Different Reparability of the Chromosomal and Cytoplasmic Deoxyribonucleic Acid in Escherichia coli Damaged by γ and Ultraviolet Irradiation

We have assessed the relative efficiencies by which chromosomal and extrachromosomal DNAs are repaired in irradiated bacteria. Repair-proficient Escherichia coli C600 cells lysogenic for, or infected with, the thermoinducible phage λcI857 ind were exposed to γ or ultraviolet radiation and then tested for colony- and plaque-forming ability. The results show that the bacterial cell is about 5 times more sensitive to γ rays and about 1.5 times more sensitive to ultraviolet light, if compared to either (1) the prophage that is irradiated in the bacterial chromosome and, on heat induction, repaired in the cytoplasm, or (2) the infecting phage that is irradiated and repaired in the cytoplasm. Since the bacterial DNA is about 80 times larger than the phage DNA, it is inferred that repair processes operating along the chromosomal DNA are one order of magnitude more efficient than those operating along the extrachromosomal DNA. This conclusion is reinforced by the fact that the absence of repair in the system Escherichia coli AB2480 uvrA recA-λcI857 ind red gives the expected ratio of 80/1 for the ultraviolet sensitivity of cells and that of intracellular phage.