New Insights Into Acidithiobacillus thiooxidans Sulfur Metabolism Through Coupled Gene Expression, Solution Chemistry, Microscopy, and Spectroscopy Analyses

Here, we experimentally expand understanding of the reactions and enzymes involved in Acidithiobacillus thiooxidans ATCC 19377 S⁰ and ${\text{S}}_2{\text{O}}_3^{2-}$ metabolism by developing models that integrate gene expression analyzed by RNA-Seq, solution sulfur speciation, electron microscopy and spectroscopy. The A. thiooxidans ${\text{S}}_2{\text{O}}_3^{2-}$ metabolism model involves the conversion of ${\text{S}}_2{\text{O}}_3^{2-}$ to ${\text{SO}}_4^{2-}$ , S⁰ and ${\text{S}}_4{\text{O}}_6^{2-},$ mediated by the sulfur oxidase complex (Sox), tetrathionate hydrolase (TetH), sulfide quinone reductase (Sqr), and heterodisulfate reductase (Hdr) proteins. These same proteins, with the addition of rhodanese (Rhd), were identified to convert S⁰ to ${\text{SO}}_3^{2-}$ , ${\text{S}}_2{\text{O}}_3^{2-}$ and polythionates in the A. thiooxidans S⁰ metabolism model. Our combined results shed light onto the important role specifically of TetH in ${\text{S}}_2{\text{O}}_3^{2-}$ metabolism. Also, we show that activity of Hdr proteins rather than Sdo are likely associated with S⁰ oxidation. Finally, our data suggest that formation of intracellular ${\text{S}}_2{\text{O}}_3^{2-}$ is a critical step in S⁰ metabolism, and that recycling of internally generated ${\text{SO}}_3^{2-}$ occurs, through comproportionating reactions that result in ${\text{S}}_2{\text{O}}_3^{2-}$ . Electron microscopy and spectroscopy confirmed intracellular production and storage of S⁰ during growth on both S⁰ and ${\text{S}}_2{\text{O}}_3^{2-}$ substrates.