Restriction enzymes were discovered 40 years ago during investigations into the phenomenon of host-specific restriction and modification of bacterial viruses. Restriction enzymes protect bacteria from infections by viruses, and it is generally accepted that this is their role in nature. They function as microbial immune systems. When a strain of E. coli lacking a restriction enzyme is infected with a virus, most virus particles can initiate a successful infection. When the same strain contains a restriction enzyme, however, the probability of successful infection plummets. The presence of additional enzymes has a multiplicative effect; a cell with four or five independent restriction enzymes could be virtually impregnable.
Among the first restriction enzymes to be purified were EcoRI and EcoRII from Escherichia coli, and HindII and HindIII 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.Restriction enzymes are exceedingly varied; they range in size from the diminutive PvuII (157 amino acids) to the giant CjeI (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...
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