Correlation of photoreactivation of ultraviolet-induced killing with dimer splitting before and after DNA replication in an excisionless strain of Escherichia coli

Abstract— Splitting of thymine‐containing dimers was compared quantitatively with photoreactivation (PR) of killing induced by ultraviolet radiation (254 nm) in a uvrA (excisionless) strain of E. coli. Immediately after irradiation, the splitting rate (number of dimers split/genome/unit PR dose) agreed well with the PR rate of the cells (rate of recovery from photoreactivable lethal damage converted into an 'estimated' number of dimers split/genome/unit PR dose). After 4 h of incubation of cells in nutrient medium, the maximal fraction of splittable dimers decreased, as did the maximal fraction of photoreactivable lethal damage. However, the initial splitting rate after incubation was equal to that before incubation. During the 4‐h incubation, the heavily irradiated uvrA cells did not divide but became filamentous and their DNA increased about 70 per cent. It is concluded that roughly half of the dimers in DNA that has replicated after ultraviolet irradiation are split as efficiently as those in DNA that has not replicated.