Glycyl Radical Enzymes and Sulfonate Metabolism in the Microbiome.

Sulfonates include diverse natural products and anthropogenic chemicals and are widespread in the environment. Many bacteria can degrade sulfonates and obtain sulfur, carbon, and energy for growth, playing important roles in the biogeochemical sulfur cycle. Cleavage of the inert sulfonate C-S bond involves a variety of enzymes, cofactors, and oxygen-dependent and oxygen-independent catalytic mechanisms. Sulfonate degradation by strictly anaerobic bacteria was recently found to involve C-S bond cleavage through O2-sensitive free radical chemistry, catalyzed by glycyl radical enzymes (GREs). The associated discoveries of new enzymes and metabolic pathways for sulfonate metabolism in diverse anaerobic bacteria have enriched our understanding of sulfonate chemistry in the anaerobic biosphere. An anaerobic environment of particular interest is the human gut microbiome, where sulfonate degradation by sulfate- and sulfite-reducing bacteria (SSRB) produces H2S, a process linked to certain chronic diseases and conditions.

DsrMKJOP is the terminal reductase complex in anaerobic sulfate respiration.

Microbial dissimilatory sulfate reduction (DSR) is a key process in the Earth biogeochemical sulfur cycle. In spite of its importance to the sulfur and carbon cycles, industrial processes, and human health, it is still not clear how reduction of sulfate to sulfide is coupled to energy conservation. A central step in the pathway is the reduction of sulfite by the DsrAB dissimilatory sulfite reductase, which leads to the production of a DsrC-trisulfide. A membrane-bound complex, DsrMKJOP, is present in most organisms that have DsrAB and DsrC, and its involvement in energy conservation has been inferred from sequence analysis, but its precise function was so far not determined. Here, we present studies revealing that the DsrMKJOP complex of the sulfate reducer Archaeoglobus fulgidus works as a menadiol:DsrC-trisulfide oxidoreductase. Our results reveal a close interaction between the DsrC-trisulfide and the DsrMKJOP complex and show that electrons from the quinone pool reduce consecutively the DsrM hemes b, the DsrK noncubane [4Fe-4S]3+/2+ catalytic center, and finally the DsrC-trisulfide with concomitant release of sulfide. These results clarify the role of this widespread respiratory membrane complex and support the suggestion that DsrMKJOP contributes to energy conservation upon reduction of the DsrC-trisulfide in the last step of DSR.

The DsrD functional marker protein is an allosteric activator of the DsrAB dissimilatory sulfite reductase.

Dissimilatory sulfur metabolism was recently shown to be much more widespread among bacteria and archaea than previously believed. One of the key pathways involved is the dsr pathway that is responsible for sulfite reduction in sulfate-, sulfur-, thiosulfate-, and sulfite-reducing organisms, sulfur disproportionators and organosulfonate degraders, or for the production of sulfite in many photo- and chemotrophic sulfur-oxidizing prokaryotes. The key enzyme is DsrAB, the dissimilatory sulfite reductase, but a range of other Dsr proteins is involved, with different gene sets being present in organisms with a reductive or oxidative metabolism. The dsrD gene codes for a small protein of unknown function and has been widely used as a functional marker for reductive or disproportionating sulfur metabolism, although in some cases this has been disputed. Here, we present in vivo and in vitro studies showing that DsrD is a physiological partner of DsrAB and acts as an activator of its sulfite reduction activity. DsrD is expressed in respiratory but not in fermentative conditions and a ΔdsrD deletion strain could be obtained, indicating that its function is not essential. This strain grew less efficiently during sulfate and sulfite reduction. Organisms with the earliest forms of dsrAB lack the dsrD gene, revealing that its activating role arose later in evolution relative to dsrAB.

Structural evolution of the ancient enzyme, dissimilatory sulfite reductase.

Dissimilatory sulfite reductase is an ancient enzyme that has linked the global sulfur and carbon biogeochemical cycles since at least 3.47 Gya. While much has been learned about the phylogenetic distribution and diversity of DsrAB across environmental gradients, far less is known about the structural changes that occurred to maintain DsrAB function as the enzyme accompanied diversification of sulfate/sulfite reducing organisms (SRO) into new environments. Analyses of available crystal structures of DsrAB from Archaeoglobus fulgidus and Desulfovibrio vulgaris, representing early and late evolving lineages, respectively, show that certain features of DsrAB are structurally conserved, including active siro-heme binding motifs. Whether such structural features are conserved among DsrAB recovered from varied environments, including hot spring environments that host representatives of the earliest evolving SRO lineage (e.g., MV2-Eury), is not known. To begin to overcome these gaps in our understanding of the evolution of DsrAB, structural models from MV2.Eury were generated and evolutionary sequence co-variance analyses were conducted on a curated DsrAB database. Phylogenetically diverse DsrAB harbor many conserved functional residues including those that ligate active siro-heme(s). However, evolutionary co-variance analysis of monomeric DsrAB subunits revealed several False Positive Evolutionary Couplings (FPEC) that correspond to residues that have co-evolved despite being too spatially distant in the monomeric structure to allow for direct contact. One set of FPECs corresponds to residues that form a structural path between the two active siro-heme moieties across the interface between heterodimers, suggesting the potential for allostery or electron transfer within the enzyme complex. Other FPECs correspond to structural loops and gaps that may have been selected to stabilize enzyme function in different environments. These structural bioinformatics results suggest that DsrAB has maintained allosteric communication pathways between subunits as SRO diversified into new environments. The observations outlined here provide a framework for future biochemical and structural analyses of DsrAB to examine potential allosteric control of this enzyme.

Redox states of Desulfovibrio vulgaris DsrC, a key protein in dissimilatory sulfite reduction.

Dissimilatory reduction of sulfite is carried out by the siroheme enzyme DsrAB, with the involvement of the protein DsrC, which has two conserved redox-active cysteines. DsrC was initially believed to be a third subunit of DsrAB. Here, we report a study of the distribution of DsrC in cell extracts to show that, in the model sulfate reducer Desulfovibrio vulgaris, the majority of DsrC is not associated with DsrAB and is thus free to interact with other proteins. In addition, we developed a cysteine-labelling gel-shift assay to monitor the DsrC redox state and behaviour, and procedures to produce the different redox forms. The oxidized state of DsrC with an intramolecular disulfide bond, which is proposed to be a key metabolic intermediate, could be successfully produced for the first time by treatment with arginine.

Global diversity and inferred ecophysiology of microorganisms with the potential for dissimilatory sulfate/sulfite reduction.

Sulfate/sulfite-reducing microorganisms (SRM) are ubiquitous in nature, driving the global sulfur cycle. A hallmark of SRM is the dissimilatory sulfite reductase encoded by the genes dsrAB. Based on analysis of 950 mainly metagenome-derived dsrAB-carrying genomes, we redefine the global diversity of microorganisms with the potential for dissimilatory sulfate/sulfite reduction and uncover genetic repertoires that challenge earlier generalizations regarding their mode of energy metabolism. We show: (i) 19 out of 23 bacterial and 2 out of 4 archaeal phyla harbor uncharacterized SRM, (ii) four phyla including the Desulfobacterota harbor microorganisms with the genetic potential to switch between sulfate/sulfite reduction and sulfur oxidation, and (iii) the combination as well as presence/absence of different dsrAB-types, dsrL-types and dsrD provides guidance on the inferred direction of dissimilatory sulfur metabolism. We further provide an updated dsrAB database including > 60% taxonomically resolved, uncultured family-level lineages and recommendations on existing dsrAB-targeted primers for environmental surveys. Our work summarizes insights into the inferred ecophysiology of newly discovered SRM, puts SRM diversity into context of the major recent changes in bacterial and archaeal taxonomy, and provides an up-to-date framework to study SRM in a global context.

Reduced protein sequence patterns in identifying key structural elements of dissimilatory sulfite reductase homologs.

Methanogenic archaea carry homologs of dissimilatory sulfite reductase (Dsr), called Dsr Like proteins (DsrLP). Dsr reduces sulfite to sulfide, a key step in an Earth's ancient metabolic process called dissimilatory sulfate reduction. The DsrLPs do not function as Dsr, and a computational approach is needed to develop hypotheses for guiding wet bench investigations on DsrLP's function. To make the computational analysis process efficient, the DsrLP amino acid sequences were transformed using only eight alphabets functionally representing twenty amino acids. The resultant reduced amino acid sequences were analyzed to identify conserved signature patterns in DsrLPs. Many of these patterns mapped on critical structural elements of Dsr and some were associated tightly with particular DsrLP groups. A search into the UniProtKB database identified several proteins carrying DsrLP's signature patterns; cysteine desulfurase, nucleosidase, and uroporphyrinogen III methylase were such matches. These outcomes provided clues to the functions of DsrLPs and highlighted the utility of the computational approach used.

Diversity and biogenesis contribution of sulfate-reducing bacteria in arsenic-contaminated soils from realgar deposits.

Microbial sulfate reduction, a vital mechanism for microorganisms living in anaerobic, sulfate-rich environments, is an essential aspect of the sulfur biogeochemical cycle. However, there has been no detailed investigation of the diversity and biogenesis contribution of sulfate-reducing bacteria in arsenic-contaminated soils from realgar deposits. To elucidate this issue, soil samples from representative abandoned realgar deposits were collected. Microcosm assays illustrated that all three samples (2-1, 2-2, and 2-3) displayed efficient sulfate and As(V)-respiring activities. Furthermore, a total of 28 novel sequence variants of dissimilatory sulfite reductase genes and 2 new families of dsrAB genes were successfully identified. A novel dissimilatory sulfate-reducing bacterium, Desulfotomaculum sp. JL1, was also isolated from soils, and can efficiently respiratory reduce As(V) and sulfate in 4 and 5 days, respectively. JL1 can promote the generation of yellow precipitates in the presence of multiple electron acceptors (both contain sulfate and As(V) in the cultures), which indicated the biogenesis contribution of sulfate-reducing bacteria to the realgar mine. Moreover, this area had unique microbial communities; the most abundant populations belonged to the phyla Proteobacteria, Chloroflexi, and Acidobacteriota, which were attributed to the unique geochemistry characteristics, such as total organic carbon, total As, NO3-, and SO42-. The results of this study provide new insight into the diversity and biogenesis contributions of sulfate-reducing bacteria in arsenic-contaminated soils from realgar deposits.

Functional Microbial Communities in Hybrid Linear Flow Channel Reactors for Desulfurization of Tannery Effluent.

Recent research has demonstrated that hybrid linear flow channel reactors (HLFCRs) can desulfurize tannery effluent via sulfate reduction and concurrent oxidation of sulfide to elemental sulfur. The reactors can be used to pre-treat tannery effluent to improve the efficiency of downstream anaerobic digestion and recover sulfur. This study was conducted to gain insight into the bacterial communities in HLFCRs operated in series and identify structure-function relationships. This was accomplished by interpreting the results obtained from amplicon sequencing of the 16S rRNA gene and quantification of the dissimilatory sulfite reducing (dsrB) gene. In an effort to provide a suitable inoculum, microbial consortia were harvested from saline estuaries and enriched. However, it was found that bioaugmentation was not necessary because native communities from tannery wastewater were selected over exogenous communities from the enriched consortia. Overall, Dethiosulfovibrio sp. and Petrimonas sp. were strongly selected (maximum relative abundances of 29% and 26%, respectively), while Desulfobacterium autotrophicum (57%), and Desulfobacter halotolerans (27%) dominated the sulfate reducing bacteria. The presence of elemental sulfur reducing genera such as Dethiosulfovibrio and Petrimonas is not desirable in HLFCRs, and strategies to counter their selection need to be considered to ensure efficiency of these systems for pre-treatment of tannery effluent.

Protein dynamics and molecular motions study in relation to molecular interaction between proteins from sulfur oxidizing proteobacteria Allochromatium vinosum.

The gamma-proteobacteria Allochromatium vinosum DSM 180T (A. vinosum) encodes the sulfur oxidizing dsr operon comprising of 15 genes. Dsr proteins are involved in oxidation of sulfur globules produced as an obligatory intermediate during the sulfur oxidation process. The dsrA and dsrB gene products are known to function as a α2ß2 hetero-tetramer and the protein complex plays the catalytic role in sulfur oxidation process. DsrC has a highly conserved C-terminal domain that forms a flexible arm, where two strictly conserved cysteines were found to act as a substrate donating residue for DsrAB instead of being a subunit of this redox enzyme. Therefore, to elucidate the molecular mechanism of the sulfur oxidation process here an attempt was made to study the dynamics, stability and binding mechanisms of DsrAB and DsrC proteins through computational docking and molecular dynamics (MD) simulations. This structure function relationship investigation revealed that the C-terminal domain of DsrC interacts with DsrA of DsrAB protein complex for catalytic functions. Some basic amino acid residues of DsrC are found to form the catalytic pockets along with DsrAB protein complex where the sulfur anions bind to get oxidized. Structural dynamics and fluctuations as well as the secondary structural alterations study revealed the possible regions responsible for protein-protein interactions. Principal Component Analysis (PCA) of protein motions displayed that the collective motions of DsrAB-DsrC complex was higher and more anti-correlated than the unbound DsrAB form. The present molecular insight study would therefore help researchers to predict the plausible biochemical mechanism of sulfur oxidation process in sulfur metabolic pathways in near future. Communicated by Ramaswamy H. Sarma.

Seawater recirculation through subducting sediments sustains a deeply buried population of sulfate-reducing bacteria.

Subseafloor sulfate concentrations typically decrease with depth as this electron acceptor is consumed by respiring microorganisms. However, studies show that seawater can flow through hydraulically conductive basalt to deliver sulfate upwards into deeply buried overlying sediments. Our previous work on IODP Site C0012A (Nankai Trough, Japan) revealed that recirculation of sulfate through the subducting Philippine Sea Plate stimulated microbial activity near the sediment-basement interface (SBI). Here, we describe the microbial ecology, phylogeny, and energetic requirements of population of aero-tolerant sulfate-reducing bacteria in the deep subseafloor. We identified dissimilatory sulfite reductase gene (dsr) sequences 93% related to oxygen-tolerant Desulfovibrionales species across all reaction zones while no SRB were detected in drilling fluid control samples. Pore fluid chemistry revealed low concentrations of methane (<0.25 mM), while hydrogen levels were consistent with active bacterial sulfate reduction (0.51-1.52 nM). Solid phase total organic carbon (TOC) was also considerably low in these subseafloor sediments. Our results reveal the phylogenetic diversity, potential function, and physiological tolerance of a community of sulfate-reducing bacteria living at ~480 m below subducting seafloor.

Diversity decoupled from sulfur isotope fractionation in a sulfate-reducing microbial community.

The extent of fractionation of sulfur isotopes by sulfate-reducing microbes is dictated by genomic and environmental factors. A greater understanding of species-specific fractionations may better inform interpretation of sulfur isotopes preserved in the rock record. To examine whether gene diversity influences net isotopic fractionation in situ, we assessed environmental chemistry, sulfate reduction rates, diversity of putative sulfur-metabolizing organisms by 16S rRNA and dissimilatory sulfite reductase (dsrB) gene amplicon sequencing, and net fractionation of sulfur isotopes along a sediment transect of a hypersaline Arctic spring. In situ sulfate reduction rates yielded minimum cell-specific sulfate reduction rates < 0.3 × 10-15 moles cell-1  day-1 . Neither 16S rRNA nor dsrB diversity indices correlated with relatively constant (38‰-45‰) net isotope fractionation (ε34 Ssulfide-sulfate ). Measured ε34 S values could be reproduced in a mechanistic fractionation model if 1%-2% of the microbial community (10%-60% of Deltaproteobacteria) were engaged in sulfate respiration, indicating heterogeneous respiratory activity within sulfate-reducing populations. This model indicated enzymatic kinetic diversity of Apr was more likely to correlate with sulfur fractionation than DsrB. We propose that, above a threshold Shannon diversity value of 0.8 for dsrB, the influence of the specific composition of the microbial community responsible for generating an isotope signal is overprinted by the control exerted by environmental variables on microbial physiology.

Phylogenetic diversity of sulfate-reducing bacteria of sediments of Chilika Lake, India, determined through analysis of the dissimilatory sulfite reductase (dsr AB) gene.

In this study, the sulfate-reducing bacteria, (SRB) were identified and reported for the first time through analysis of functional gene dsrAB, from the DNA of sediment samples collected from 10 sites of the Chilika lake. The finding illustrates Forty six Operational Taxonomic Units (OTUs), identified from the DGGE which were obtained from the 10 sediment samples. Of these, 34 OTUs exhibited around 78-96% sequence similarity and 12 OTUs showed 97 to 100% sequence similarity to the dsrAB gene of reported type strains of SRB. The sequence information obtained revealed the presence and distribution of diverse types of SRB which include phylotypes related to Desulfovibrio, Desulfonatronovibrio, Desulfomicrobium, Desulfobotulous and Desulfobacca. Upon comparison of dsrAB gene sequences of SRB obtained through this study with those collected from the GenBank, and through the dendrogram constructed, it was observed that except 13 OTUs that clustered closely with the reported type strains, all other 36 OTUs clustered distantly and had no representative member of SRB. This indicated the presence of phylogenetically diverse groups of SRB inhabiting the lake Chilika.

The functional diversity of the prokaryotic sulfur carrier protein TusA.

Persulfide groups participate in a wide array of biochemical pathways and are chemically very versatile. The TusA protein has been identified as a central element supplying and transferring sulfur as persulfide to a number of important biosynthetic pathways, like molybdenum cofactor biosynthesis or thiomodifications in nucleosides of tRNAs. In recent years, it has furthermore become obvious that this protein is indispensable for the oxidation of sulfur compounds in the cytoplasm. Phylogenetic analyses revealed that different TusA protein variants exists in certain organisms, that have evolved to pursue specific roles in cellular pathways. The specific TusA-like proteins thereby cannot replace each other in their specific roles and are rather specific to one sulfur transfer pathway or shared between two pathways. While certain bacteria like Escherichia coli contain several copies of TusA-like proteins, in other bacteria like Allochromatium vinosum a single copy of TusA is present with an essential role for this organism. Here, we give an overview on the multiple roles of the various TusA-like proteins in sulfur transfer pathways in different organisms to shed light on the remaining mysteries of this versatile protein.

Sulfur Isotope Effects of Dissimilatory Sulfite Reductase.

The precise interpretation of environmental sulfur isotope records requires a quantitative understanding of the biochemical controls on sulfur isotope fractionation by the principle isotope-fractionating process within the S cycle, microbial sulfate reduction (MSR). Here we provide the only direct observation of the major ((34)S/(32)S) and minor ((33)S/(32)S, (36)S/(32)S) sulfur isotope fractionations imparted by a central enzyme in the energy metabolism of sulfate reducers, dissimilatory sulfite reductase (DsrAB). Results from in vitro sulfite reduction experiments allow us to calculate the in vitro DsrAB isotope effect in (34)S/(32)S (hereafter, [Formula: see text]) to be 15.3 ± 2‰, 2σ. The accompanying minor isotope effect in (33)S, described as [Formula: see text], is calculated to be 0.5150 ± 0.0012, 2σ. These observations facilitate a rigorous evaluation of the isotopic fractionation associated with the dissimilatory MSR pathway, as well as of the environmental variables that govern the overall magnitude of fractionation by natural communities of sulfate reducers. The isotope effect induced by DsrAB upon sulfite reduction is a factor of 0.3-0.6 times prior indirect estimates, which have ranged from 25 to 53‰ in (34)εDsrAB. The minor isotope fractionation observed from DsrAB is consistent with a kinetic or equilibrium effect. Our in vitro constraints on the magnitude of [Formula: see text] is similar to the median value of experimental observations compiled from all known published work, where (34)ε r-p = 16.1‰ (r-p indicates reactant vs. product, n = 648). This value closely matches those of MSR operating at high sulfate reduction rates in both laboratory chemostat experiments ([Formula: see text] 17.3 ± 1.5‰, 2σ) and in modern marine sediments ([Formula: see text] 17.3 ± 3.8‰). Targeting the direct isotopic consequences of a specific enzymatic processes is a fundamental step toward a biochemical foundation for reinterpreting the biogeochemical and geobiological sulfur isotope records in modern and ancient environments.

Complete genomes of freshwater sulfur oxidizers Sulfuricella denitrificans skB26 and Sulfuritalea hydrogenivorans sk43H: genetic insights into the sulfur oxidation pathway of betaproteobacteria.

Despite detailed studies of marine sulfur-oxidizing bacteria, our knowledge concerning their counterparts in freshwater lake ecosystems is limited. Genome sequencing of the freshwater sulfur-oxidizing betaproteobacteria Sulfuricella denitrificans skB26 and Sulfuritalea hydrogenivorans sk43H have been completed. Strain skB26 possessed a circular plasmid of 86.6-kbp in addition to its chromosome, and an approximate 18-kbp region of the plasmid was occupied by an arxA-like operon, encoding a new clade of anaerobic arsenite oxidase. Multilocus sequence analysis showed that strain skB26 could not be assigned to any existing order; thus a novel order, Sulfuricellales, is proposed. The genomes of strains skB26 and sk43H were examined, focusing on the composition and the phylogeny of genes involved in the oxidation of inorganic sulfur compounds. Strains skB26 and sk43H shared a common pathway, which consisted of Sqr, SoxEF, SoxXYZAB, Dsr proteins, AprBA, Sat, and SoeABC. Comparative genomics of betaproteobacterial sulfur oxidizers showed that this pathway was also shared by the freshwater sulfur oxidizers Thiobacillus denitrificans and Sideroxydans lithotrophicus. It also revealed the presence of a conserved gene cluster, which was located immediately upstream of the betaproteobacterial dsr operon.

Effects of Spartina alterniflora invasion on the communities of methanogens and sulfate-reducing bacteria in estuarine marsh sediments.

The effect of plant invasion on the microorganisms of soil sediments is very important for estuary ecology. The community structures of methanogens and sulfate-reducing bacteria (SRB) as a function of Spartina alterniflora invasion in Phragmites australis-vegetated sediments of the Dongtan wetland in the Yangtze River estuary, China, were investigated using 454 pyrosequencing and quantitative real-time PCR (qPCR) of the methyl coenzyme M reductase A (mcrA) and dissimilatory sulfite-reductase (dsrB) genes. Sediment samples were collected from two replicate locations, and each location included three sampling stands each covered by monocultures of P. australis, S. alterniflora and both plants (transition stands), respectively. qPCR analysis revealed higher copy numbers of mcrA genes in sediments from S. alterniflora stands than P. australis stands (5- and 7.5-fold more in the spring and summer, respectively), which is consistent with the higher methane flux rates measured in the S. alterniflora stands (up to 8.01 ± 5.61 mg m(-2) h(-1)). Similar trends were observed for SRB, and they were up to two orders of magnitude higher than the methanogens. Diversity indices indicated a lower diversity of methanogens in the S. alterniflora stands than the P. australis stands. In contrast, insignificant variations were observed in the diversity of SRB with the invasion. Although Methanomicrobiales and Methanococcales, the hydrogenotrophic methanogens, dominated in the salt marsh, Methanomicrobiales displayed a slight increase with the invasion and growth of S. alterniflora, whereas the later responded differently. Methanosarcina, the metabolically diverse methanogens, did not vary with the invasion of, but Methanosaeta, the exclusive acetate utilizers, appeared to increase with S. alterniflora invasion. In SRB, sequences closely related to the families Desulfobacteraceae and Desulfobulbaceae dominated in the salt marsh, although they displayed minimal changes with the S. alterniflora invasion. Approximately 11.3 ± 5.1% of the dsrB gene sequences formed a novel cluster that was reduced upon the invasion. The results showed that in the sediments of tidal salt marsh where S. alterniflora displaced P. australis, the abundances of methanogens and SRB increased, but the community composition of methanogens appeared to be influenced more than did the SRB.

Regulation of dissimilatory sulfur oxidation in the purple sulfur bacterium allochromatium vinosum.

In the purple sulfur bacterium Allochromatium vinosum, thiosulfate oxidation is strictly dependent on the presence of three periplasmic Sox proteins encoded by the soxBXAK and soxYZ genes. It is also well documented that proteins encoded in the dissimilatory sulfite reductase (dsr) operon, dsrABEFHCMKLJOPNRS, are essential for the oxidation of sulfur that is stored intracellularly as an obligatory intermediate during the oxidation of thiosulfate and sulfide. Until recently, detailed knowledge about the regulation of the sox genes was not available. We started to fill this gap and show that these genes are expressed on a low constitutive level in A. vinosum in the absence of reduced sulfur compounds. Thiosulfate and possibly sulfide lead to an induction of sox gene transcription. Additional translational regulation was not apparent. Regulation of soxXAK is probably performed by a two-component system consisting of a multi-sensor histidine kinase and a regulator with proposed di-guanylate cyclase activity. Previous work already provided some information about regulation of the dsr genes encoding the second important sulfur-oxidizing enzyme system in the purple sulfur bacterium. The expression of most dsr genes was found to be at a low basal level in the absence of reduced sulfur compounds and enhanced in the presence of sulfide. In the present work, we focused on the role of DsrS, a protein encoded by the last gene of the dsr locus in A. vinosum. Transcriptional and translational gene fusion experiments suggest a participation of DsrS in the post-transcriptional control of the dsr operon. Characterization of an A. vinosum ΔdsrS mutant showed that the monomeric cytoplasmic 41.1-kDa protein DsrS is important though not essential for the oxidation of sulfur stored in the intracellular sulfur globules.

Mechanisms and evolution of oxidative sulfur metabolism in green sulfur bacteria.

Green sulfur bacteria (GSB) constitute a closely related group of photoautotrophic and thiotrophic bacteria with limited phenotypic variation. They typically oxidize sulfide and thiosulfate to sulfate with sulfur globules as an intermediate. Based on genome sequence information from 15 strains, the distribution and phylogeny of enzymes involved in their oxidative sulfur metabolism was investigated. At least one homolog of sulfide:quinone oxidoreductase (SQR) is present in all strains. In all sulfur-oxidizing GSB strains except the earliest diverging Chloroherpeton thalassium, the sulfide oxidation product is further oxidized to sulfite by the dissimilatory sulfite reductase (DSR) system. This system consists of components horizontally acquired partly from sulfide-oxidizing and partly from sulfate-reducing bacteria. Depending on the strain, the sulfite is probably oxidized to sulfate by one of two different mechanisms that have different evolutionary origins: adenosine-5'-phosphosulfate reductase or polysulfide reductase-like complex 3. Thiosulfate utilization by the SOX system in GSB has apparently been acquired horizontally from Proteobacteria. SoxCD does not occur in GSB, and its function in sulfate formation in other bacteria has been replaced by the DSR system in GSB. Sequence analyses suggested that the conserved soxJXYZAKBW gene cluster was horizontally acquired by Chlorobium phaeovibrioides DSM 265 from the Chlorobaculum lineage and that this acquisition was mediated by a mobile genetic element. Thus, the last common ancestor of currently known GSB was probably photoautotrophic, hydrogenotrophic, and contained SQR but not DSR or SOX. In addition, the predominance of the Chlorobium-Chlorobaculum-Prosthecochloris lineage among cultured GSB could be due to the horizontally acquired DSR and SOX systems. Finally, based upon structural, biochemical, and phylogenetic analyses, a uniform nomenclature is suggested for sqr genes in prokaryotes.