Cytochromes c constitute a layer of protection against nitric oxide but not nitrite

Abstract

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 c-type cytochromes (cytochromes c) constitute a primary NO protection system in Shewanella oneidensis, a Gram-negative environmental bacterium renowned for respiratory versatility due to its high cytochrome c content. By using mutants producing cytochromes c 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 c is negligible. This NO-protecting system has no effect on nitrite inhibition. In the absence of cytochromes c, other NO targets and protective proteins, such as NnrS, emerge to show physiological influences during the NO stress. We further demonstrate that cytochromes c also play a similar role in Escherichia coli, albeit only modestly. Our data thus identify the in vivo function of an important group of proteins in alleviating NO stress.