Abstract
Dose-induction functions for several λ-lysogenic strains of Escherichia coli with different reactivation mechanisms have been measured. It is found that dark-reactivation (DR) reactivates inducing lesions but K reactivation does not. For DR-positive strains the fraction induced is proportional to the UV dose under conditions of complete DR, while for DR-negative strains the fraction induced is proportional to about the fourth power of the UV dose. It is concluded that several dark-reactivable lesions cooperate in λ induction, while a single nonreactivable lesion can probably induce the prophage. Therefore the pathway to induction from reactivable UV lesions appears to be different from the pathway to induction from nonreactivable ones. Low UV doses kill Bs-1(λ) and BB(λ) cells without inducing the prophage, but higher doses induce these dead cells. It is concluded that killing and induction by UV are independent processes. The formal target size for induction is much smaller than that for killing.
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