[Methyl-3H]Thymidine in DNA induces lesions which are recognized by a mammalian DNA-repair endonuclease

It has been known for some years that single-stranded breaks are formed in 3H-labelled DNA during prolonged storage and when such breaks are formed in an intact cell its DNA-repair mechanism is able to restore the damaged DNA. Indirectly it has been shown that a great number of lesions are formed in 3H-labelled DNA, but their chemical structure is not known. Nothing has been published which elucidates the enzymatic steps which take place in restoring tritium damaged DNA. Quite recently a mammalian DNA-repair endonuclease was partially purified and characterized. This enzyme appeared to recognize both AP-lesions and non-AP-lesions induced by agents like OsO4 and UV-light. The present work reports that the same endonuclease incises [methyl-3H]thymidine-labelled DNA which has been stored at 40C. This suggests that repair of some tritium-induced DNA lesions can take place through the incision-excision DNA-repair mechanism. The lesion recognized has not been identified, although the lesion could be a thymine glycol. (Auth.)