Endonuclease from Escherichia coli that acts specifically upon duplex DNA damaged by ultraviolet light, osmium tetroxide, acid, or x-rays.

An endonuclease which is active upon DNA exposed to ultraviolet light at a photoproduct other than thymine dimers has been extensively purified from Escherichia coli. The small (2.7 S) enzyme is active in the presence of EDTA, has a neutral pH optimum, and is inhibited by tRNA and 1 M NaCl. It has no detectable exonuclease, DNA-N-glycosidase, or ribonuclease activities. The enzyme also nicks duplex DNA exposed to OsO4, x-rays, or acid, but it does not act upon undamaged DNA or irradiated single-stranded DNA. The majority of sites of action in DNA exposed to ultraviolet light or OsO4 appear to be alkali-stable, but those in DNA exposed to x-rays or acid are not. The incisions created by the endonuclease contain 5'-phosphate termini. The enzyme is possibly the same as E. coli endonuclease III described by Radman (Radman, M. (1976) J. Biol. Chem. 251, 1438-1445), but it is distinguishable from the other endodeoxyribonucleases described from that organism.