Endonuclease III: an endonuclease from Escherichia coli that introduces single polynucleotide chain scissions in ultraviolet-irradiated DNA.

An endonuclease that makes single polynucleotide chain scissions in UV-irradiated DNA has been purified from Escherichia coli. The activity has the following properties: (1) unirradiated DNA is attacked very little if at all; (2) single-stranded DNA is not attacked, whether irradiated or not; (3) there is no requirement for divalent cations, and the activity is not affected by addition of EDTA; (4) the pH optimum is approximately 7; (5) the activity is inhibited by 1 M NaCl, single-stranded DNA, transfer RNA, and unirradiated double-stranded DNA; (6) the sedimentation coefficient, S20, W, is approximately 2.6; (7) it is a basic protein. The enzyme is tentatively named E. coli endonuclease III. The physiological function of the endonuclease has not yet been established.