Enzymatic Breakage of Deoxyribonucleic Acid

An endonuclease activity was purified about 150-fold from T4 phage-infected Escherichia coli B. This enzyme has been named T4 endonuclease IV and introduces breaks preferentially into single stranded DNA.The enzyme produces oligonucleotides as small as 150 nucleotides long from fd DNA.These oligonucleotides contain 3'-hydroxyl and St-phosphate termini.T4 endonuclease IV produces breaks exclusively adjacent to cytosine residues in both fd and denatured X DNA.This enzyme has no effect on denatured T4 DNA, whether glucosylated or nonglucosylated, and may therefore be involved in the breakdown of host cell DNA which occurs after T4 phage infection.In an attempt to purify the endonuclease activity induced after infection of Escherichia coli with T4 phage, an assay was devised which used as substrate the circular single stranded DNA of bacteriophage fd.The enzyme purified on the basis of this assay had little activity on native DNA, in contrast to T4 endonuclease II (1).This second enzyme has been named endonuclease IV.' A description of the purification and properties of this enzyme is the subject of this report.The combined action of endonuclease II, a TCinduced exonuclease, and endonuclease IV was found to cause double strand breakage of X DNA.Since neither endonuclease has any action on T4 DNA, a mechanism is proposed in which the initial stages in the breakdown of host cell DNA (3,4) might be brought about by the action of these enzymes.