Analysis of spontaneous base substitutions generated in mutator strains of Bacillus subtilis.

In the current studies, we investigated base substitutions in the Bacillus subtilis mutT, mutM, and mutY DNA error-prevention system. In the wild type strain, spontaneous mutations were mainly transitions, either G:C --> A:T or A:T --> G:C. Although both transitions and transversions were observed in mutY and mutM mutants, mutM/mutY double mutants contain strictly G:C --> T:A transversions. In the mutT strain, A:T --> C:G transversion was not observed, and over-expression of the B. subtilis mutT gene had no effect on the mutation rate in the Escherichia coli mutT strain. Using 8-oxo-dGTP-induced mutagenesis, transitions especially A:T --> G:C were predominant in the wild type and mutY strains. In contrary, transversion was high on mutY and double mutant (mutM mutY). Finally, the opuBC and yitG genes were identified from the B. subtilis chromosome as mutator genes that prevented the transition base substitutions.