Radiation-Induced DNA-Strand Scissions in Bacterial Cells and Their Repair
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概要
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In correlation with the radiation killing of bacterial cells, DNA-strand scissions and their repair during postirradiation incubation were investigated by the sedimentation analysis using neutral (for doublestrand scissions) and alkaline (for single-strand scissions)sucrose gradients.Radioresistant strains, <I>Micrococcus radiodurans</I> and <I>Escherichia coli</I> B/r, were employed as the test organisms. Double-strand scissions in DNA were estimated to occur at the rate of one double cut per 800 eV. This rate is in a good agreement of the values reported for other types of bacterial and mammaliancells. The rejoining of these double-strand scissions was observed during the repair process of the postirradiation incubation (Fig. 1) and the mean rejoiningtime was found to be about 50 minutes. This rejoining repair was inhibited by adding chloramphenicol, tetracycline or actinomycin D to the postirradiation incubation medium (Fig. 1). <I>M. radiodurans</I> cells were also found to loss their colony forming ability when the postirradiation incubation was carried out in the presenceof these inhibitors of protein or RNA synthesis(Fig. 2). Thus, it is suggested that the high resistance character of <I>M. radiodurans</I> to gamma rays may be due to the efficient capacity of this rejoining repair. Although the similar results were obtained with <I>E. coli</I> B/r, the radiation lethal effect could not be fully explained by the result of the sedimentation analysis. The sedimentation patterns of DNA from irradiated <I>E. coli</I> B/r(Fig. 3) indicated that there might be a possibility of the repair of double-strand scissions in DNA detectedby the current technique. Studies on these problems should be done to establish the relationship between radiation-induced DNA-strand breaks and the radiation lethal effects.
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