Two pathways for thiosulfate oxidation in the alphaproteobacterial chemolithotroph Paracoccus thiocyanatus SST

Chemolithotrophic bacteria oxidize various sulfur species for energy and electrons, thereby operationalizing biogeochemical sulfur cycles in nature. The best-studied pathway of bacterial sulfur-chemolithotrophy involves direct oxidation of thiosulfate (S₂O₃^2-) to sulfate (SO₄^2-) without any free intermediate. This pathway mediated by SoxXAYZBCD is apparently the exclusive mechanism of thiosulfate oxidation in facultatively chemolithotrophic alphaproteobacteria. Here we explore the molecular mechanisms of sulfur oxidation in the thiosulfate- and tetrathionate(S₄O₆^2-)-oxidizing alphaproteobacterium Paracoccus thiocyanatus SST, and compare them with the prototypical Sox process of Paracoccus pantotrophus. Our results reveal a unique case where an alphaproteobacterium has Sox as its secondary pathway of thiosulfate oxidation converting ∼10% of the thiosulfate supplied, whilst ∼90% of the substrate is oxidized via a pathway that produces tetrathionate as an intermediate. Sulfur oxidation kinetics of a deletion mutant showed that thiosulfate-to-tetrathionate conversion, in SST, is catalyzed by a thiosulfate dehydrogenase (TsdA) homolog that has far-higher substrate-affinity than the Sox system of this bacterium, which in turn is also less efficient than the P. pantotrophus Sox. Deletion of soxB abolished sulfate-formation from thiosulfate/tetrathionate, while thiosulfate-to-tetrathionate conversion remained unperturbed. Physiological studies revealed the involvement of glutathione in SST tetrathionate oxidation. However, zero impact of the insertional mutation of a thiol dehydrotransferase (thdT) homolog, together with the absence of sulfite as an intermediate, indicated that SST tetrathionate oxidation is mechanistically novel, and distinct from its betaproteobacterial counterpart mediated by glutathione, ThdT, SoxBCD and sulfite:acceptor oxidoreductase. The present findings highlight extensive functional diversification of sulfur-oxidizing enzymes across phylogenetically close, as well as distant, bacteria.