Thiosulfate oxidation in sulfur‐reducing Shewanella oneidensis and its unexpected influences on the cytochrome c content

Abstract

Summary Thiosulfate, an important form of sulfur compounds, can serve as both electron donor and acceptor in various microorganisms. In Shewanella oneidensis, a bacterium renowned for respiratory versatility, thiosulfate reduction has long been recognized but whether it can catalyse thiosulfate oxidation remains elusive. In this study, we discovered that S. oneidensis is capable of thiosulfate oxidation, a process specifically catalysed by two periplasmic cytochrome c (cyt c) proteins, TsdA and TsdB, which act as the catalytic subunit and the electron transfer subunit respectively. In the presence of oxygen, oxidation of thiosulfate has priority over reduction. Intriguingly, thiosulfate oxidation negatively regulates the cyt c content in S. oneidensis cells, largely by reducing intracellular levels of cAMP, which as the cofactor modulates activity of global regulator Crp required for transcription of many cyt c genes. This unexpected finding provides an additional dimension to interplays between the respiration regulator and the respiratory pathways in S. oneidensis. Moreover, the data presented here identified S. oneidensis as the first bacterium known to date owning both functional thiosulfate reductase and dehydrogenase, and importantly, genomics analyses suggested that the number of bacterial species possessing this feature is rather limited.

Generation and Physiology of Hydrogen Sulfide and Reactive Sulfur Species in Bacteria

“…Ttr is composed of three subunits: TtrA contains a molybdopterin guanine dinucleotide cofactor and a [4Fe–4S] cluster, TtrB is associated with four [4Fe–4S] clusters, and TtrC is an integral membrane protein functioning to oxidize quinol ( Figure 4 ). In addition, TsdA has been identified as a bidirectional enzyme that catalyzes the interconversion of tetrathionate and thiosulfate [ 14 , 95 , 96 , 97 ] ( Figure 3 B,C). For oxidation of thiosulfate, the enzyme is called thiosulfate dehydrogenase (Tsd), which will be discussed later in thiosulfate oxidation.…”

“…Nevertheless, the primary role that the Sox system plays is to catalyze oxidation of zero-valent sulfur (ZVS) from a variety of sulfur compounds [ 72 ]. Among them, thiosulfate is considered particularly important, as it is a main product of most S 2− /S 0 oxidation and fulfills a key role in the sulfur cycle [ 14 ]. In Paracoccus pantrophus , whose Sox system has been intensively studied, the sox gene cluster comprises at least two transcriptional units with fifteen genes, seven of which, soxXYZABCD , encode proteins essential for sulfur oxidation in vitro [ 49 , 129 , 130 ].…”

“…The crystal structure of TsdA reveals His/Cys iron coordination for Heme 1, the active site of the enzyme, to which a thiosulfate covalently can be bound via the cys [ 96 , 145 , 146 ]. The diheme cyt c TsdB, which may be fused to TsdA, is the electron-accepting unit for transport electrons abstracted from the oxidation to cyt c heme-copper oxidase (HCO) of the respiratory chain for energy generation, although the exception has been found [ 14 , 96 ] ( Figure 3 B,C). Comparison of the structure of C. jejuni TsdA predicted by AlphaFold to a structurally available counterpart catalyzing thiosulfate oxidation reveals a high level of structural similarity.…”

“…However, C. jejuni TsdA has a long random coil in the N-terminus ( Figure 3 C). According to phylogenetic and genomics analyses, the enzymes identified as thiosulfate dehydrogenase are distributed more broadly [ 14 , 96 , 145 ] ( Figure 4 ).Intriguingly, some bacteria, such as S. oneidensis , are equipped with independent enzymatic systems catalyzing oxidation and reduction of thiosulfate [ 14 ].…”

“…Persulfide and polysulfide can then be formed from H 2 S through various non-enzymatic reactions involving ROS. While bacteria preserve these pathways to generate H 2 S, many of them can also do so through reduction of a variety of inorganic and organic compounds containing sulfur of high higher valence, releasing H 2 S as well as other RSS as the terminal products or intermediates [ 13 , 14 ]. In parallel, H 2 S can be oxidized by diverse prokaryotes, and even mitochondria, through a combination of abiotic and biotic reactions to diverse sulfur compounds of any possible valence, many of which are RSS [ 8 , 9 ].…”