Role of an inducible single-domain hemoglobin in mediating resistance to nitric oxide and nitrosative stress in Campylobacter jejuni and Campylobacter coli

Campylobacter jejuni expresses two hemoglobins, each of which exhibits a heme pocket and structural signatures in common with vertebrate and plant globins. One of these, designated Cgb, is homologous to Vgb from Vitreoscilla stercoraria and does not possess the reductase domain seen in the flavohemoglobins. A Cgb-deficient mutant of C. jejuni was hypersensitive to nitrosating agents (S-nitrosoglutathione [GSNO] or sodium nitroprusside) and a nitric oxide-releasing compound (spermine NONOate). The sensitivity of the Cgb-deficient mutant to methyl viologen, hydrogen peroxide, and organic peroxides, however, was the same as for the wild type. Consistent with the protective role of Cgb against NO-related stress, cgb expression was minimal in standard laboratory media but strongly and specifically induced after exposure to nitrosative stress. In contrast, the expression of Cgb was independent of aeration and the presence of superoxide. In the absence of preinduction by exposure to nitrosative stress, no difference was seen in the degree of respiratory inhibition by NO or the half-life of the NO signal when cells of the wild type and the cgb mutant were compared. However, cells expressing GSNO-upregulated levels of Cgb exhibited robust NO consumption and respiration that was relatively NO insensitive compared to the respiration of the cgb mutant. Based on similar studies in Campylobacter coli, we also propose an identical role for Cgb in this closely related species. We conclude that, unlike the archetypal single-domain globin Vgb, Cgb forms a specific and inducible defense against NO and nitrosating agents.