OVERPRODUCTION OF THE RBO GENE-PRODUCT FROM DESULFOVIBRIO SPECIES SUPPRESSES ALL DELETERIOUS EFFECTS OF LACK OF SUPEROXIDE-DISMUTASE IN ESCHERICHIA-COLI

In an attempt to isolate the superoxide dismutase (SOD) gene from the anaerobic sulfate-reducing bacterium Desulfoarculus baarsii, a DNA fra gment was isolated which functionally complemented an Escherichia coli mutant (sodA sodB) deficient in cytoplasmic SODs. This region carries two open reading frames with sequences which are very similar to that of the rbo-rub operon from Desulfovibrio vulgaris, Independent expres sion of the rbo and tub genes from ptac showed that expression of rbo was responsible for the observed phenotype, rbo overexpression suppres sed all deleterious effects of SOD deficiency in E. coli, including in activation by superoxide of enzymes containing 4Fe-4S clusters and DNA damage produced via the superoxide enhanced Fenton reaction. Thus. rb o restored to the sodA sodB mutant the ability to grow on minimal medi um without the addition of branched amino acids, and growth on glucona te and succinate carbon sources was no longer impaired, The spontaneou s mutation rate, which is elevated in SOD-deficient mutants, returned to the wild-type level in the presence of Rbo, which also restored aer obic viability of sodA sodB recA mutants. Rbo from Desulfovibrio vulga ris, but not Desulfovibrio gigas desulforedoxin, which corresponds to the NH2-terminal domain of Rbo, complemented sod mutants, The physiolo gical role of Rbo in sulfate reducing bacteria is unknown. In E. coli, Rbo may permit the bacterium to avoid superoxide stress by maintainin g functional (reduced) superoxide sensitive 4Fe-4S clusters, It would thereby restore enzyme activities and prevent the release of iron that occurs after cluster degradation and presumably leads to DNA damage.