Lethal and mutagenic action of 3200–4000 Å light

Cells of a thymine-requiring auxotrophic mutant of Escherichia coli B have been irradiated with 2537 Å light (ultraviolet) and 3200–4000 Å light (black light) while being held in aerosols of various relative humidity (R.H.) levels. When cells were held in aerosols of 70% R.H. or lower they became susceptible to damage by black light and much of this damage could be prevented by the compound myo-inositol. The damage inflicted on cells by black light was not photorepairable by the usual methods, suggesting that the lesions produced are different from those produced by ultraviolet light. In addition, the ability of cells to undergo photorepair after irradiation with 2537 Å light was found to decrease rapidly when the cells were irradiated in a dry or near-dry state, indicating that the lesions produced under these conditions are different from those produced in wet cells.Sensitization of the cells to both kinds of radiations by the presence of bromodeoxyuridine (BUDR) in their deoxyribonucleic acid was apparent only when the cells were irradiated in a wet or semidry state, suggesting that sensitization involves a photostimulated hydrolysis of BUDR. Black light was found to be more mutagenic to cells held in a semidried state than was 2537 Å light. It is concluded that the irradiation of cells with 2537 Å light or with black light when they are in the dry state produces a lesion which is non-photorepairable and which is both lethal and mutagenic.