Role of pyridine nucleotides in 5-fluorouracil-mediated reactivation of ultraviolet radiation damage.

Ultraviolet irradiation (520 ergs/mm(2) at 254 nm) causes the respiration of Escherichia coli B/r cells to cease after about 90 min postirradiation incubation in a minimal medium containing glycerol as the sole source of carbon. The cessation of respiration is associated with loss of pyridine nucleotides. Agents which interfere with postirradiation transcription and translation prevent cessation of respiration. We have studied the effects of one of these agents, 5-fluorouracil (FU), on respiration, pyridine nucleotide levels, viability, capacity to support phage growth, and the repair of irradiated deoxyribonucleic acid (DNA). Addition of FU to cells immediately after irradiation results in the continuance of respiration at a linear rate and the maintenance of high levels of pyridine nucleotides. Cellular viability increases dramatically during the first 60 min of postirradiation incubation in the presence of FU. The ability of irradiated cells to support the growth of phage T4 is also greatly increased. FU treatment has no effect on the kinetics of pyrimidine dimer excision or the degradation of DNA. However, treated cells repair single-strand breaks resulting from early steps in excision repair slightly more efficiently than do untreated cells. The results support the hypothesis that one of the causes of death in these irradiated cells is the disappearance of pyridine nucleotides, coenzymes of certain respiratory dehydrogenases, and, in the case of nicotinamide adenine dinucleotide, for polynucleotide ligase, the enzyme responsible for the final step in the repair of DNA.