Truncated hemoglobin GlbO from Mycobacterium leprae alleviates nitric oxide toxicity

As a consequence of reductive genome evolution, the obligate intracellular pathogen Mycobacterium leprae has minimized the repertoire of genes implicated in defense against reactive oxygen and nitrogen species. Genes for multiple hemoglobin types coexist in mycobacterial genomes, but M. leprae has retained only glbO, encoding a group-II truncated hemoglobin. Mycobacterium tuberculosis GlbO has been involved in oxygen transfer and respiration during hypoxia, but a role in protection from nitric oxide (NO) has not been documented yet. Here, we report that the in vitro reaction of oxygenated recombinant M. leprae GlbO with NO results in an immediate stoichiometric formation of nitrate, concomitant with heme-protein oxidation. Overexpression of GlbO alleviates the growth inhibition of Escherichia colihmp (flavohemoglobin gene) mutants in the presence of NO-donors, partly complementing the defect in Hmp synthesis. A promoter element upstream of glbO was predicted in silico, and confirmed by using a glbO::lacZ transcriptional fusion in the heterologous Mycobacterium smegmatis system. The glbO::lacZ fusion was expressed through the whole growth cycle of M. smegmatis, and moderately induced by NO. We propose that M. leprae, by retaining the unique truncated hemoglobin GlbO, may have coupled O2 delivery to the terminal oxidase with a defensive mechanism to scavenge NO from respiratory enzymes. These activities would help to sustain the obligate aerobic metabolism required for intracellular survival of leprosy bacilli.