Thiosulfate-dependent chemolithoautotrophic growth of Bradyrhizobium japonicum

Thiosulfate-oxidizing sox gene homologues were found at four loci (I, II, III IV) on the genome of Bradyrhizobium japonicum USDA110, a symbiotic nitrogen-fixing bacterium in soil. In fact, B. japonicum USDA110 can oxidize thiosulfate and grow under a chemolithotrophic condition. The deletion mutation of the SoxY₁ gene at the sox locus I, homologous to the sulfur-oxidizing (Sox) system in Alphaproteobacteria, left B. japonicum unable to oxidize thiosulfate and grow under chemolithotrophic conditions, whereas the deletion mutation of the soxY₂ gene at sox locus II, homologous to the Sox system in green sulfur bacteria, produced phenotypes similar to those of wild-type USDA110. Thiosulfate-dependent O₂ respiration was observed only in USDA110 and the soxY₂ mutant and not in the SoxY₁ mutant. In the cells, 1 mol of thiosulfate was stoichiometrically converted to approximately 2 mol of sulfate and consumed approximately 2 mol of O ₂. B. japonicum USDA110 showed ¹⁴CO₂ fixation under chemolithotrophic growth conditions. The CO₂ fixation of resting cells was significantly dependent on thiosulfate addition. These results show that USDA110 is able to grow chemolithoautotrophically using thiosulfate as an electron donor, oxygen as an electron acceptor, and carbon dioxide as a carbon source, which likely depends on sox locus I including the SoxY ₁ gene on USDA110 genome. Thiosulfate oxidation capability is frequently found in members of the Bradyrhizobiaceae, which phylogenetic analysis showed to be associated with the presence of sox locus I homologues, including the SoxY₁ gene of B. japonicum USDA110.