NirA is a Cyanide-Tolerant Nitrite Reductase Which Protects Pseudomonas aeruginosa From Self-Poisoning

Host production of nitric oxide in response to P. aeruginosa results in accumulation of nitrite and nitrate at the infection site, with both utilised for anaerobic respiration to support survival. Nitric oxide and nitrite also act as aerobic respiratory inhibitors. P. aeruginosa must overcome these toxic metabolites alongside self-produced cyanide to persist at the infection site. We previously identified a novel nitrite reductase (NirA) that supports P. aeruginosa virulence in a wide range of infection models. In this work, we demonstrate that mutation of nirA inhibits growth of P. aeruginosa at reduced oxygen tensions in the presence of nitrite or nitrate, with this phenotype shown to be dependent on cyanide. NirA is a siroheme-dependent enzyme, a classical target for inhibition with cyanide. Biochemical characterisation confirms that NirA is a novel cyanide-tolerant nitrite reductase, which supports reduction of nitrite in the presence of cyanide. We hypothesise that NirA enables detoxification of nitrite to prevent build-up of multiple respiratory inhibitors and facilitate cyanide-resistant aerobic respiration at low oxygen tensions. Through targeting effectors of these resistance mechanisms, we could promote P. aeruginosa self-poisoning and prevent adaptation to the reduced oxygen environment typically encountered by P. aeruginosa in biofilms and during infection.