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, involving direct oxidation of thiosulfate to sulfate (without any free intermediate) by the SoxXAYZBCD multienzyme system, 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-oxidizing alphaproteobacterium Paracoccus thiocyanatus SST, and compare them with the prototypical Sox process characterized in Paracoccus pantotrophus. Our results revealed the 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 Tetrathionate-Intermediate pathway. Knock-out mutation, followed by the study of sulfur oxidation kinetics, 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, remarkably, is also less efficient than the P. pantotrophus Sox. soxB-deletion in SST 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 knock-out of a thiol dehydrotransferase (thdT) homolog, together with no production of sulfite as an intermediate, indicated that tetrathionate oxidation in SST is mechanistically novel, and distinct from its betaproteobacterial counterpart mediated by glutathione, ThdT, SoxBCD and sulfite:acceptor oxidoreductase. All the present findings collectively highlight extensive functional diversification of sulfur-oxidizing enzymes across phylogenetically close, as well as distant, bacteria.