Mutagenic DNA repair in escherichia coli. Pt. 6

In the non-filamenting tif-1 strain WP44sub(s)NF trp a dramatic enhancement of both UV and gamma ray mutability to Trp+ was observed when irradiated bacteria were incubated on plates at 430C. This enhanced mutability was progressively suppressed when the initial plating density exceeded 108 bacteria per plate and was not demonstrable in liquid media. Under optimal conditions more mutants were induced by gamma radiation than could reasonably be accounted for by the initial number of radiation-induced lesions in the DNA, implying the existence of some mechanism for amplifying the radiation effect. Moreover, the tif-enhanced mutation frequency could be obtained if incubation at restrictive temperature was delayed for up to 60 min in nutrient broth after irradiation, at a time when all known reparable DNA damage had been repaired and the number of viable bacteria had more than doubled. In plates the effect of high temperature was still fully demonstrable 120 min after irradiation. A compatible interpretation of the results would be that radiation causes a persisting physiological disturbance in the cell and that this enhances the spontaneous mutator effect occuring in tif-1 bacteria subjected to subsequent thermal shock. (orig./MG)