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 hav e 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 (in cluding those that repair alkylation damage [alkA tag strain], oxidative da mage [fpg nei nth strain], and deaminated cytosine [ung strain]) showed ess entially 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 c ritical for defense against NO.. In addition, recA mutant cells were exquis itely sensitive to NO.-induced killing. Both SOS-deficient (lexA3) and Holl iday 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-stra nd end repair (recBCD strains) were very sensitive to NO., which suggests t hat NO. exposure leads to the formation of double-strand ends. One conseque nce of these double-strand ends is that NO. induces homologous recombinatio n at a genetically engineered substrate. Taken together, it is now clear th at, in addition to the known point mutagenic effects of NO., it is also imp ortant to consider recombination events among the spectrum of genetic chang es that NO. can induce. Furthermore, the importance of recombinational repa ir for cellular survival of NO. exposure reveals a potential susceptibility factor for invading microbes.