Spontaneous Lac− to Lac+ mutant frequencies of the wild-type and mutM, mutY and mutM mutY derivatives with or without the norM-encoding plasmid.

amean (± SEM) of the mutant frequencies for 4 independent experiments are shown. P-values were obtained for pairwise comparisons by the nonparametric Mann–Whitney U test. The mutant frequency of the wild-type strain, NR10834 [pCA24N], is significantly different from those of the strains ΔmutY::bla [pCA24N] (P = 0.016), ΔmutY::bla [pCnorM] (P = 0.009), mutM::Tn10 [pCnorM] (P = 0.014), mutM mutY [pCA24N] (P = 0.009) and mutM mutY [pCnorM] (P = 0.009).