Mutagenic DNA repair in Escherichia coli

Summary

In the non-filamenting tif-1 strain WP44_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 43°. 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. On plates the effect of high temperature was still fully demonstrable 120 min after irradiation. The results are hard to reconcile with the hypothesis that incubation of tif-1 bacteria at restrictive temperature causes the induction of a repair system acting on DNA damaged by gamma radiation. A more compatible interpretation would be that radiation causes a persisting physiological disturbance in the cell and that this enhances the spontaneous mutator effect occurring in tif-1 bacteria subjected to subsequent thermal shock.