Aerobic life requires the presence of antioxidant enzymes, such as superoxide dismutase, catalase, and peroxidase to eliminate deleterious oxygen derivatives. Treponema pallidum, a microaerophilic bacterium responsible for venereal syphilis, is an interesting organism because it lacks all of the above-mentioned enzymes, as deduced from its recently sequenced genome. In this paper, we describe a gene in T. pallidum with sequence homologies to a new class of antioxidant systems, named superoxide reductases, recently isolated from sulfate-reducing bacteria (Lombard, M., Fontecave, M., Touati, D., and Nivière, V. (2000) J. Biol. Chem. 275, 115-121). We report that (i) expression of the T. pallidum gene fully restored to a superoxide dismutase-deficient Escherichia coli mutant the ability to grow under aerobic conditions; (ii) the corresponding protein displays a strong superoxide reductase activity; and (iii) the T. pallidum protein contains only one mononuclear nonheme ferrous center, able to reduce superoxide selectively and efficiently, whereas previously characterized superoxide reductase from Desulfoarculus baarsii contains an additional rubredoxin-like ferric center. These results suggest that T. pallidum antioxidant defenses rely on a new class of superoxide reductase and raise the question of the importance of superoxide reductases in mechanisms for detoxifying superoxide radicals.