A novel type of hemoglobin confers host-derived nitric oxide resistance to the opportunistic pathogen Acinetobacter baumannii.

Acinetobacter baumannii is an opportunistic Gram-negative pathogen responsible for various infections, such as those of the bloodstream and lungs, which often resist antibiotic therapy. In the course of an infection, the human innate immune system's phagocytic cells are activated producing nitric oxide (NO) that cause bacterial injury. While the antimicrobial effects of nitrosative stress and the bacterial resistance mechanisms are well-characterized for several pathogens, the adaptations of Acinetobacter spp. to NO have not been studied. In this work, we define the transcriptional response of A. baumannii to nitrosative stress induced by NO donor exposure. A. baumannii triggers the expression of several transporters, including those involved in iron and siderophore synthesis. One of the most significantly NO-induced genes is a putative flavohemoglobin. The loss of function of this gene in the mutant strain led to decreased fitness of A. baumannii under NO stress. We also identified the A. baumannii nitric oxide sensor NsrR and demonstrated that NsrR regulates the hemoglobin gene. Combining biochemical, kinetic, and structural prediction studies we show that A. baumannii hemoglobin exhibits nitric oxide dioxygenase and reductase activities and has an atypical structural domain composition. Moreover, we reveal that Acinetobacter hemoglobins have evolved into an independent branch and are phylogenetically distant from other bacterial hemoglobins. Altogether, our findings demonstrate that A. baumannii hemoglobins represent a novel class of NO-detoxifying defense proteins that evolve from flavohemoglobin.