Radiation-Induced Reorientation of Chromosome Replication Sequence: Generality in Escherichia coli; Independence of Prophage or 5-Bromouracil Toxicity

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Abstract
Irradiation with X-rays or UV light induces an altered chromosome replication sequence in E. coli strains 15T-(555-7), B/r and C. Since strains B/r and C showed no evidence of inducible prophage when examined by the same procedures that showed strain 15T-(555-7) to be lysogenic, postirradiation alteration of the chromosome replication sequence may not be related to the induction of prophage by radiaction. The accuracy of measuring replication sequence by 5-bromouracil density labeling was determined by experiments with 13C and 15n. An inhibitory effect of 5-bromouracil on DNA synthesis was observed to a variable extent in the strains studied. Postirradiation inhibition of the rate and extent of total DNA replication by 5-bromouracil varied with the strain, but the replication sequence after irradiation was accurately measured by 5-bromouracil density labeling. When studies were performed with strain 15T-(555-7) density-labeled with 13C and 15N, similar postiradiation replication sequences were obtained in 12C and 14N medium containing either 5-bromouracil or thymine.