PHOTOREACTIVATION OF ULTRAVIOLET-IRRADIATED ESCHERICHIA COLI, WITH SPECIAL REFERENCE TO THE DOSE-REDUCTION PRINCIPLE AND TO ULTRAVIOLET-INDUCED MUTATION

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Abstract
Visible light will cause the recovery of microbial cells from u.-v. (2537 A)-induced injury which would otherwise be fatal. Light-induced recovery, or photoreactlvation occurs in at least 4 spp., Escherichia coli B/r, Streptomyces griseus ATC 3326, Penicillium notatum, and Saccharomyces cerevisiae. The active wave lengths He below 5100 A. In E. coli B/r, the u.-v. dose-survival curves for cell suspensions kept dark after irradiation, and those illuminated with visible light after irradiation, have the same shape. From the similarity in shape of the curves was evolved the dose-reduction theory, the effect of a constant amt. of reactivating light on survival in a suspension of E. coli B/r irradiated with varying doses of u.-v. radiation, is the same as if it decreased the effective u.-v. dose by a constant factor. For E. coli B/r this means that the amt. of reactiving light used in one expt. reduced the effective u.-v. dose (as measured by killing) by 60%. E. coli B/r cells incubated in broth at 37[degree] C in the dark after u.-v. irradiation exponentially lost their ability to be photoreactivated. The ability to recover disappeared entirely after 2-3 hrs. incubation. Induced mutants occur in photoreactivated cells. Only one u.-v. dose was studied. Reactivating light apparently reduces the frequency of mutants which are phenotypically expressed a few minutes after u.-v. irradiation, but has little or no effect on delayed mutants, those phenotypically expressed only after a longer period of incubation after u.-v. irradiation. No clear-cut answer was obtained as to whether reactivating light affected u.-v. mutagenesis in the same manner as it affected the lethal action of u.-v. light.