Abstract

The single-strand specific endonuclease purified from Neurospora crassa mycelia was found by alkaline sucrose gradient centrifugation to cause phosphodiester bond cleavage (“nicking”) of ultraviolet light-irradiated bacteriophage ФX-174 replicative form (RF) DNAs. The level of “nicking” was directly proportional to the dose of ultraviolet light given. Only single-strand scissions were detected in ultraviolet light-irradiated superhelical RF I DNA treated with a low concentration of endonuclease, but double-strand scissions occurred as well when the enzyme concentration was 9–15 times higher. The results confirm the existence of single-strand regions in ultraviolet light-irradiated native DNA.

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