Gamma Endonuclease of Micrococcus luteus: Action on Irradiated DNA

Gamma endonuclease is a ${\rm Mg}^{2+}\text{-independent}$ enzyme of Micrococcus luteus that recognizes and cleaves DNA at a variety of altered pyrimidines produced by ionizing radiation. The production of enzyme-recognizable sites (ERS) by ionizing radiation under different irradiation conditions was measured. Ionizing radiation produced the greatest number of ERS when irradiations were performed under anoxic conditions in the presence of the free radical scavenger KI. Since dihydrothymine is a major pyrimidine lesion produced in DNA during anoxic irradiation, the ability of γ endonuclease to excise this lesion was assessed. Dihydrothymine was released from DNA irradiated under anoxic conditions in a radiation dose-dependent manner, consistent with γ endonuclease's known DNA glycosylase activity. Gamma endonuclease was also shown to cleave heavily uv-irradiated DNA. When the sequence specificity of γ-endonuclease cleavage was studied using uv-irradiated DNA, cleavage was seen specifically at cytosines. T...