SUMMARY: The effect of various modifying agents has been studied on the synergistic interaction of u.v.-and γ-radiation in the induction of prototrophic revertants of tryptophan-requiring B/r WP2. The component of γ-radiation damage involved in the interaction shows a large effect of oxygen during irradiation (dose modifying factor of up to 3) and is thus more like lethal than mutational damage. The interaction mutants respond to various post-irradition treatments in the same way as u.v.-induced, but not γ-induced, mutants, i.e. they are photoreactivable, their frequency is increased by acriflavine and nutrient broth after irradiation and they are susceptible to mutation frequency decline in minimal medium. It is postulated that damage induced in DNA by γ-radiation inhibits the repair of premutational lesions induced by u.v. thus increasing the yield of u.v.-induced mutants. The possible nature of the system involved in this repair (which is not that responsible for the excision of thymine dimers from DNA) is discussed.


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