Genetic recombination in Bacillus subtilis 168: effects of recU and recS mutations on DNA repair and homologous recombination.

Bacillus subtilis recombination-deficient mutants were constructed by inserting a selectable marker (cat gene) into the yppB and ypbC coding regions. The yppB:cat and ypbC:cat null alleles rendered cells sensitive to DNA-damaging agents, impaired plasmid transformation (25- and 100-fold), and moderately affected chromosomal transformation when present in an otherwise Rec+ B. subtilis strain. The yppB gene complemented the defect of the recG40 strain. yppB and ypbC and their respective null alleles were termed "recU" and "recU1" (recU:cat) and "recS" and "recS1" (recS:cat), respectively. The recU and recS mutations were introduced into rec-deficient strains representative of the alpha (recF), beta (addA5 addB72), gamma (recH342), and epsilon (recG40) epistatic groups. The recU mutation did not modify the sensitivity of recH cells to DNA-damaging agents, but it did affect inter- and intramolecular recombination in recH cells. The recS mutation did not modify the sensitivity of addAB cells to DNA-damaging agents, and it marginally affected recF, recH, and recU cells. The recS mutation markedly reduced (about 250-fold) intermolecular recombination in recH cells, and there were reductions of 10- to 20-fold in recF, addAB, and recU cells. Intramolecular recombination was blocked in recS recF, recS addAB, and recS recU cells. RecU and RecS have no functional counterparts in Escherichia coli. Altogether, these data indicate that the recU and recS proteins are required for DNA repair and intramolecular recombination and that the recF (alpha epistatic group), addAB (beta), recH (gamma), recU (epsilon), and recS genes provide overlapping activities that compensate for the effects of single mutation. We tentatively placed recS within a new group, termed "zeta".