Pretreatment with near-ultraviolet (334 nm) light (photoprotection treatment), or posttreatment with potassium cyanide or 2,4-dinitrophenol, inhibits growth of Escherichia coli B and increases its survival after far-ultraviolet (254 nm) irradiation. In general, recovery is proportional to the extent of growth inhibition, whether this is induced by photoprotection treatment, by cyanide, or by dinitrophenol. Cyanide and dinitrophenol recovery overlap with photoprotection. Posttreatment with 2,4,6-trinitrophenol increases the survival of far-ultraviolet-irradiated cells, but inhibits growth less, for the same survival, than the other treatments. The actions of trinitrophenol and photoprotection are additive, and appear to be independent. It is concluded that cyanide, dinitrophenol, and photoprotection affect far-ultraviolet damage primarily through the common mechanism of growth inhibition, which permits more time for dark repair. Trinitrophenol increases far-ultraviolet survival by some unknown mechanism.

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