Recombinational Repair Is Critical for Survival of Escherichia coli Exposed to Nitric Oxide

Nitric oxide (NO(⋅)) is critical to numerous biological processes, including signal transduction and macrophage-mediated immunity. In this study, we have explored the biological effects of NO(⋅)-induced DNA damage on Escherichia coli. The relative importance of base excision repair, nucleotide excision repair (NER), and recombinational repair in preventing NO(⋅)-induced toxicity was determined. E. coli strains lacking either NER or DNA glycosylases (including those that repair alkylation damage [alkA tag strain], oxidative damage [fpg nei nth strain], and deaminated cytosine [ung strain]) showed essentially wild-type levels of NO(⋅) resistance. However, apyrimidinic/apurinic (AP) endonuclease-deficient cells (xth nfo strain) were very sensitive to killing by NO(⋅), which indicates that normal processing of abasic sites is critical for defense against NO(⋅). In addition, recA mutant cells were exquisitely sensitive to NO(⋅)-induced killing. Both SOS-deficient (lexA3) and Holliday junction resolvase-deficient (ruvC) cells were very sensitive to NO(⋅), indicating that both SOS and recombinational repair play important roles in defense against NO(⋅). Furthermore, strains specifically lacking double-strand end repair (recBCD strains) were very sensitive to NO(⋅), which suggests that NO(⋅) exposure leads to the formation of double-strand ends. One consequence of these double-strand ends is that NO(⋅) induces homologous recombination at a genetically engineered substrate. Taken together, it is now clear that, in addition to the known point mutagenic effects of NO(⋅), it is also important to consider recombination events among the spectrum of genetic changes that NO(⋅) can induce. Furthermore, the importance of recombinational repair for cellular survival of NO(⋅) exposure reveals a potential susceptibility factor for invading microbes.