A novel bacterial sulfite dehydrogenase that requires three c -type cytochromes for electron transfer.

c -type Cytochromes ( c -Cyts), primarily as electron carriers and oxidoreductases, play a key role in energy transduction processes in virtually all living organisms. Many bacteria, such as Shewanella oneidensis , are particularly rich in c -Cyts, supporting respiratory versatility not seen in eukaryotes. Unfortunately, a large number of c -Cyts are underexplored, and their biological functions remain unknown. In this study, we identify SorCABD of S. oneidensis as a novel sulfite dehydrogenase (SDH), which catalyzes the oxidation of sulfite to sulfate. In addition to catalytic subunit SorA, this enzymatic complex includes three c -Cyt subunits, which all together carry out electron transfer. The electrons extracted from sulfite oxidation are ultimately delivered to oxygen, leading to oxygen reduction, a process relying on terminal oxidase cyt cbb 3 . Genomic analysis suggests that the homologs of this SDH are present in a small number of bacterial genera, Shewanella and Vibrio in particular. Because these bacteria are generally capable of reducing sulfite under anaerobic conditions, the co-existence of a sulfite oxidation system implies that they may play especially important roles in the transformation of sulfur species in natural environments.Importance c -type Cytochromes ( c -Cyts) endow bacteria with high flexibility in their oxidative/respiratory systems, allowing them to extracellularly transform diverse inorganic and organic compounds for survival and growth. However, a large portion of the bacterial c -Cyts remain functionally unknown. Here, we identify three c -Cyts that work together as essential electron transfer partners for the catalytic subunit of a novel SDH in sulfite oxidation in Shewanella oneidensis . This characteristic makes S. oneidensis the first organism known to be capable of oxidizing and reducing sulfite. The findings suggest that Shewanella , along with a small number of other aquatic bacteria, would serve as a particular driving force in the biogeochemical sulfur cycle in nature.

Competing Interests: The authors declare no conflict of interest.