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J Bacteriol. 1968 October; 96(4): 1150-1158
Copyright © 1968 American Society for Microbiology. All Rights Reserved.

Effects of Ionizing Radiation on Synchronous Cultures of Escherichia coli B/r

^a Virus Laboratory, University of California, Berkeley, California 94720

ABSTRACT

The sensitivity of Escherichia coli B/r to X-irradiation is correlated with the replication cycle of deoxyribonucleic acid (DNA). The sensitivity to X-irradiation in the wild type can be attributed to the presence of nuclear targets plus DNA repair mechanisms. The effects of nuclear targets are observed in the recombination-deficient (rec–) mutant B/r, but the sensitivity reflected by changes in the slope of killing curves is absent. A study of different growth conditions indicates that maximal resistance to X rays occurs toward the middle of the division cycle. Evidence is offered that branched chromosomes respond as one-hit targets to X-irradiation. The killing effects of heavy-ion bombardment on E. coli are due primarily to ionizing radiation.

¹ Present address: Department of Microbiology, University of British Columbia, Vancouver 8, B.C., Canada.