Phosphorothioate DNA as an antioxidant in bacteria

Diverse bacteria contain DNA with sulfur incorporated stereo-specifically into their DNA backbone at specific sequences (phosphorothioa-tion). We found that in vitro oxidation of phosphoro-thioate (PT) DNA by hydrogen peroxide (H2O2) or peracetic acid has two possible outcomes: DNA backbone cleavage or sulfur removal resulting in restoration of normal DNA backbone. The physio-logical relevance of this redox reaction was investigated by challenging PT DNA hosting Salmonella enterica cells using H2O2. DNA phospho-rothioation was found to correlate with increasing resistance to the growth inhibition by H2O2. Resistance to H2O2 was abolished when each of the three dnd genes, required for phosphorothioa-tion, was inactivated. In vivo, PT DNA is more resist-ant to the double-strand break damage caused by H2O2 than PT-free DNA. Furthermore, sulfur on the modified DNA was consumed and the DNA was con-verted to PT-free state when the bacteria were incubated with H2O2. These findings are consistent with a hypothesis that phosphorothioation modifica-tion endows DNA with reducing chemical property, which protects the hosting bacteria against peroxide, explaining why this modification is main-tained by diverse bacteria