Rubredoxin:oxygen oxidoreductase enhances survival of Desulfovibrio vulgaris hildenborough under microaerophilic conditions

Genes for superoxide reductase (Sor), rubredoxin (Rub), and rubredoxin:oxygen oxidoreductase (Roo) are located in close proximity in the chromosome of Desulfovibrio vulgaris Hildenborough. Protein blots confirmed the absence of Roo from roo mutant and sor-rub-roo (srr) mutant cells and its presence in sor mutant and wild-type cells grown under anaerobic conditions. Oxygen reduction rates of the roo and srr mutants were 20 to 40% lower than those of the wild type and the sor mutant, indicating that Roo functions as an O2 reductase in vivo. Survival of single cells incubated for 5 days on agar plates under microaerophilic conditions (1% air) was 85% for the sor, 4% for the roo, and 0.7% for the srr mutant relative to that of the wild type (100%). The similar survival rates of sor mutant and wild-type cells suggest that O2 reduction by Roo prevents the formation of reactive oxygen species (ROS) under these conditions; i.e., the ROS-reducing enzyme Sor is only needed for survival when Roo is missing. In contrast, the sor mutant was inactivated much more rapidly than the roo mutant when liquid cultures were incubated in 100% air, indicating that O2 reduction by Roo and other terminal oxidases did not prevent ROS formation under these conditions. Competition of Sor and Roo for limited reduced Rub was suggested by the observation that the roo mutant survived better than the wild type under fully aerobic conditions. The roo mutant was more strongly inhibited than the wild type by the nitric oxide (NO)-generating compound S-nitrosoglutathione, indicating that Roo may also serve as an NO reductase in vivo.

Function of oxygen resistance proteins in the anaerobic, sulfate-reducing bacterium Desulfovibrio vulgaris hildenborough

Two mutant strains of Desulfovibrio vulgaris Hildenborough lacking either the sod gene for periplasmic superoxide dismutase or the rbr gene for rubrerythrin, a cytoplasmic hydrogen peroxide (H(2)O(2)) reductase, were constructed. Their resistance to oxidative stress was compared to that of the wild-type and of a sor mutant lacking the gene for the cytoplasmic superoxide reductase. The sor mutant was more sensitive to exposure to air or to internally or externally generated superoxide than was the sod mutant, which was in turn more sensitive than the wild-type strain. No obvious oxidative stress phenotype was found for the rbr mutant, indicating that H(2)O(2) resistance may also be conferred by two other rbr genes in the D. vulgaris genome. Inhibition of Sod activity by azide and H(2)O(2), but not by cyanide, indicated it to be an iron-containing Sod. The positions of Fe-Sod and Sor were mapped by two-dimensional gel electrophoresis (2DE). A strong decrease of Sor in continuously aerated cells, indicated by 2DE, may be a critical factor in causing cell death of D. vulgaris. Thus, Sor plays a key role in oxygen defense of D. vulgaris under fully aerobic conditions, when superoxide is generated mostly in the cytoplasm. Fe-Sod may be more important under microaerophilic conditions, when the periplasm contains oxygen-sensitive, superoxide-producing targets.