Endonuclease II of Escherichia coli is exonuclease III.

Exonuclease III, a phosphatase-exonuclease specific for bihelical DNA, wn the preparation was endonuclease II, an activity specific for DNA that has been partially depurinated by treatment with methyl methanesulfonate. The two activities, which could not be separated by electrophoresis, by sedimentation, or by gel filtration, were associated with a single monomeric protein of 28,000 daltons. To explain how a relatively small protein could have such diverse activities, it is proposed that one site on the enzyme can recognize interstrand spaces created either by depurination or by spontaneous terminal unwinding of a DNA duplex.