A restriction enzyme from Hemophilus influenzae. I. Purification and general properties.

The major breakthrough in molecular biology that permitted the “cutting and pasting” of DNA fragments was the discovery by Smith and Wilcox of a restriction enzyme (they called it endonuclease R) from Hemophilus influenzae (now called Haemophilus influenzae). These investigators reported the isolation of an enzyme that was capable of cleaving double-stranded native T7 phage DNA and other foreign double-stranded DNA but not native DNA from H. influenzae or denatured foreign DNA. In the discussion section of their paper, the authors speculated that (i) the native host DNA was probably protected by modifications to the DNA and (ii) enzyme R recognized specific DNA sequences and that these sequences had to be 5 to 6 bases in length. These hypotheses were later proven to be correct. The impact of this discovery was recognized quickly, and Smith was awarded the Nobel Prize in Physiology and Medicine in 1978. ALISON O’BRIEN