Efficient repair of hydrogen peroxide-induced DNA damage by Escherichia coli requires SOS induction of RecA and RuvA proteins.

The survival of Escherichia coli following treatment with a low dose (1–3 mM) of hydrogen peroxide (H2O2) that causes extensive mode-one killing of DNA repair mutants is stimulated by the induction of the SOS regulon. Results for various mutants indicate that induction of recA and RecA protein-mediated recombination are critical factors contributing to the repair of H2O2-induced oxidative DNA damage. However, because DNA damage activates RecA protein's coprotease activity essential to cleavage of LexA repressor protein and derepression of all SOS genes, it is unclear to what extent induction of RecA protein stimulates this repair. To make this determination, we examined mode-one killing of ΔrecA cells carrying plasmid-borne recA (Ptac-recA+) and constitutively expressing a fully induced level of wild-type RecA protein when SOS genes other than recA are non-inducible in a lexA3 (Ind−) genetic background or inducible in a lexA+ background. At a H2O2 dose resulting in maximal killing, ΔrecA lexA3 (Ind−) cells with Ptac-recA+ show 40-fold greater survival than lexA3 (Ind−) cells with chromosomal recA having a low, non-induced level of RecA protein. However, they still show 10- to 15-fold lower survival than wild-type cells and ΔrecA lexA+ cells with Ptac-recA+. To determine if the inducible RuvA protein stimulates survival, we examined a ruvA60 mutant that is defective for the repair of UV-induced DNA damage. This mutant also shows 10- to 15-fold lower survival than wild-type cells. We conclude that while induction of RecA protein has a pronounced stimulatory effect on the recombinational repair of H2O2-induced oxidative DNA damage, the induction of other SOS proteins such as RuvA is essential for wild-type repair.