EVIDENCE FOR TWO MECHANISMS OF PHOTOREACTIVATION IN ESCHERICHIA COLI B.
Escherichia coli B phr(-), which is not photoreactivable under certain conditions, has been shown to exhibit photoreactivation of killing in the logarithmic growth phase at 3341 A. Dependence of the reaction upon (a) wavelength, (b) dose, and (c) dose rate of the reactivating radiation, as well as upon (d) temperature during reactivation treatment, is very similar to that of photoprotection. We conclude that this photoreactivation is similar in mechanism to photoprotection, believed to be an indirect repair process, the initial step of which is non-enzymatic and leads to a growth-division delay. We therefore call the present phenomenon "indirect photoreactivation." Similar studies suggest that indirect photoreactivation of killing occurs also in the parent strain, E. coli B (Harm). It has often been supposed that all photoreactivation results from a photoenzymatic reaction similar to that found to operate in vitro on transforming DNA. Our data provide the first evidence for two distinct types of photoreactivation of cell killing, one of which appears not to involve photoenzymes. These experiments also show that photoprotection results from intracellular events that can be induced by treatment after, as well as before, far ultraviolet irradiation.