Cytochromes Constitute a Layer of Protection against Nitric Oxide but Not Nitrite.

Nitric oxide (NO) is a radical gas that reacts with various biological molecules in complex ways to inhibit growth as a bacteriostatic agent. NO is nearly ubiquitous because it can be generated both biotically and abiotically. To protect the cell from NO damage, bacteria have evolved many strategies, with the production of detoxifying enzymatic systems being the most efficient. Here, we report that -type cytochromes (cytochromes ) constitute a primary NO protection system in , a Gram-negative environmental bacterium renowned for respiratory versatility due to its high cytochrome content. By using mutants producing cytochromes at varying levels, we found that the content of these proteins is inversely correlated with the growth inhibition imposed by NO, whereas the effect of each individual cytochrome is negligible. This NO-protecting system has no effect on nitrite inhibition. In the absence of cytochromes , other NO targets and protective proteins, such as NnrS, emerge to show physiological influences during the NO stress. We further demonstrate that cytochromes also play a similar role in , albeit only modestly. Our data thus identify the function of an important group of proteins in alleviating NO stress. It is widely accepted that the antibacterial effects of nitrite are attributable to nitric oxide (NO) formation, suggesting a correlation of bacterial susceptibilities to these two chemicals. However, compared to , is highly sensitive to nitrite but resistant to NO, implying the presence of robust NO-protective systems. Here, we show that -type cytochromes (cytochromes ) play a main role in protecting against damages from NO but not from nitrite. In their absence, impacts of proteins that promote NO tolerance and that are targets of NO inhibition become evident. Our data thus reveal the specific activity of cytochromes in alleviating the stress caused by NO but not nitrite.