Nitric oxide tunes secreted metabolite bioactivity

The radical nitric oxide (·NO) is short-lived but has imprinted itself on many aspects of physiology and disease. ·NO's rapid production and consumption, coupled with its intrinsic reactivity, drive its biological importance; thus, defining mechanisms and targets of ·NO reactivity is necessary to assess its fate and impact. Cellular small molecules are a major class of ·NO-reactive targets, possessing a variety of molecular functionalities that can react with ·NO. Yet the capacity for secreted small molecules to react with ·NO, as well as the biological consequences of such reactivity, have received little attention. Here, we explore the reactivity of ·NO with phenazine metabolites, microbially-derived secreted small molecules that possess antibiotic properties and can modulate their microenvironment. Using Pseudomonas aeruginosa as a model phenazine producer, we find that ·NO reacts with specific phenazines to yield stable, chemically-distinct products. These chemical transformations significantly attenuate phenazine antibiotic properties, including against the phenazine nonproducer Staphylococcus aureus, a competitor with P. aeruginosa for niches in the context of infection. By contrast, P. aeruginosa experiences rapid loss in viability when phenazines and ·NO react. This toxicity occurs even in the presence of S. aureus, which displays resistance to nitrosylated phenazines, implicating a specific toxicity dependent on the formation of the phenazine-NO adduct. These findings highlight the capacity of ·NO to transform metabolite activity and suggest that ·NO can tune microbial interactions in complex environments by a mechanism of action hitherto unappreciated.