Summary: (Müller) became resistant to killing by hydrogen peroxide (HO) when pretreated with non-lethal concentrations of HO. When rapidly growing cells were pretreated with 100 μm-HO, they became 7-10-fold more resistant to 20 mM-HO than were naive cells. Activities of several oxidative defense enzymes were measured in cells treated with 100 μm-HO in either exponential or stationary phase growth. The specific activity of catalase in crude extracts of cells pretreated in either phase increased about 40%, Peroxidase activity, in cell extracts and culture supernatants, respectively, of cells treated in the stationary growth phase increased two times and four times. Glucose-6-phosphate dehydrogenase increased by 60% at the exponential growth phase. Glutathione reductase increased 80% after treatment in the exponential phase and 4-fold in the stationary growth phase. However, superoxide dismutase activity decreased by 70%. Two mutants resistant to HO were isolated after mutagenesis of spores with -methyl-'-nitro--nitrosoguanidine. In addition to a dramatic increase in the survival rate in 20 mM-HO, both mutants exhibited increased activities of all the above enzymes except superoxide dismutase. The pleiotropic phenotype of the mutants suggests that there exists a global regulation of oxidative response in


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