Replication of the bacterial chromosome: location of new initiation sites after irradiation.

New loci of replication along the bacterial chromosome are observed after irradiation of Escherichia coli. It was conjectured that, after X-irradiation, the new initiation site was random with respect to the fixed-origin, whereas, after ultraviolet light exposure, it was selective and appeared to be from the fixed-origin. Evidence presented here shows that, after X-irradiation of E. coli, the new initiation site(s) for the onset of deoxyribonucleic acid replication is induced at chromosomal regions not restricted to the fixed-origin. After ultraviolet light exposure, the new initiation site is preferentially from the fixed-origin. In these studies amino acid starvation was used to synchronize chromosome replication and to allow for differential radioisotopic labeling of the chromosomal origin and terminus. To facilitate interpretation, growing cells actively replicating their chromosome were compared with cells lacking growth points at the time of irradiation. The role of these new replication sites in the observed kinetics of deoxyribonucleic acid replication following X-ray or ultraviolet light exposure is discussed.