cells that destroy foreign DNA molecules. Among the first of these "restriction enzymes" to be purified were EcoR I and EcoR II from Escherichia coli, and Hind II and Hind III 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 archae 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. Restriction enzymes are exceedingly varied; they range in size from the diminutive Pvu II (157 amino acids) to the giant Cje I (1250 amino acids) and beyond. Among over 3,000 activities that have been purified and characterized, more than 250 different sequence-specificities have been discovered. Of these, over 30% were discovered and characterized at New England Biolabs.Restriction Endonucleases: An Overview  Restriction enzymes were discovered about 30 years ago during investigations into the phenomenon of host-specific restriction and modification of bacterial viruses.  Bacteria initially resist infections by new viruses, and this "restriction" of viral growth stemmed from endonucleases within the cells that destroy foreign DNA molecules. Among the first of these "restriction enzymes" to be purified were EcoR I and EcoR II from Escherichia coli, and Hind II and Hind III 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 archae 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. Restriction enzymes are exceedingly varied; they range in size from the diminutive Pvu II (157 amino acids) to the giant Cje I (1250 amino acids) and beyond. Among over 3,000 activities that have been purified and characterized, more than 250 different sequence-specificities have been discovered. Of these, over 30% were discovered and characterized at New England Biolabs