SUMMARY: A study was made of the kinetics of killing by ultraviolet (u.v.) radiation (2537 A) and subsequent photo-reactivation (with black light) of strain 15TAU (requires thymine, arginine, uracil) by using () logarithmic-phase cultures, () early-stationary-phase cultures, () logarithmic-phase cultures after 90 min. of incubation in the absence of arginine and uracil (— AU cultures). The stationary and — AU cultures showed the same enhanced resistance to u.v. killing. In all three cultures (1) the photo-reactivable sectors were the same, (2) the rate of photoreactivation was the same function of u.v. dose, (3) the amount of light required for maximum photo-reactivation was a linear function of the u.v. dose. We conclude that the initial lethal lesions produced by a given u.v. dose were qualitatively and quantitatively the same in all three cultures, despite differences in u.v. survival. This implies that the stationary-phase cultures and the — AU cultures were more radiation resistant solely because they were better able to cope with the initial lethal damage. The survival curves can be satisfactorily described mathematically in terms of this model.


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