DNA Modification, Restriction Endonucleases: Type I Enzymes

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The original genetic studies of type I DNA restriction and modification (R/M) enzymes were published in 1953 by Bertani and Weigle, the same year as Watson and Crick presented their structural model of B-form DNA. Bertani and Weigle demonstrated that some strains of Escherichia coli could reduce (restrict) the ability of bacteriophage to propagate through the bacterial population. Arber and colleagues later showed that the surviving phage acquired a modification specific to the host bacterial strain, which was lost upon subsequent passage through different hosts. The biological results were eventually linked to an enzyme-specific modification of DNA by type I R/M enzymes. These enzymes were designated type I because they were the first restriction enzymes to be purified and characterized biochemically by Meselson and Yuan. The type II restriction enzymes prepared some years later had the very desirable property of cutting DNA into defined pieces and became the backbone of the genetic engineering revolution. In contrast, the type I restriction enzymes, which cut DNA into unpredictable pieces, have provided important molecular models for complex biological mechanisms.