Effects of Mild Heating on Recovery from Ultraviolet Inactivation in Escherichia coli B/r. Il. Removal of Heat-induced Effects by Subsequent Incubatlon

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When heat (52℃ for 20 min) and ultraviolet (UV, 750 erg mm^<-2>)-treated E. coli B/r cells were subjected to liquid holding and then illuminated with photoreactivating light, their ability to be photoreactivated (photoreactivability) was not changed during 30 hr holding. However, when heattreated cells were incubated in growth medium for 3 hr, exposed to UV, and then subjected to liquid holding for 6 hr, photoreactivability disappeared completely. Such disappearance of photoreactivability was also observed when heat treated cells were incubated in growth medium before UV exposure in the presence of hydroxyurea or chloramphenicol, but not when incubated in buffer. These results suggest that heat-induced lesions, which inhibit dimer excision, are repaired during incubation in growth medium. It was found in a previous paper that sublethal heat treatment (52℃ for 20 min) of starved stationary phase cells of E. coli B/r inhibited excision of pyrimidine dimers formed by subsequent ultraviolet light (UV) irradiation of the cells. Therefore, it is suggested that the heat treatment produces some lesions responsible for the inhibition of dimer excision.
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