Lactobacillus exonuclease mutants : II. Sensitivity toward ultraviolet irradiation and alkylating agents

Abstract

Mutants of Lactobacillus acidophilus R-26, an organism which requires a source of deoxyribonucleoside for its growth, were selected for inability to grow in a basal medium supplemented with nitrophenyl esters of thymidine 3′- or 5′-monophosphates. These esters are specific substrates in vitro for exonucleases of the bacterium. Among the selected strains were found exonuclease-altered mutants, whose extracts failed to hydrolyze normally the 3′- or 5′-nitrophenyl diesters, respectively (Hotchkiss, Nozawa & Gabor, 1971). It is found that 5′-exonuclease mutants are sensitive to u.v.-irradiation, nitrosoguanidine and methyl methanesulfonate treatment. Spontaneous and nitrosoguanidine-induced mutability of these mutants is lower than that of the wild strain, indicating that the 5′-exonuclease function may be involved in the DNA repair system. By contrast, 3′-exonuclease mutants do not exhibit such sensitivity.