Action of gamma endonuclease on clustered lesions in irradiated DNA

Irradiation of DNA in situ i.e. in phage particles or in the cell leads to alterations of single DNA nucleotides as well as to clustered lesions such as double strand breaks or unpaired DNA regions the latter being sensitive to digestion by S1 nuclease. A contribution will be made to the configuration of such S1-nuclease-sensitive sites (S1 sites). DNA from irradiated lambda phage containing S1 sites was treated with gamma endonuclease from M. luteus which is known to split the nucleotide strand at the position of oxidized pyrimidine base. It was found that the gamma endonuclease induces double-strand breaks at some of the S1 sites indicating double base damage within this site. However, half of the S1 sites are not converted into a double-strand break by the gamma endonuclease, indicating base damage only on one strand within the unpaired region.