Deoxyribonucleic Acid Repair Replication after Ultraviolet Light or X-Ray Exposure of Bacteria

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Abstract
A comparison of repair synthesis after ultraviolet light (UV) or X-ray exposure was made in Escherichia coli strains 15T − (555–7) and B/r by use of a D, 15 N, 13 C density labeling system. During the initial 15 min of incubation after UV irradiation, both a “repair” synthesis and a reduced semiconservative deoxyribonucleic acid (DNA) synthesis occurred. In the so-called “physiological” dose range used, the latter was greater than the former. X-irradiation of cells, at doses producing similar levels of cell death as in the UV-exposed cultures, did not lead to a similar repair replication process. However, a density heterogeneity of the DNA synthesized in the initial 10 min after exposure was observed. This is interpreted in terms of X ray-induced DNA degradation. Normal cells showed only a semiconservative type of replication and, therefore, within the limits of resolution of the system used (the incorporation of 1,000 to 5,000 nucleotides per replicating chromosome could be measured), DNA in normal cells did not appear to undergo a repair synthesis involving thymine exchange. These results indicate that not all repair mechanisms mimic that found after UV exposure.