Multiply damaged sites (MDSs) consist of two or more damages within 20 base pairs (bps) and are introduced into DNA by ionizing radiation. Using a plasmid assay, we previously demonstrated that repair in Escherichia coli generated a double strand break (DSB) from two closely opposed uracils when uracil DNA glycosylase initiated repair. To identify the enzymes that converted the resulting apurinic/apyrimidinic (AP) sites to DSBs, repair was examined in bacteria deficient in AP site cleavage. Since exonuclease III (xth) and endonuclease IV (nfo) mutant bacteria were able to introduce DSBs at the MDSs, we generated unique bacterial mutants deficient in UvrA, Xth and Nfo. However, the additional disruption of nucleotide excision repair (NER) did not prevent DSB formation. xth- nfo- nfi- bacteria also converted the MDSs to DSBs, ruling out endonuclease V as the candidate AP endonuclease. By using MDSs containing tetrahydrofuran (an AP site analog), it was determined that even in the absence of Xth, Nfo, NER and AP lyase cleavage, DSBs were formed from closely opposed AP sites. This finding implies that there is an unknown enzyme/repair pathway for MDSs, and multiple underlying repair systems in cells that can process closely opposed DNA damage into lethal lesions following exposure to ionizing radiation.

Multiply damaged sites (MDSs) are generated in DNA by ionizing radiation. In vitro studies predict that base excision repair in cells will convert MDSs to lethal double strand breaks (DSBs) when two opposing base damages are situated >/=2 bp apart. If the lesions are situated immediately 5' or 3' to each other, repair is predicted to occur sequentially due to inhibition of the DNA glycosylase by a single strand break repair intermediate. In this study, we examined how the distance between two opposing lesions alters the mutation frequency of an 8-oxodG in an MDS, and whether repair generates DSBs and deletions in bacteria. The 8-oxodG mutation frequency declined in MutY-deficient bacteria when the opposing 8-oxodG was 6 bp away, and was similar to a single 8-oxodG when the lesions were separated by 14 bp. However, the number of deletions detected for the MDSs was equivalent to the undamaged sequence. Using a separate assay, MDSs consisting of two 8-oxodG or an 8-oxodG opposite a uracil were not converted to DSBs in the absence of DNA replication in wild-type and transcription-coupled repair-deficient bacteria. This is the first study showing that DSB-repair intermediates and deletions are not formed during repair of clustered 8-oxodGs in cells.