Roles of Endonuclease V, Uracil-DNA Glycosylase, and Mismatch Repair in Bacillus subtilis DNA Base-Deamination-Induced Mutagenesis

The disruption ofung , the unique uracil-DNA-glycosylase-encoding gene inBacillus subtilis , slightly increased the spontaneous mutation frequency to rifampin resistance (Rifr ), suggesting that additional repair pathways counteract the mutagenic effects of uracil in this microorganism. An alternative excision repair pathway is involved in this process, as the loss of YwqL, a putative endonuclease V homolog, significantly increased the mutation frequency of theung null mutant, suggesting that Ung and YwqL both reduce the mutagenic effects of base deamination. Consistent with this notion, sodium bisulfite (SB) increased the Rifr mutation frequency of the singleung and doubleung ywqL strains, and the absence of Ung and/or YwqL decreased the ability ofB. subtilis to eliminate uracil from DNA. Interestingly, the Rifr mutation frequency of singleung andmutSL (mismatch repair [MMR] system) mutants was dramatically increased in aung knockout strain that was also deficient in MutSL, suggesting that the MMR pathway also counteracts the mutagenic effects of uracil. Since the mutation frequency of theung mutSL strain was significantly increased by SB, in addition to Ung, the mutagenic effects promoted by base deamination in growingB. subtilis cells are prevented not only by YwqL but also by MMR. Importantly, in nondividing cells ofB. subtilis , the accumulations of mutations in three chromosomal alleles were significantly diminished following the disruption ofung andywqL . Thus, under conditions of nutritional stress, the processing of deaminated bases inB. subtilis may normally occur in an error-prone manner to promote adaptive mutagenesis.