Structural and molecular genetic insight into a widespread sulfur oxidation pathway

Many environmentally important photo- and chemolithoautotrophic bacteria accumulate globules of polymeric, water-insoluble sulfur as a transient product during oxidation of reduced sulfur compounds. Oxidation of this sulfur requires the concerted action of Dsr proteins. However, individual functions and interplay of these proteins are largely unclear. We proved with a ΔdsrE mutant experiment that the cytoplasmic α₂β₂γ₂-structured protein DsrEFH is absolutely essential for the oxidation of sulfur stored in the intracellular sulfur globules of the purple sulfur bacterial model organism Allochromatium vinosum. The ability to degrade stored sulfur was fully regained upon complementation with dsrEFH in trans. The crystal structure of DsrEFH was determined at 2.5 Å resolution to assist functional assignment in detail. In conjunction with phylogenetic analyses, two different types of putative active sites were identified in DsrE and DsrH and shown to be characteristic for sulfur-oxidizing bacteria. Conserved Cys78 of A. vinosum DsrE corresponds to the active cysteines of Escherichia coli YchN and TusD. TusBCD and the protein TusE are parts of sulfur relay system involved in thiouridine biosynthesis. DsrEFH interacts with DsrC, a TusE homologue encoded in the same operon. The conserved penultimate cysteine residue in the carboxy-terminus of DsrC is essential for the interaction. Here, we show that Cys78 of DsrE is strictly required for interaction with DsrC while Cys20 in the putative active site of DsrH is dispensable for that reaction. In summary, our findings point at the occurrence of sulfur transfer reactions during sulfur oxidation via the Dsr proteins.