Genetic recombination in Bacillus subtilis 168: effect of recN, recF, recH and addAB mutations on DNA repair and recombination.

A recN- (recN1) strain of Bacillus subtilis was constructed. The effects of this and recF, recH and addAB mutations on recombination proficiency were tested. Mutations in the recN, recF, recH and addAB genes, when present in an otherwise Rec+ B. subtilis strain, did not affect genetic exchange. Strains carrying different combinations of mutations in these genes were constructed and examined for their sensitivity to 4-nitroquinoline-1-oxide (4NQO) and recombination proficiency. The recH mutation did not affect the 4NQO sensitivity of recN and recF cells and it only marginally affected that of addA addB cells. However, it reduced genetic recombination in these cells 10(2)- to 10(4)-fold. The addA addB mutations increased the 4NQO sensitivity of recF and recN cells, but completely blocked genetic recombination of recF cells and marginally affected recombination in recN cells. The recN mutation did not affect the recombinational capacity of recF cells. These data indicate that the recN gene product is required for DNA repair and recombination and that the recF, recH and addAB genes provide overlapping activities that compensate for the effects of single mutants proficiency. We proposed that the recF, recH, recB and addA gene products define four different epistatic groups.