We compared dimer excision in viable and nonviable cells fractions separated from Escherichia coli B/r cultures exposed to ultraviolet (UV) irradiation. For cells grown on minimal medium with glycerol as a carbon source, both fractions from the irradiated (20 J/m2, 5% survival) culture excised 60 to 70% of the thymine dimers from prelabeled DNA within 120 min. This percentage was, within experimental error, the same as that obtained from unseparated cultures. When isolated viable and nonviable populations were given a second UV exposure (20 J/m2) both types of cells were again able to excise dimers. The UV survival curve for the isolated viable population indicates that these cells are no more sensitive to radiation than exponentially growing cells not previously exposed to UV. The extent of dimer excision after UV irradiation was also the same in viable and nonviable cells separated from cultures grown on a glucose minimal medium in which both populations excised about 85% of the dimers within 120 min. These results show that the extent of removal of pyrimidine dimer from deoxyribonucleic acid is not precisely correlated with survival of repair-competent bacterial cells after exposure to UV light.
DNA Repair DNA, Bacterial