Desulfonation of propanesulfonic acid by comamonas acidovorans strain P53: evidence for an alkanesulfonate sulfonatase and an atypical sulfite dehydrogenase

Pterin function in bacteria

Pterins are widely conserved biomolecules that play essential roles in diverse organisms. First described as enzymatic cofactors in eukaryotic systems, bacterial pterins were discovered in cyanobacteria soon after. Several pterin structures unique to bacteria have been described, with conjugation to glycosides and nucleotides commonly observed. Despite this significant structural diversity, relatively few biological functions have been elucidated. Molybdopterin, the best studied bacterial pterin, plays an essential role in the function of the Moco cofactor. Moco is an essential component of molybdoenzymes such as sulfite oxidase, nitrate reductase, and dimethyl sulfoxide reductase, all of which play important roles in bacterial metabolism and global nutrient cycles. Outside of the molybdoenzymes, pterin cofactors play important roles in bacterial cyanide utilization and aromatic amino acid metabolism. Less is known about the roles of pterins in nonenzymatic processes. Cyanobacterial pterins have been implicated in phenotypes related to UV protection and phototaxis. Research describing the pterin-mediated control of cyclic nucleotide metabolism, and their influence on virulence and attachment, points to a possible role for pterins in regulation of bacterial behavior. In this review, we describe the variety of pterin functions in bacteria, compare and contrast structural and mechanistic differences, and illuminate promising avenues of future research.

Methanesulfonate (MSA) Catabolic Genes from Marine and Estuarine Bacteria

Quantitatively, methanesulfonate (MSA) is a very relevant compound in the global biogeochemical sulfur cycle. Its utilization by bacteria as a source of carbon and energy has been described and a specific enzyme, methanesulfonate monooxygenase (MSAMO), has been found to perform the first catabolic step of its oxidation. Other proteins seemingly involved in the import of MSA into bacterial cells have been reported. In this study, we obtained novel sequences of genes msmA and msmE from marine, estuary and soil MSA-degraders (encoding the large subunit of the MSAMO enzyme and the periplasmic component of the import system, respectively). We also obtained whole-genome sequences of two novel marine Filomicrobium strains, Y and W, and annotated two full msm operons in these genomes. Furthermore, msmA and msmE sequences were amplified from North Atlantic seawater and analyzed. Good conservation of the MsmA deduced protein sequence was observed in both cultured strains and metagenomic clones. A long spacer sequence in the Rieske-type [2Fe-2S] cluster-binding motif within MsmA was found to be conserved in all instances, supporting the hypothesis that this feature is specific to the large (α) subunit of the MSAMO enzyme. The msmE gene was more difficult to amplify, from both cultivated isolates and marine metagenomic DNA. However, 3 novel msmE sequences were obtained from isolated strains and one directly from seawater. With both genes, our results combined with previous metagenomic analyses seem to imply that moderate to high-GC strains are somehow favored during enrichment and isolation of MSA-utilizing bacteria, while the majority of msm genes obtained by cultivation-independent methods have low levels of GC%, which is a clear example of the misrepresentation of natural populations that culturing, more often than not, entails. Nevertheless, the data obtained in this work show that MSA-degrading bacteria are abundant in surface seawater, which suggests ecological relevance for this metabolic group of bacteria.

Functional dissection of the multi-domain di-heme cytochrome c(550) from Thermus thermophilus

In bacteria, oxidation of sulfite to sulfate, the most common strategy for sulfite detoxification, is mainly accomplished by the molybdenum-containing sulfite:acceptor oxidoreductases (SORs). Bacterial SORs are very diverse proteins; they can exist as monomers or homodimers of their core subunit, as well as heterodimers with an additional cytochrome c subunit. We have previously described the homodimeric SOR from Thermus thermophilus HB8 (SOR(TTHB8)), identified its physiological electron acceptor, cytochrome c(550), and demonstrated the key role of the latter in coupling sulfite oxidation to aerobic respiration. Herein, the role of this di-heme cytochrome c was further investigated. The cytochrome was shown to be composed of two conformationally independent domains, each containing one heme moiety. Each domain was separately cloned, expressed in E. coli and purified to homogeneity. Stopped-flow experiments showed that: i) the N-terminal domain is the only one accepting electrons from SOR(TTHB8); ii) the N- and C-terminal domains are in rapid redox equilibrium and iii) both domains are able to transfer electrons further to cytochrome c(552), the physiological substrate of the ba(3) and caa(3) terminal oxidases. These findings show that cytochrome c(550) functions as a electron shuttle, without working as an electron wire with one heme acting as the electron entry and the other as the electron exit site. Although contribution of the cytochrome c(550) C-terminal domain to T. thermophilus sulfur respiration seems to be dispensable, we suggest that di-heme composition of the cytochrome physiologically enables storage of the two electrons generated from sulfite oxidation, thereof ensuring efficient contribution of sulfite detoxification to the respiratory chain-mediated energy generation.

Molecular genetics and biochemistry of N-acetyltaurine degradation by Cupriavidus necator H16

Cupriavidus necator H16 (DSM 428), whose genome has been sequenced, was found to degrade N-acetyltaurine as a sole source of carbon and energy for growth. Utilization of the compound was quantitative. The degradative pathway involved an inducible N-acetyltaurine amidohydrolase (NaaS), which catalysed the cleavage of N-acetyltaurine to acetate and taurine. The degradation of the latter compound is via an inducible, degradative pathway that involves taurine dehydrogenase [EC 1.4.2.-], sulfoacetaldehyde acetyltransferase [EC 2.3.3.15], phosphotransacetylase [EC 2.4.1.8], a sulfite exporter [TC 9.A.29.2.1] and sulfite dehydrogenase [EC 1.8.2.1]. Induction of the expression of representative gene products, encoded by at least four gene clusters, was confirmed biochemically. The acetate released by NaaS was activated to acetyl-CoA by an inducible acetate-CoA ligase [EC 6.2.1.1]. NaaS was purified to homogeneity; it had a K(m) value of 9.4 mM for N-acetyltaurine, and it contained tightly bound Zn and Fe atoms. The denatured enzyme has a molecular mass of about 61 kDa (determined by SDS-PAGE) and the native enzyme was apparently monomeric. Peptide-mass fingerprinting identified the locus tag as H16_B0868 in a five-gene cluster, naaROPST (H16_B0865-H16_B0869). The cluster presumably encodes a LysR-type transcriptional regulator (NaaR), a membrane protein (NaaO), a solute : sodium symporter-family permease [TC 2.A.21] (NaaP), the metal-dependent amidohydrolase (NaaS) and a putative metallochaperone (COG0523) (NaaT). Reverse-transcription PCR indicated that naaOPST were inducibly transcribed.

How are "Atypical" Sulfite Dehydrogenases Linked to Cell Metabolism? Interactions between the SorT Sulfite Dehydrogenase and Small Redox Proteins

Sulfite dehydrogenases (SDHs) are enzymes that catalyze the oxidation of the toxic and mutagenic compound sulfite to sulfate, thereby protecting cells from adverse effects associated with sulfite exposure. While some bacterial SDHs that have been characterized to date are able to use cytochrome c as an electron acceptor, the majority of these enzymes prefer ferricyanide as an electron acceptor and have therefore been termed "atypical" SDHs. Identifying the natural electron acceptor of these enzymes, however, is crucial for understanding how the "atypical" SDHs are integrated into cell metabolism. The SorT sulfite dehydrogenase from Sinorhizobium meliloti is a representative of this enzyme type and we have investigated the interactions of SorT with two small redox proteins, a cytochrome c and a Cu containing pseudoazurin, that are encoded in the same operon and are co-transcribed with the sorT gene. Both potential acceptor proteins have been purified and characterized in terms of their biochemical and electrochemical properties, and interactions and enzymatic studies with both the purified SorT sulfite dehydrogenase and components of the respiratory chain have been carried out. We were able to show for the first time that an "atypical" sulfite dehydrogenase can couple efficiently to a cytochrome c isolated from the same organism despite being unable to efficiently reduce horse heart cytochrome c, however, at present the role of the pseudoazurin in SorT electron transfer is unclear, but it is possible that it acts as an intermediate electron shuttle between. The SorT system appears to couple directly to the respiratory chain, most likely to a cytochrome oxidase.

Racemase activity effected by two dehydrogenases in sulfolactate degradation by Chromohalobacter salexigens: purification of (S)-sulfolactate dehydrogenase

Chromohalobacter salexigens DSM 3043, whose genome has been sequenced, is known to degrade (R,S)-sulfolactate as a sole carbon and energy source for growth. Utilization of the compound(s) was shown to be quantitative, and an eight-gene cluster (Csal_1764-Csal_1771) was hypothesized to encode the enzymes in the degradative pathway. It comprised a transcriptional regulator (SuyR), a Tripartite Tricarboxylate Transporter-family uptake system for sulfolactate (SlcHFG), two sulfolactate dehydrogenases of opposite sulfonate stereochemistry, namely novel SlcC and ComC [(R)-sulfolactate dehydrogenase] [EC 1.1.1.272] and desulfonative sulfolactate sulfo-lyase (SuyAB) [EC 4.4.1.24]. Inducible reduction of 3-sulfopyruvate, inducible SuyAB activity and induction of an unknown protein were detected. Separation of the soluble proteins from induced cells on an anion-exchange column yielded four relevant fractions. Two different fractions reduced sulfopyruvate with NAD(P)H, a third yielded SuyAB activity, and the fourth contained the unknown protein. The latter was identified by peptide-mass fingerprinting as SlcH, the candidate periplasmic binding protein of the transport system. Separated SuyB was also identified by peptide-mass fingerprinting. ComC was partially purified and identified by peptide-mass fingerprinting. The (R)-sulfolactate that ComC produced from sulfopyruvate was a substrate for SuyAB, which showed that SuyAB is (R)-sulfolactate sulfo-lyase. SlcC was purified to homogeneity. This enzyme also formed sulfolactate from sulfopyruvate, but the latter enantiomer was not a substrate for SuyAB. SlcC was obviously ( S)-sulfolactate dehydrogenase.

Bifurcated degradative pathway of 3-sulfolactate in Roseovarius nubinhibens ISM via sulfoacetaldehyde acetyltransferase and (S)-cysteate sulfolyase

Data from the genome sequence of the aerobic, marine bacterium Roseovarius nubinhibens ISM were interpreted such that 3-sulfolactate would be degraded as a sole source of carbon and energy for growth via a novel bifurcated pathway including two known desulfonative enzymes, sulfoacetaldehyde acetyltransferase (EC 2.3.3.15) (Xsc) and cysteate sulfo-lyase (EC 4.4.1.25) (CuyA). Strain ISM utilized sulfolactate quantitatively with stoichiometric excretion of the sulfonate sulfur as sulfate. A combination of enzyme assays, analytical chemistry, enzyme purification, peptide mass fingerprinting, and reverse transcription-PCR data supported the presence of an inducible, tripartite sulfolactate uptake system (SlcHFG), and a membrane-bound sulfolactate dehydrogenase (SlcD) which generated 3-sulfopyruvate, the point of bifurcation. 3-Sulfopyruvate was in part decarboxylated by 3-sulfopyruvate decarboxylase (EC 4.1.1.79) (ComDE), which was purified. The sulfoacetaldehyde that was formed was desulfonated by Xsc, which was identified, and the acetyl phosphate was converted to acetyl-coenzyme A by phosphate acetyltransferase (Pta). The other portion of the 3-sulfopyruvate was transaminated to (S)-cysteate, which was desulfonated by CuyA, which was identified. The sulfite that was formed was presumably exported by CuyZ (TC 9.B.7.1.1 in the transport classification system), and a periplasmic sulfite dehydrogenase is presumed. Bioinformatic analyses indicated that transporter SlcHFG is rare but that SlcD is involved in three different combinations of pathways, the bifurcated pathway shown here, via CuyA alone, and via Xsc alone. This novel pathway involves ComDE in biodegradation, whereas it was discovered in the biosynthesis of coenzyme M. The different pathways of desulfonation of sulfolactate presumably represent final steps in the biodegradation of sulfoquinovose (and exudates derived from it) in marine and aquatic environments.

The DUF81 protein TauE in Cupriavidus necator H16, a sulfite exporter in the metabolism of C2 sulfonates

The degradation of taurine, isethionate and sulfoacetate in Cupriavidus necator (Ralstonia eutropha) H16 was shown by enzyme assays to be inducible, and each pathway involved sulfoacetaldehyde, which was subject to phosphatolysis by a common sulfoacetaldehyde acetyltransferase (Xsc, H16_B1870) to yield acetyl phosphate and sulfite. The neighbouring genes encoded phosphate acetyltransferase (Pta, H16_B1871) and a hypothetical protein [domain of unknown function (DUF)81, H16_B1872], with eight derived transmembrane helices. RT-PCR showed inducible transcription of these three genes, and led to the hypothesis that H16_B1872 and orthologous proteins represent a sulfite exporter, which was named TauE.

Roseovariussp. strain 217: aerobic taurine dissimilation via acetate kinase and acetate-CoA ligase

The genome sequence of Roseovarius sp. strain 217 indicated that many pathway enzymes found in other organisms for the degradation of taurine are represented, but that a novel, apparently energy-dependent pathway is involved in the conversion of acetyl phosphate to acetyl CoA. Thus, an ABC transporter for taurine could be postulated, while inducible taurine: pyruvate aminotransferase, alanine dehydrogenase, sulfoacetaldehyde acetyltransferase and sulfite dehydrogenase could be assayed. Whereas phosphate acetyltransferase has been found in other organisms, none was indicated in the genome sequence and no activity was found in cell-free extracts. Instead, acetate kinase was active as was acetate-CoA ligase.

N-acetyltaurine dissimilated via taurine by Delftia acidovorans NAT

The naturally occurring sulfonate N-acetyltaurine was synthesized chemically and its identity was confirmed. Aerobic enrichment cultures for bacteria able to utilize N-acetyltaurine as sole source of fixed nitrogen or as sole source of carbon were successful. One representative isolate, strain NAT, which was identified as a strain of Delftia acidovorans, grew with N-acetyltaurine as carbon source and excreted stoichiometric amounts of sulfate and ammonium. Inducible enzyme activities were measured in crude extracts of this organism to elucidate the degradative pathway. Cleavage of N-acetyltaurine by a highly active amidase yielded acetate and taurine. The latter was oxidatively deaminated by taurine dehydrogenase to ammonium and sulfoacetaldehyde. This key intermediate of sulfonate catabolism was desulfonated by the known reaction of sulfoacetaldehyde acetyltransferase to sulfite and acetyl phosphate, which was further degraded to enter central metabolism. A degradative pathway including transport functions is proposed.

The sulfonated osmolyte N-methyltaurine is dissimilated by Alcaligenes faecalis and by Paracoccus versutus with release of methylamine

Selective enrichments yielded bacterial cultures able to utilize the osmolyte N-methyltaurine as sole source of carbon and energy or as sole source of fixed nitrogen for aerobic growth. Strain MT1, which degraded N-methyltaurine as a sole source of carbon concomitantly with growth, was identified as a strain of Alcaligenes faecalis. Stoichiometric amounts of methylamine, whose identity was confirmed by matrix-assisted, laser-desorption ionization time-of-flight mass spectrometry, and of sulfate were released during growth. Inducible N-methyltaurine dehydrogenase, sulfoacetaldehyde acetyltransferase (Xsc) and a sulfite dehydrogenase could be detected. Taurine dehydrogenase was also present and it was hypothesized that taurine dehydrogenase has a substrate range that includes N-methyltaurine. Partial sequences of a tauY-like gene (encoding the putative large component of taurine dehydrogenase) and an xsc gene were obtained by PCR with degenerate primers. Strain N-MT utilized N-methyltaurine as a sole source of fixed nitrogen for growth and could also utilize the compound as sole source of carbon. This bacterium was identified as a strain of Paracoccus versutus. This organism also expressed inducible (N-methyl)taurine dehydrogenase, Xsc and a sulfite dehydrogenase. The presence of a gene cluster with high identity to a larger cluster from Paracoccus pantotrophus NKNCYSA, which is now known to dissimilate N-methyltaurine via Xsc, allowed most of the overall pathway, including transport and excretion, to be defined. N-Methyltaurine is thus another compound whose catabolism is channelled directly through sulfoacetaldehyde.

Inducible transcription of genes involved in taurine uptake and dissimilation by Silicibacter pomeroyi DSS-3T

A largely untested hypothesis for the bacterial dissimilation of taurine was explored in Silicibacter pomeroyi DSS-3, whose genome has been sequenced. Substrate-specific transcription of candidate genes encoding taurine uptake and dissimilation (tauABC, tpa, ald, xsc, pta) was found, which corresponded to the induction of Tpa, Ald, Xsc and Pta, that was observed.

L-cysteate sulpho-lyase, a widespread pyridoxal 5'-phosphate-coupled desulphonative enzyme purified from Silicibacter pomeroyi DSS-3(T)

Quantitative utilization of L-cysteate (2-amino-3-sulphopropionate) as the sole source of carbon and energy for growth of the aerobic, marine bacterium Silicibacter pomeroyi DSS-3(T) was observed. The sulphonate moiety was recovered in the medium largely as sulphite, and the appropriate amount of the ammonium ion was also observed. Genes [suyAB (3-sulpholactate sulpho-lyase)] encoding the known desulphonation reaction in cysteate degradation were absent from the genome, but a homologue of a putative sulphate exporter gene (suyZ) was found, and its neighbour, annotated as a D-cysteine desulphhydrase, was postulated to encode pyridoxal 5'-phosphate-coupled L-cysteate sulpho-lyase (CuyA), a novel enzyme. Inducible CuyA was detected in cysteate-grown cells. The enzyme released equimolar pyruvate, sulphite and the ammonium ion from L-cysteate and was purified to homogeneity by anion-exchange, hydrophobic-interaction and gel-filtration chromatography. The N-terminal amino acid sequence of this 39-kDa subunit confirmed the identification of the cuyA gene. The native enzyme was soluble and homomultimeric. The K(m)-value for L-cysteate was high (11.7 mM) and the enzyme also catalysed the D-cysteine desulphhydrase reaction. The gene cuyZ, encoding the putative sulphite exporter, was co-transcribed with cuyA. Sulphite was exported despite the presence of a ferricyanide-coupled sulphite dehydrogenase. CuyA was found in many bacteria that utilize cysteate.

Dissimilation of C3-sulfonates

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 C(3)-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.

Isolation and properties of methanesulfonate-degrading Afipia felis from Antarctica and comparison with other strains of A. felis

Three novel strains of methylotrophic Afipia felis were isolated from several locations on Signy Island, Antarctica, and a fourth from estuary sediment from the River Douro, Portugal. They were identified as strains of the alpha-2 proteobacterium A. felis by 16S rRNA gene sequence analysis. Two strains tested were shown to contain the fdxA gene, diagnostic for A. felis. All strains grew with methanesulfonate (and two strains with dimethylsulfone) as sole carbon substrate. Growth on methanesulfonate required methanesulfonate monooxygenase (MSAMO), using NADH as the reductant and stimulated by reduced flavin nucleotides and Fe(II). Polymerase chain reaction amplification of DNA from an Antarctic strain showed a typical msmA gene for the alpha-hydroxylase of MSAMO, and both Antarctic and Portuguese strains contained mxaF, the methanol dehydrogenase large subunit gene. This is the first report of methanesulfonate-degrading bacteria from the Antarctic and of methylotrophy in Afipia, and the first description of any bacterium able to use both methanesulfonate and dimethylsulfone. In contrast, the type strain of A. felis DSM 7326(T) was not methylotrophic, but grew in defined mineral medium with a wide range of single simple organic substrates. Free-living Afipia strains occurring widely in the natural environment may be significant as methylotrophs, degrading C(1)-sulfur compounds, including the recalcitrant organosulfur compound methanesulfonate.

Dissimilation of cysteate via 3-sulfolactate sulfo-lyase and a sulfate exporter in Paracoccus pantotrophus NKNCYSA

Paracoccus pantotrophusNKNCYSA utilizes (R)-cysteate (2-amino-3-sulfopropionate) as a sole source of carbon and energy for growth, with either nitrate or molecular oxygen as terminal electron acceptor, and the specific utilization rate of cysteate is about 2 mkat (kg protein)−1. The initial degradative reaction is catalysed by an (R)-cysteate : 2-oxoglutarate aminotransferase, which yields 3-sulfopyruvate. The latter was reduced to 3-sulfolactate by an NAD-linked sulfolactate dehydrogenase [3·3 mkat (kg protein)−1]. The inducible desulfonation reaction was not detected initially in cell extracts. However, a strongly induced protein with subunits of 8 kDa (α) and 42 kDa (β) was found and purified. The corresponding genes had similarities to those encoding altronate dehydratases, which often require iron for activity. The purified enzyme could then be shown to convert 3-sulfolactate to sulfite and pyruvate and it was termed sulfolactate sulfo-lyase (Suy). A high level of sulfite dehydrogenase was also induced during growth with cysteate, and the organism excreted sulfate. A putative regulator, OrfR, was encoded upstream ofsuyABon the reverse strand. Downstream ofsuyABwassuyZ, which was cotranscribed withsuyB. The gene, an allele oftauZ, encoded a putative membrane protein with transmembrane helices (COG2855), and is a candidate to encode the sulfate exporter needed to maintain homeostasis during desulfonation.suyAB-like genes are widespread in sequenced genomes and environmental samples where, in contrast to the current annotation, several presumably encode the desulfonation of 3-sulfolactate, a component of bacterial spores.

Enzymes and genes of taurine and isethionate dissimilation in Paracoccus denitrificans

Growth of the alpha-proteobacterium Paracoccus denitrificans NKNIS with taurine or isethionate as sole source of carbon involves sulfoacetaldehyde acetyltransferase (Xsc), which is presumably encoded by an xsc gene in subgroup 3, none of whose gene products has been characterized. The genome of the alpha-proteobacterium Rhodobacter sphaeroides 2.4.1 was interpreted to contain a nine-gene cluster encoding the inducible dissimilation of taurine, and this deduced pathway included a regulator, a tripartite ATP-independent transporter, taurine dehydrogenase (TDH; presumably TauXY) as well as Xsc (subgroup 3), a hypothetical protein and phosphate acetyltransferase (Pta). A similar cluster was found in P. denitrificans NKNIS, in contrast to an analogous cluster encoding an ATP-binding cassette transporter in Paracoccus pantotrophus. Inducible TDH, Xsc and Pta were found in extracts of taurine-grown cells of strain NKNIS. TDH oxidized taurine to sulfoacetaldehyde and ammonium ion with cytochrome c as electron acceptor. Whereas Xsc and Pta were soluble enzymes, TDH was located in the particulate fraction, where inducible proteins with the expected masses of TauXY (14 and 50 kDa, respectively) were detected by SDS-PAGE. Xsc and Pta were separated by anion-exchange chromatography. Xsc was effectively pure; the molecular mass of the subunit (64 kDa) and the N-terminal amino acid sequence confirmed the identification of the xsc gene. Inducible isethionate dehydrogenase (IDH), Xsc and Pta were assayed in extracts of isethionate-grown cells of strain NKNIS. IDH was located in the particulate fraction, oxidized isethionate to sulfoacetaldehyde with cytochrome c as electron acceptor and correlated with the expression of a 62 kDa protein. Strain NKNIS excreted sulfite and sulfate during growth with a sulfonate and no sulfite dehydrogenase was detected. There is considerable biochemical, genetic and regulatory complexity in the degradation of these simple molecules.

Sulphoacetaldehyde acetyltransferase yields acetyl phosphate: purification from Alcaligenes defragrans and gene clusters in taurine degradation

The facultatively anaerobic bacterium Alcaligenes defragrans NKNTAU was found to oxidize taurine (2-aminoethanesulphonate) with nitrate as the terminal electron acceptor. Taurine was transaminated to 2-sulphoacetaldehyde. This was not converted into sulphite and acetate by a "sulphoacetaldehyde sulpho-lyase" (EC 4.4.1.12), but into sulphite and acetyl phosphate, which was identified by three methods. The enzyme, which required the addition of phosphate, thiamin diphosphate and Mg(2+) ions for activity, was renamed sulphoacetaldehyde acetyltransferase (Xsc; EC 2.3.1.-). Inducible Xsc was expressed at high levels, and a three-step 11-fold purification yielded an essentially homogeneous soluble protein, which was a homotetramer in its native form; the molecular mass of the subunit was found to be between about 63 kDa (SDS/PAGE) and 65.3 kDa (matrix-assisted laser-desorption ionization-time-of-flight MS). The N-terminal and two internal amino acid sequences were determined, and PCR primers were generated. The xsc gene was amplified and sequenced; the derived molecular mass of the processed protein was 65.0 kDa. The downstream gene presumably encoded the inducible phosphate acetyltransferase (Pta) found in crude extracts. The desulphonative enzymes ("EC 4.4.1.12") from Achromobacter xylosoxidans NCIMB 10751 and Desulfonispora thiosulfatigenes GKNTAU were shown to be Xscs. We detected at least three subclasses of xsc in Proteobacteria and in Gram-positive bacteria, and they comprised a distinct group within the acetohydroxyacid synthase supergene family. Genome sequencing data revealed xsc genes in Burkholderia fungorum (80% sequence identity) and Sinorhizobium meliloti (61%) with closely linked pta genes. Different patterns of regulation for the transport and dissimilation of taurine were hypothesized for S. meliloti and B. fungorum.

Enzymology and molecular biology of prokaryotic sulfite oxidation

Despite its toxicity, sulfite plays a key role in oxidative sulfur metabolism and there are even some microorganisms which can use it as sole electron source. Sulfite is the main intermediate in the oxidation of sulfur compounds to sulfate, the major product of most dissimilatory sulfur-oxidizing prokaryotes. Two pathways of sulfite oxidation are known: (1) direct oxidation to sulfate catalyzed by a sulfite:acceptor oxidoreductase, which is thought to be a molybdenum-containing enzyme; (2) indirect oxidation under the involvement of the enzymes adenylylsulfate (APS) reductase and ATP sulfurylase and/or adenylylsulfate:phosphate adenylyltransferase with APS as an intermediate. The latter pathway allows substrate phosphorylation and occurs in the bacterial cytoplasm. Direct oxidation appears to have a wider distribution; however, a redundancy of pathways has been described for diverse photo- or chemotrophic, sulfite-oxidizing prokaryotes. In many pro- and also eukaryotes sulfite is formed as a degradative product from molecules containing sulfur as a heteroatom. In these organisms detoxification of sulfite is generally achieved by direct oxidation to sulfate.

Purification and partial characterization of the hydroxylase component of the methanesulfonic acid mono-oxygenase from methylosulfonomonas methylovora strain M2

The reductase enzyme and the hydroxylase enzyme of the three-component methanesulfonic acid mono-oxygenase (MSAMO) from Methylosulfonomonas methylovora were purified. Purification of the reductase from M. methylovora using a range of chromatographic techniques was accompanied by complete loss of activity. Expression of the reductase as a glutathionine S-transferase fusion protein in Escherichia coli cells was successful as judged from the size of the polypeptide band obtained on induction with isopropyl thio-beta-D-galactoside. Subsequent affinity purification of the fusion protein, however, led to a protein extract containing only glutathionine S-transferase protein, indicating that the fusion protein was unstable in vitro. The hydroxylase component of the MSAMO was purified from M. methylovora to near electrophoretic homogeneity using Q-Sepharose, hydroxyapatite and Mono Q chromatography. SDS/PAGE of the purified hydroxylase showed a single band at approximately 43.7 kDa for the alpha-subunit and a double band at approximately 23 kDa for the beta-subunit. MS scans obtained with matrix-assisted laser desorption/ionization and electrospray ionization showed single peaks for both subunits, with a mass of 48 145.4 Da for alpha, 20 479.1 Da for beta, and 68 624.5 for the alphabeta-monomer. Gel filtration revealed a mass of 209 kDa, suggesting an alpha3beta3 structure for the native enzyme. Purified hydroxylase enzyme exhibited absorbance maxima at 330 nm, 460 nm and 570 nm, indicating the presence of iron-sulfur centres. The protein preparations contained 1 mol sulfide and 3-4 mol iron per mol alphabeta-monomer. Chromium, cobalt, copper, lead, nickel, molybdenum, tungsten and vanadium were not found. Flavins were also absent. Antibodies raised against the native hydroxylase enzyme cross-reacted with cell-free extract from M. methylovora cells grown with methanesulfonate, but not with extract from cells grown with methanol, confirming that MSAMO was specifically induced during growth on methanesulfonate.