Effect of the recU suppressors sms and subA on DNA repair and homologous recombination in Bacillus subtilis

In Bacillus subtilis, mutant alleles of the genes sms and subA partially suppress the recombination phenotype of recU cells. When present in an otherwise Rec(+) strain, Deltasms and DeltasubA alleles render cells slightly sensitive to DNA-damaging agents, and impair chromosomal transformation (3- to 10-fold reduction), but do not affect plasmid transformation (less than 1.5-fold reduction). The Deltasms and DeltasubA alleles were introduced into rec-deficient strains representative of the epistatic groups alpha (recF strain), beta (addA addB), gamma (recH), epsilon (recB, DeltarecU and recD strains) and zeta (DeltarecS). Both the Deltasms and DeltasubA mutations were found to increase sensitivity to DNA-damaging agents in recF, DeltarecS and addAB cells. In contrast, the Deltasms mutations decreased the sensitivity of recB, DeltarecU, recD and recH cells, and the DeltasubA mutation decreased the sensitivity of recB and DeltarecU cells to DNA-damaging agents. Functions classified within the epistatic groups (alpha, epsilon and zeta are required for intramolecular recombination, measured as plasmid transformation. The Deltasms and DeltasubA mutations, which partially suppressed the recombinational repair phenotype of mutants for functions within epistatic group E, enhanced plasmid transformation of recU (recB, recD) and recS cells by 10- to 20-fold. In the absence of the proteins Sms and SubA, the recombination machinery is apparently redirected towards (an) alternative pathway(s). Furthermore, the shared ability of the Deltasms and DeltasubA mutations to act as indirect suppressors of recB, recU and recD mutations supports the classification of the recBUD genes within epistatic group epsilon.