ROLES OF URACIL-DNA GLYCOSYLASE AND APYRIMIDINIC ENDONUCLEASES IN THE MOLECULAR 5-BROMO-2'-DEOXYURIDINE PHOTOSENSITIZATION IN ESCHERICHIA-COLI K-12

The molecular mechanism for 5-bromo-2'-deoxyuridine (BrdU) photosensitization was studied in thymine-requiring wild-type and uracil-DNA glycosylase (UDG)-deficient ung mutant celts of Escherichia coli K-12. Wild-type cells were more sensitive to BrdU photosensitization than ung mutant cells. UV induced the identical numbers of alkaline sucrose single-strand breaks (SSB) in 5-bromouracil-DNA (BrU-DNA) of both the wild type and ung mutant. The ung mutant cells repaired SSB almost completely, whereas the wild-type cells with UDG produced more adverse SSB by 90 min after UV. Neutral aprose gel electrophoresis of minipreps indicated that UV induced (1) more smears of host BrU-DNA possibly by more double-strand breaks (DSB) and (2) a greater decline of pBR322 Form I BrU-DNA in the wild-type cells than the ung cells. These results indicated a greater induction of SSB by apyrimidinic (AP) endonucleases in wild-type cells. The ung/wild ratios (= 1.7-1.9) for cellular and plasmid BrdU sensitizations after growth in 50% BrdU were similar. The extents of UDG-dependent and UDG-independent sensitizations in wild-type cells were approximately 40 and approximately 60%, respectively. The xth nfo double mutant defective in both exonuclease III and endonuclease IV was more sensitive to BrdU photosensitization than the wild type, indicating that an excess of AP sites remaining after uracil excision in the xth nfo mutant causes a greater BrdU photosensitization than SSB by AP endonucleases in wild-type cells. Conversely, the xth nfo ung triple mutant was more resistant to BrdU photosensitization than the xth nfo double mutant, so that UV-induced uracil residues in the BrU-DNA are tolerated and do not appear to be directly responsible for BrdU photosensitization.