Sulfite oxidation in the purple sulfur bacterium Allochromatium vinosum: identification of SoeABC as a major player and relevance of SoxYZ in the process

Abstract

In phototrophic sulfur bacteria, sulfite is a well-established intermediate during reduced sulfur compound oxidation. Sulfite is generated in the cytoplasm by the reverse-acting dissimilatory sulfite reductase DsrAB. Many purple sulfur bacteria can even use externally available sulfite as a photosynthetic electron donor. Nevertheless, the exact mode of sulfite oxidation in these organisms is a long-standing enigma. Indirect oxidation in the cytoplasm via adenosine-59-phosphosulfate (APS) catalysed by APS reductase and ATP sulfurylase is neither generally present nor essential. The inhibition of sulfite oxidation by tungstate in the model organism Allochromatium vinosum indicated the involvement of a molybdoenzyme, but homologues of the periplasmic molybdopterin-containing SorAB or SorT sulfite dehydrogenases are not encoded in genome-sequenced purple or green sulfur bacteria. However, genes for a membrane-bound polysulfide reductase-like iron-sulfur molybdoprotein (SoeABC) are universally present. The catalytic subunit of the protein is predicted to be oriented towards the cytoplasm. We compared the sulfide-and sulfite-oxidizing capabilities of A. vinosum WT with single mutants deficient in SoeABC or APS reductase and the respective double mutant, and were thus able to prove that SoeABC is the major sulfite-oxidizing enzyme in A. vinosum and probably also in other phototrophic sulfur bacteria. The genes also occur in a large number of chemotrophs, indicating a general importance of SoeABC for sulfite oxidation in the cytoplasm. Furthermore, we showed that the periplasmic sulfur substrate-binding protein SoxYZ is needed in parallel to the cytoplasmic enzymes for effective sulfite oxidation in A. vinosum and provided a model for the interplay between these systems despite their localization in different cellular compartments.

New Proteins Involved in Sulfur Trafficking in the Cytoplasm of Allochromatium vinosum

“…Besides proteins of central carbon metabolism, several important enzymes of the sulfur oxidation pathway in A. vinosum contain FeS-clusters, including DsrAB, DsrL, and the membrane complex DsrMKJOP (1, 2, 31, 32). Another example is the iron-sulfur molybdoenzyme SoeABC, the major player in the oxidation of the DsrAB product sulfite (11). It can, therefore, not a priori been excluded that the close genomic association of the rhd-tusA-dsrE2 genes with sulfur oxidation genes is related with a function of the former in biosynthesis of co-factors of the latter.…”

“…In most representatives of the phototrophic green sulfur bacteria Chlorobiaceae the complete set dsrE2-rhd-tusA is situated immediately upstream of the dsr gene cluster. Remarkably, the whole cluster is preceded by genes encoding the SoeABC-related PSRLC3 complex for which a role in the oxidation of sulfite in the cytoplasm is very probable (10,11). Notably, the only Chlorobium species with incomplete rhd-tusA-dsrE2 clusters, Chlorobaculum parvum and Chlorobium ferrooxidans, are unable to convert accumulated sulfur globules to sulfate (12) and completely incapable of living on reduced sulfur compounds (13), respectively.…”

“…Although the complete rhd-tusA-dsrE2 cluster is present in Chromatiaceae, an Alvin_2599 homolog is not present in phototrophic members of the Ectothiorhodospiraceae. In some of these organisms, tusA and dsrE2 are immediately linked with hdr genes; in others tusA resides close to the dsr and soeABC genes (9,11,14). The only phototrophic ␣-proteobacterium that contains tusA-dsrE2 is Rhodomicrobium vannielii, and in fact this is also the only genome-sequenced phototrophic member of this class that contains genes encoding the complete Dsr pathway.…”

A Comparative Quantitative Proteomic Study Identifies New Proteins Relevant for Sulfur Oxidation in the Purple Sulfur Bacterium Allochromatium vinosum

“…The latter very probably serves as a direct substrate for reverse-acting sulfite reductase, DsrAB (19)(20)(21); Y. Stockdreher, M. Sturm, M. Josten, H. G. Sahl, N. Dobler, R. Zigann, and C. Dahl, submitted for publication). The product of the oxidative step catalyzed by DsrAB is sulfite, which is oxidized to the final product, sulfate, either directly via the membrane-bound ironsulfur molybdoprotein SoeABC or by an indirect pathway involving formation of adenosine-5-phosphosulfate (APS) catalyzed by APS reductase and ATP sulfurylase (5,22,23).…”

Structural basis for specificity and promiscuity in a carrier protein/enzyme system from the sulfur cycle

“…As well as partnering the canonical Sox enzymes, SoxYZ probably also interacts with SoxEF/FccAB (12), a flavocytochrome c with a poorly defined function in the Sox mechanism (13). Additionally, SoxYZ has been implicated in sulfite oxidation in A. vinosum where it is suggested sulfite is carried as a SoxY-cysteine-S-sulfinate derivative, which then interacts with unidentified partners (14).…”