Summary: Continuous cultures of the tryptophan-requiring strain of known as wp2 were continuously exposed to gamma radiation for periods of several generation times. The frequency of prototrophic revertants increased steadily during irradiation at a rate which was proportional to the radiation dose rate. The change of revertant frequency per unit dose (rad) was equal to the induced reversion rate per mutable unit. This mutation rate (ρ) was independent of population density but slightly dependent upon the presence of supplements, such as nutrient broth, to the normal minimal medium with tryptophan. There was a marked dependence of ρ upon culture temperature, the values at 16°, 22° and 37° being 1·2 × 10, 2·4 × 10 and 3·9 × 10 per mutable unit per rad. It is probable that only a fraction of radiation-induced changes in the genetic material which could give rise to phenotypic reversions are actually expressed, this fraction being dependent on the post-irradiation temperature. The proportion of the population inactivated by radiation was intentionally kept at 10 % or less in order to avoid difficulties in the interpretation of results. Other possible sources of error in ρ have been reviewed.


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