The biosynthesis of sulfoquinovosyldiacylglycerol: studies with groundnut (Arachis hypogaea) leaves

Genome-guided insights into the versatile metabolic capabilities of the mercaptosuccinate-utilizing β-proteobacterium Variovorax paradoxus strain B4

Variovorax paradoxus B4 is able to utilize 2-mercaptosuccinate (MS) as sole carbon, sulfur and energy source. The whole genome of V. paradoxus B4 was sequenced, annotated and evaluated with special focus on genomic elements related to MS metabolism. The genome encodes two chromosomes harbouring 5 795 261 and 1 353 255 bp. A total of 6753 putative protein-coding sequences were identified. Based on the genome and in combination with results from previous studies, a putative pathway for the degradation of MS could be postulated. The putative molybdopterin oxidoreductase identified during transposon mutagenesis probably catalyses the conversion of MS first into sulfinosuccinate and then into sulfosuccinate by successive transfer of oxygen atoms. Subsequently, the cleavage of sulfosuccinate yields oxaloacetate and sulfite, while the latter is oxidized to sulfate. The expression of the putative molybdopterin oxidoreductase was induced by MS, but not by gluconate, as confirmed by reverse transcriptase polymerase chain reaction. Further, in silico studies combined with experiments and comparative genomics revealed high metabolic diversity of strain B4. It bears a high potential as plant growth-promoting bacterium and as candidate for degradation and detoxification of xenobiotics and other hardly degradable substances. Additionally, the strain is of special interest for production of polythioesters with sulfur-containing precursors as MS.

Sulfoacetate is degraded via a novel pathway involving sulfoacetyl-CoA and sulfoacetaldehyde in Cupriavidus necator H16

Bacterial degradation of sulfoacetate, a widespread natural product, proceeds via sulfoacetaldehyde and requires a considerable initial energy input. Whereas the fate of sulfoacetaldehyde in Cupriavidus necator (Ralstonia eutropha) H16 is known, the pathway from sulfoacetate to sulfoacetaldehyde is not. The genome sequence of the organism enabled us to hypothesize that the inducible pathway, which initiates sau (sulfoacetate utilization), involved a four-gene cluster (sauRSTU; H16_A2746 to H16_A2749). The sauR gene, divergently orientated to the other three genes, probably encodes the transcriptional regulator of the presumed sauSTU operon, which is subject to inducible transcription. SauU was tentatively identified as a transporter of the major facilitator superfamily, and SauT was deduced to be a sulfoacetate-CoA ligase. SauT was a labile protein, but it could be separated and shown to generate AMP and an unknown, labile CoA-derivative from sulfoacetate, CoA, and ATP. This unknown compound, analyzed by MALDI-TOF-MS, had a relative molecular mass of 889.7, which identified it as protonated sulfoacetyl-CoA (calculated 889.6). SauS was deduced to be sulfoacetaldehyde dehydrogenase (acylating). The enzyme was purified 175-fold to homogeneity and characterized. Peptide mass fingerprinting confirmed the sauS locus (H16_A2747). SauS converted sulfoacetyl-CoA and NADPH to sulfoacetaldehyde, CoA, and NADP(+), thus confirming the hypothesis.

Sulfoacetate released during the assimilation of taurine-nitrogen by Neptuniibacter caesariensis: purification of sulfoacetaldehyde dehydrogenase

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.

Accumulation of sulfoquinovosyl-1-O-dihydroxyacetone in a sulfolipid-deficient mutant of Rhodobacter sphaeroides inactivated in sqdC

The biosynthesis of the sulfolipid sulfoquinovosyl diacylglycerol in the purple bacterium Rhodobacter sphaeroides requires at least four genes:sqdA, sqdB, sqdC, and sqdD. As part of our strategy aimed at the elucidation of the function of the different sqd gene products, we insertionally inactivated sqdC of R. sphaeroides. The resulting sqdC null mutant showed only a 90% reduction in sulfolipid content. Apparently, the sqdC gene product is required for optimal sulfolipid biosynthesis, but either catalyzes no essential reaction in the pathway or can be functionally replaced to a certain extent by a different protein. The mutant accumulated a 35S-labeled compound that was purified to homogeneity from cell extracts. Matrix-assisted laser desorption mass spectrometry and nuclear magnetic resonance spectroscopy provided conclusive structural evidence to identify the compound as alpha-D-sulfoquinovosyl-1-O-dihydroxyacetone that exists in two interconvertible, keto and hemiacetal forms. Incubation of wild-type protein extracts with the labeled compound did not result in the incorporation into sulfolipid as would be expected for an intermediate of the pathway. Based on our results we propose that the sqdC gene product mediates the substrate specificity of the UDP-sulfoquinovose:diacylglycerol sulfoquinovosyltransferase that is encoded by sqdD and that catalyzes the final reaction of sulfolipid biosynthesis.