Photochemistry of the bisbenzimidazole dye 33258 Hoechst with bromodeoxyuridine and its biological effects on BrdUrd-substituted Escherichia coli

Exposure of BrdUrd-substituted E. coli cells to 360 nm light in the presence of the bisbenzimidazole dye 33258 Hoechst increases their sensitivity dramatically. Mutant cells deficient in excision repair pf DNA damage (uvr B) are more sensitive than wild type cells, indicating that the cells are able to repair this type of damage. However, they perform only a limited amount of liquid holding recovery. Exposure of the dye with BrdUrd to near UV light in solution results in the appearance of two BrdUrd derived photoproducts. One appears to be deoxyuridine, and the other - an adduct of BrdUrd dye. The adduct is acid labile and as a result only uracil is observed in acid-hydrolyzates of DNA after exposure of BrdUrd-substituted cells to 360 nm light in the presence of 33258 Hoechst. The production of uracil is linearly dependent on light exposure. Cells in which 85% of thymidine was replaced by BrdUrd are unable to remove more than 5-10% of uracil from their DNA during postirradiation incubation. However, when only 4% of thymidine is replaced, about 50% of the uracil is removed during 30 min incubation after exposure. The results are consistent with previous work, indicating that BrdUrd interferes with repair via excision-resynthesis. A working hypothesis is suggested to explain this interference. (author)