Effect of the recC gene in Escherichia coli on frequencies of ultraviolet-induced mutants

UV induction of Lac^? mutations was compared with UV induction of Mal⁺ mutations in E. coli B/r strains differing in the recC gene. The frequency of Lac^? mutants per survivor induced by the same dose was not significantly affected by the recC gene but the percentage of pure rather than sectored Lac^? colonies was greater when the recC gene was present. On the other hand, as reported previously, frequencies of Mal⁺ mutants induced by the same UV dose were lower when the strain was recC. The reduction factor was the same as for spontaneous Mal⁺ mutants. The difference in the effect of the recC gene on the yields of Lac^? and Mal⁺ mutants can be explained by taking into account the influence of lethal sectoring, which introduces an artifact when mutants arising in the recC strain are scored selectively as in the case for Mal⁺ mutants, but not when the scoring is non-selective as for Lac^? mutants. Lethal sectoring as indicated by a discrepancy between total cell counts and numbers of colony-formers, was observed for the recC strain growing in liquid minimal medium corresponding to the agar medium used to score Mal⁺ mutants but was not observed for the rec⁺ strain. Both strains showed lethal sectoring in the liquid medium corresponding to the agar medium to score Lac^? mutants. The hypothesis concerning the role of lethal sectoring in the selective scoring of mutants arising in a recC background is supported by evidence concerning the UV induction of mutants in a polA1 background. Like the recC gene, the polA1 gene did not affect yields of Lac^? mutants. However, unlike the recC gene, the polA1 gene has previously been shown not to influence UV yields of prototrophic mutations (scored selectively) and not to cause lethal sectoring except under irrelevant conditions.