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Escherichia coli K-12 performs sulphoglycolysis; heterologous expression of enzymes encoded in a ten-gene cluster present in almost all (>91%) available E. coli genomes is used to show that sulphoquinovose is catabolised through four reactions to produce dihydroxyacetone phosphate, which powers energy conservation and growth, and a sulphonate product, which is excreted. Sulphoglycolysis as well as glycolysis in E. coli The monosaccharide sugar sulphoquinovose is a major component of the biological sulphur cycle, distributed widely in photosynthetic membranes and also in some non-photosynthetic bacteria and archaea. There is evidence for three different degradative pathways for sulphoquinovose in bacteria, but until now none had been fully characterized. This study shows that Escherichia coli K-12, the most widely-studied prokaryotic model organism, can perform sulphoglycolysis as well as standard glycolysis. A ten-gene cluster encodes all the enzymes needed to degrade sulphoquinovose to dihydroxyacetone phosphate. The same gene cluster is present in almost all available E. coli genomes and is widespread among the Enterobacteriaceae. The authors suggest that this newly defined pathway may represent a substantial part of the biogeochemical sulphur cycle, and may have a significant role in bacteria in the alimentary tract of all omnivores and herbivores, and in plant pathogens. Sulphoquinovose (SQ, 6-deoxy-6-sulphoglucose) has been known for 50 years as the polar headgroup of the plant sulpholipid 1 , 2 in the photosynthetic membranes of all higher plants, mosses, ferns, algae and most photosynthetic bacteria 3 . It is also found in some non-photosynthetic bacteria 4 , and SQ is part of the surface layer of some Archaea 5 . The estimated annual production of SQ 4 is 10,000,000,000 tonnes (10 petagrams), thus it comprises a major portion of the organo-sulphur in nature, where SQ is degraded by bacteria 6 , 7 . However, despite evidence for at least three different degradative pathways in bacteria 6 , 7 , 8 , no enzymic reaction or gene in any pathway has been defined, although a sulphoglycolytic pathway has been proposed 7 . Here we show that Escherichia coli K-12, the most widely studied prokaryotic model organism, performs sulphoglycolysis, in addition to standard glycolysis. SQ is catabolised through four newly discovered reactions that we established using purified, heterologously expressed enzymes: SQ isomerase, 6-deoxy-6-sulphofructose (SF) kinase, 6-deoxy-6-sulphofructose-1-phosphate (SFP) aldolase, and 3-sulpholactaldehyde (SLA) reductase. The enzymes are encoded in a ten-gene cluster, which probably also encodes regulation, transport and degradation of the whole sulpholipid; the gene cluster is present in almost all (>91%) available E. coli genomes, and is widespread in Enterobacteriaceae. The pathway yields dihydroxyacetone phosphate (DHAP), which powers energy conservation and growth of E. coli , and the sulphonate product 2,3-dihydroxypropane-1-sulphonate (DHPS), which is excreted. DHPS is mineralized by other bacteria, thus closing the sulphur cycle within a bacterial community.

Abstract Sulfoquinovose (SQ, 6-deoxy-6-sulfoglucose) was synthesized chemically. An HPLC-ELSD method to separate SQ and other chromophore-free sulfonates, e.g. 2,3-dihydroxypropane-1-sulfonate (DHPS), was developed. A set of 10 genome-sequenced, sulfonate-utilizing bacteria did not utilize SQ, but an isolate, Pseudomonas putida SQ1, from an enrichment culture did so. The molar growth yield with SQ was half of that with glucose, and 1 mol 3-sulfolactate (mol SQ)−1 was formed during growth. The 3-sulfolactate was degraded by the addition of Paracoccus pantotrophus NKNCYSA, and the sulfonate sulfur was recovered quantitatively as sulfate. Another isolate, Klebsiella oxytoca TauN1, could utilize SQ, forming 1 mol DHPS (mol SQ)−1; the molar growth yield with SQ was half of that with glucose. This DHPS could be degraded by Cupriavidus pinatubonensis JMP134, with quantitative recovery of the sulfonate sulfur as sulfate. We presume that SQ can be degraded by communities in the environment.

Cupriavidus necator H16 grew exponentially with ( R )-cysteate, a structural analogue of aspartate, as sole source of nitrogen in succinate-salts medium. Utilization of cysteate was quantitative and concomitant with growth and with the excretion of the deaminated product ( R )-sulfolactate, which was identified thoroughly. The deaminative pathway started with transport of ( R )-cysteate into the cell, which we attributed to an aspartate transporter. Transamination to sulfopyruvate involved an aspartate/( R )-cysteate:2-oxoglutarate aminotransferase (Aoa/Coa) and regeneration of the amino group acceptor by NADP + -coupled glutamate dehydrogenase. Reduction of sulfopyruvate to ( R )-sulfolactate was catalyzed by a ( S )-malate/( R )-sulfolactate dehydrogenase (Mdh/Sdh). Excretion of the sulfolactate could be attributed to the sulfite/organosulfonate exporter TauE, which was co-encoded and co-expressed, with sulfoacetaldehyde acetyltransferase (Xsc), though Xsc was irrelevant to the current pathway. The metabolic enzymes could be assayed biochemically. Aoa/Coa and Mdh/Sdh were highly enriched by protein separation, partly characterized, and the relevant locus-tags identified by peptide-mass fingerprinting. Finally, RT-PCR was used to confirm the transcription of all appropriate genes. We thus demonstrated that Cupriavidus necator H16 uses a patchwork pathway by recruitment of ‘housekeeping’ genes and sulfoacetaldehyde-degradative genes to scavenge for ( R )-cysteate-nitrogen.

Sulphoquinovose (SQ, 6-deoxy-6-sulphoglucose) has been known for 50 years as the polar headgroup of the plant sulpholipid (1,2) in the photosynthetic membranes of all higher plants, mosses, ferns, algae and most photosynthetic bacteria (3). It is also found in some non-photosynthetic bacteria (4), and SQ is part of the surface layer of some Archaea (5). The estimated annual production of SQ4 is 10,000,000,000 tonnes (10 petagrams), thus it comprises a major portion of the organos-ulphur in nature, where SQ is degraded by bacteria (6,7). However, despite evidence for at least three different degradative pathways in bacteria (6-8), no enzymic reaction or gene in any pathway has been defined, although a sulphoglycolytic pathway has been proposed (7). Here we show that Escherichia coliK-12, the most widely studied prokaryotic model organism, performs sulphoglycolysis, in addition to standard glycolysis. SQ is catabolised through four newly discovered reactions that we established using purified, heterologously expressed enzymes: SQ isomerase, 6-deoxy-6-sulphofructose (SF) kinase, 6-deoxy-6-sulphofructose-1-phosphate (SFP) aldolase, and 3-sulpholactaldehyde (SLA) reductase. The enzymes are encoded in a ten-gene cluster, which probably also encodes regulation, transport and degradation of the whole sulpholipid; the gene cluster is present in almost all (>91%) available E. coligenomes, and is widespread in Enterobacteriaceae. The pathway yields dihydroxyacetone phosphate (DHAP), which powers energy conservation and growth of E. coli, and the sulphonate product 2,3-dihydroxypropane-1sulphonate (DHPS), which is excreted. DHPS is mineralized by other bacteria, thus closing the sulphur cycle within a bacterial community.

Acinetobacter calcoaceticus SW1, under nitrogen limitation, assimilates the nitrogen moiety of taurine (2-aminoethanesulfonate) inducibly and excretes sulfoacetaldehyde, a product of taurine dehydrogenase (TauXY). BLAST searches of newly available genome sequences using the TauXY sequences revealed a 5-gene cluster, tauRXYPI , in Acinetobacter radioresistens SH164. We hypothesized that tauXYPI (HMPREF0018_00717–HMPREF0018_00720) encodes proteins that are orthologs of the undefined pathway from strain SW1, and that tauR (HMPREF0018_00716) encodes the relevant transcriptional regulator. Strain SH164 excreted sulfoacetaldehyde from taurine during growth. TauXY activity was expressed inducibly. Reverse transcription PCR showed that the tauRXYPI genes were transcribed inducibly. This allowed the conclusions that (i) TauP (currently annotated as permease GabP [TC 2.A.3]) is a taurine permease, and (ii) TauI (currently annotated as DUF6 drug/metabolite exporter [TC 2.A.7]) is a sulfoacetaldehyde exporter. The presumably equifunctional cluster tauRXYPI was then found in strain SW1. TauP is the third recognized taurine uptake system, and TauI is the third postulated class of sulfonate exporters, in bacteria.

Organosulfonates are widespread in the environment, both as natural products and as xenobiotics; and they generally share the property of chemical stability. A wide range of phenomena has evolved in microorganisms able to utilize the sulfur or the carbon moiety of these compounds; and recent work has centered on bacteria. This Mini-Review centers on bacterial catabolism of the carbon moiety in the C2-sulfonates and the fate of the sulfonate group. Five of the six compounds examined are subject to catabolism, but information on the molecular nature of transport and regulation is based solely on sequencing data. Two mechanisms of desulfonation have been established. First, there is the specific monooxygenation of ethanesulfonate or ethane-1,2-disulfonate. Second, the oxidative, reductive and fermentative modes of catabolism tend to yield the intermediate sulfoacetaldehyde, which is now known to be desulfonated to acetyl phosphate by a thiamin-diphosphate-dependent acetyltransferase. This enzyme is widespread and at least three subgroups can be recognized, some of them in genomic sequencing projects. These data emphasize the importance of acetyl phosphate in bacterial metabolism. A third mechanism of desulfonation is suggested: the hydrolysis of sulfoacetate.

Abstract Organosulfonates are widespread compounds, be they natural products of low or high molecular weight, or xenobiotics. Many commonly found compounds are subject to desulfonation, even if it is not certain whether all the corresponding enzymes are widely expressed in nature. Sulfonates require transport systems to cross the cell membrane, but few physiological data and no biochemical data on this topic are available, though the sequences of some of the appropriate genes are known. Desulfonative enzymes in aerobic bacteria are generally regulated by induction, if the sulfonate is serving as a carbon and energy source, or by a global network for sulfur scavenging (sulfate-starvation-induced (SSI) stimulon) if the sulfonate is serving as a source of sulfur. It is unclear whether an SSI regulation is found in anaerobes. The anaerobic bacteria examined can express the degradative enzymes constitutively, if the sulfonate is being utilized as a carbon source, but enzyme induction has also been observed. At least three general mechanisms of desulfonation are recognisable or postulated in the aerobic catabolism of sulfonates: (1) activate the carbon neighboring the C–SO−3 bond and release of sulfite assisted by a thiamine pyrophosphate cofactor; (2) destabilize the C–SO−3 bond by addition of an oxygen atom to the same carbon, usually directly by oxygenation, and loss of the good leaving group, sulfite; (3) an unidentified, formally reductive reaction. Under SSIS control, different variants of mechanism (2) can be seen. Catabolism of sulfonates by anaerobes was discovered recently, and the degradation of taurine involves mechanism (1). When anaerobes assimilate sulfonate sulfur, there is one common, unknown mechanism to desulfonate the inert aromatic compounds and another to desulfonate inert aliphatic compounds; taurine seems to be desulfonated by mechanism (1).

Taurine (2-aminoethanesulfonate) is a widespread natural product whose nitrogen moiety was recently shown to be assimilated by bacteria, usually with excretion of an organosulfonate via undefined novel pathways; other data involve transcriptional regulator TauR in taurine metabolism. A screen of genome sequences for TauR with the BLAST algorithm allowed the hypothesis that the marine gammaproteobacterium Neptuniibacter caesariensis MED92 would inducibly assimilate taurine-nitrogen and excrete sulfoacetate. The pathway involved an ABC transporter (TauABC), taurine:pyruvate aminotransferase (Tpa), a novel sulfoacetaldehyde dehydrogenase (SafD) and exporter(s) of sulfoacetate (SafE) (DUF81). Ten candidate genes in two clusters involved three sets of paralogues (for TauR, Tpa and SafE). Inducible Tpa and SafD were detected in cell extracts. SafD was purified 600-fold to homogeneity in two steps. The monomer had a molecular mass of 50 kDa (SDS-PAGE); data from gel filtration chromatography indicated a tetrameric native protein. SafD was specific for sulfoacetaldehyde with a K m-value of 0.12 mM. The N-terminal amino acid sequence of SafD confirmed the identity of the safD gene. The eight pathway genes were transcribed inducibly, which indicated expression of the whole hypothetical pathway. We presume that this pathway is one source of sulfoacetate in nature, where this compound is dissimilated by many bacteria.

Cysteate and sulfolactate are widespread natural products in the environment, while propanesulfonate, 3-aminopropanesulfonate and propane-1,3-disulfonate are xenobiotics. While some understanding of the bacterial assimilation of cysteate sulfur has been achieved, details of the dissimilation of cysteate and sulfolactate by microbes together with information on the degradation of the xenobiotics have only recently become available. This minireview centres on bacterial catabolism of the carbon moiety in these C₃-sulfonates and on the fate of the sulfonate group. Three mechanisms of desulfonation have been established. Firstly, cysteate is converted via sulfopyruvate to sulfolactate, which is desulfonated to pyruvate and sulfite; the latter is oxidized to sulfate by a sulfite dehydrogenase and excreted as sulfate in Paracoccus pantotrophus NKNCYSA. Secondly, sulfolactate can be converted to cysteate, which is cleaved in a pyridoxal 5'-phosphate-coupled reaction to pyruvate, sulfite and ammonium ions; in Silicibacter pomeroyi DSS-3, the sulfite is excreted largely as sulfite. Both desulfonation reactions seem to be widespread. The third desulfonation mechanism is oxygenolysis of, e.g. propanesulfonate(s), about which less is known.

The amphoteric surfactant N-oleoyl-N-methyltaurine, which is in use in skin-care products, was utilized by aerobic bacteria as the sole source of carbon or of nitrogen in enrichment cultures. One isolate, which was identified as Pseudomonas alcaligenes, grew with the xenobiotic compound as the sole source of carbon and energy. The sulfonate moiety, N-methyltaurine, was excreted quantitatively during growth, while the fatty acid was dissimilated. The initial degradative reaction was shown to be hydrolytic and inducible. This amidase reaction could be demonstrated with crude cell extracts. The excreted N-methyltaurine could be utilized by other bacteria in cocultures. Complete degradation of similar natural compounds in bacterial communities seems likely.