Purification and characterization of serine acetyltransferase from Escherichia coli partially truncated at the C-terminal region.
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Incubation of serine acetyltransferase (SAT) from Escherichia coli at 25 degrees C in the absence of protease inhibitors yielded a truncated SAT. The truncated SAT was much less sensitive to feedback inhibition than the wild-type SAT. Analyses of the N- and C-terminal amino acid sequences found that the truncated SAT designated as SAT delta C20 was a resultant form of the wild-type SAT cleaved between Ser 253 and Met 254, deleting 20 amino acid residues from the C-terminus. Based on these findings, we constructed a plasmid containing an altered cysE gene encoding the truncated SAT. SAT delta C20 was produced using the cells of E. coli JM70 transformed with the plasmid and purified to be homogeneous on an SDS-polyacrylamide gel. Properties of the purified SAT delta C20 were investigated in comparison with those of the wild-type SAT and Met-256-Ile mutant SAT, which was isolated by Denk and Bock but not purified (J. Gen. Microbiol., 133, 515-525 (1987)). SAT delta C20 was composed of four identical subunits like the wild-type SAT and Met-256-Ile mutant SAT. Specific activity, optimum pH for reaction, thermal stability, and stability to reagents for SAT delta C20 were similar those for the wild-type SAT and Met-256-Ile mutant SAT. However, SAT delta C20 did not form a complex with O-acetylserine sulfhydrylase-A (OASS-A), a counterpart of the cysteine synthetase and did not reduce OASS activity in contrast to the wild-type SAT and Met-256-Ile mutant SAT. (+info)
Pathways of assimilative sulfur metabolism in Pseudomonas putida.
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Cysteine and methionine biosynthesis was studied in Pseudomonas putida S-313 and Pseudomonas aeruginosa PAO1. Both these organisms used direct sulfhydrylation of O-succinylhomoserine for the synthesis of methionine but also contained substantial levels of O-acetylserine sulfhydrylase (cysteine synthase) activity. The enzymes of the transsulfuration pathway (cystathionine gamma-synthase and cystathionine beta-lyase) were expressed at low levels in both pseudomonads but were strongly upregulated during growth with cysteine as the sole sulfur source. In P. aeruginosa, the reverse transsulfuration pathway between homocysteine and cysteine, with cystathionine as the intermediate, allows P. aeruginosa to grow rapidly with methionine as the sole sulfur source. P. putida S-313 also grew well with methionine as the sulfur source, but no cystathionine gamma-lyase, the key enzyme of the reverse transsulfuration pathway, was found in this species. In the absence of the reverse transsulfuration pathway, P. putida desulfurized methionine by the conversion of methionine to methanethiol, catalyzed by methionine gamma-lyase, which was upregulated under these conditions. A transposon mutant of P. putida that was defective in the alkanesulfonatase locus (ssuD) was unable to grow with either methanesulfonate or methionine as the sulfur source. We therefore propose that in P. putida methionine is converted to methanethiol and then oxidized to methanesulfonate. The sulfonate is then desulfonated by alkanesulfonatase to release sulfite for reassimilation into cysteine. (+info)
Cysteine biosynthesis in Chlamydomonas reinhardtii. Molecular cloning and regulation of O-acetylserine(thiol)lyase.
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A cDNA, Cys1ACr, encoding an isoform of O-acetylserine(thiol) lyase has been isolated from Chlamydomonas reinhardtii, using a PCR-based approach. The inclusion of dimethylsulfoxide in the PCR reaction has been demonstrated to be essential for the correct amplification of C. reinhardtii templates with complex secondary structures caused by a high G + C content. The deduced amino acid sequence exhibited highest similarity with plant O-acetylserine(thiol)lyase isoforms, indicating that the C. reinhardtii enzyme was structurally more similar to higher plant O-acetylserine(thiol)lyase than to the corresponding prokaryotic enzymes. The N-terminal extension present in Cys1ACr showed several characteristics of an organellar transit peptide, with a length typical for C. reinhardtii. Southern blot analysis suggested that the C. reinhardtii genome may contain a single copy of the organellar O-acetylserine(thiol)lyase gene. O-acetylserine(thiol)lyase activity was strongly induced by sulfur-deficient conditions (up to sevenfold the level observed in a sulfur-repleted cell culture) and required the presence of a nitrogen source. Northern blot analysis showed a different pattern of regulation of Cys1ACr to that observed at the activity level. To obtain an increase of transcript abundance a longer period of sulfur limitation was required, reaching a maximum level of approximately threefold Cys1ACr mRNA when compared with the level of a sulfate-grown culture. (+info)
O-Acetylserine sulfhydrylase from Methanosarcina thermophila.
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Cysteine is the major source of fixed sulfur for the synthesis of sulfur-containing compounds in organisms of the Bacteria and Eucarya domains. Though pathways for cysteine biosynthesis have been established for both of these domains, it is unknown how the Archaea fix sulfur or synthesize cysteine. None of the four archaeal genomes sequenced to date contain open reading frames with identities to either O-acetyl-L-serine sulfhydrylase (OASS) or homocysteine synthase, the only sulfur-fixing enzymes known in nature. We report the purification and characterization of OASS from acetate-grown Methanosarcina thermophila, a moderately thermophilic methanoarchaeon. The purified OASS contained pyridoxal 5'-phosphate and catalyzed the formation of L-cysteine and acetate from O-acetyl-L-serine and sulfide. The N-terminal amino acid sequence has high sequence similarity with other known OASS enzymes from the Eucarya and Bacteria domains. The purified OASS had a specific activity of 129 micromol of cysteine/min/mg, with a K(m) of 500 +/- 80 microM for sulfide, and exhibited positive cooperativity and substrate inhibition with O-acetyl-L-serine. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a single band at 36 kDa, and native gel filtration chromatography indicated a molecular mass of 93 kDa, suggesting that the purified OASS is either a homodimer or a homotrimer. The optimum temperature for activity was between 40 and 60 degrees C, consistent with the optimum growth temperature for M. thermophila. The results of this study provide the first evidence for a sulfur-fixing enzyme in the Archaea domain. The results also provide the first biochemical evidence for an enzyme with the potential for involvement in cysteine biosynthesis in the Archaea. (+info)
Cysteine biosynthesis pathway in the archaeon Methanosarcina barkeri encoded by acquired bacterial genes?
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The pathway of cysteine biosynthesis in archaea is still unexplored. Complementation of a cysteine auxotrophic Escherichia coli strain NK3 led to the isolation of the Methanosarcina barkeri cysK gene [encoding O-acetylserine (thiol)-lyase-A], which displays great similarity to bacterial cysK genes. Adjacent to cysK is an open reading frame orthologous to bacterial cysE (serine transacetylase) genes. These two genes could account for cysteine biosynthesis in this archaeon. Analysis of recent genome data revealed the presence of bacteria-like cysM genes [encoding O-acetylserine (thiol)-lyase-B] in Pyrococcus spp., Sulfolobus solfataricus, and Thermoplasma acidophilum. However, no orthologs for these genes can be found in Methanococcus jannaschii, Methanobacterium thermoautotrophicum, and Archaeoglobus fulgidus, implying the existence of unrecognizable genes for the same function or a different cysteine biosynthesis pathway. (+info)
Regulation of sulfate assimilation by nitrogen in Arabidopsis.
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Using Arabidopsis, we analyzed the effect of omission of a nitrogen source and of the addition of different nitrogen-containing compounds on the extractable activity and the enzyme and mRNA accumulation of adenosine 5'-phosphosulfate reductase (APR). During 72 h without a nitrogen source, the APR activity decreased to 70% and 50% of controls in leaves and roots, respectively, while cysteine (Cys) and glutathione contents were not affected. Northern and western analysis revealed that the decrease of APR activity was correlated with decreased mRNA and enzyme levels. The reduced APR activity in roots could be fully restored within 24 h by the addition of 4 mM each of NO(3)(-), NH(4)(+), or glutamine (Gln), or 1 mM O-acetylserine (OAS). (35)SO(4)(2-) feeding showed that after addition of NH(4)(+), Gln, or OAS to nitrogen-starved plants, incorporation of (35)S into proteins significantly increased in roots; however, glutathione and Cys labeling was higher only with Gln and OAS or with OAS alone, respectively. OAS strongly increased mRNA levels of all three APR isoforms in roots and also those of sulfite reductase, Cys synthase, and serine acetyltransferase. Our data demonstrate that sulfate reduction is regulated by nitrogen nutrition at the transcriptional level and that OAS plays a major role in this regulation. (+info)
beta-Cyanoalanine synthase is a mitochondrial cysteine synthase-like protein in spinach and Arabidopsis.
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beta-Cyano-alanine synthase (CAS; EC 4.4.1.9) plays an important role in cyanide metabolism in plants. Although the enzymatic activity of beta-cyano-Ala synthase has been detected in a variety of plants, no cDNA or gene has been identified so far. We hypothesized that the mitochondrial cysteine synthase (CS; EC 4.2.99. 8) isoform, Bsas3, could actually be identical to CAS in spinach (Spinacia oleracea) and Arabidopsis. An Arabidopsis expressed sequence tag database was searched for putative Bsas3 homologs and four new CS-like isoforms, ARAth;Bsas1;1, ARAth;Bsas3;1, ARAth;Bsas4;1, and ARAth;Bsas4;2, were identified in the process. ARAth;Bsas3;1 protein was homologous to the mitochondrial SPIol;Bsas3;1 isoform from spinach, whereas ARAth;Bsas4;1 and ARAth;Bsas4;2 proteins defined a new class within the CS-like proteins family. In contrast to spinach SPIol;Bsas1;1 and SPIol;Bsas2;1 recombinant proteins, spinach SPIol;Bsas3;1 and Arabidopsis ARAth;Bsas3;1 recombinant proteins exhibited preferred substrate specificities for the CAS reaction rather than for the CS reaction, which identified these Bsas3 isoforms as CAS. Immunoblot studies supported this conclusion. This is the first report of the identification of CAS synthase-encoding cDNAs in a living organism. A new nomenclature for CS-like proteins in plants is also proposed. (+info)
Cysteine synthase (O-acetylserine (thiol) lyase) substrate specificities classify the mitochondrial isoform as a cyanoalanine synthase.
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A cyanoalanine synthase and two isoforms (A, cytosolic and B, chloroplastic) of cysteine synthase (O:-acetylserine (thiol) lyase) were isolated from spinach. N-terminal amino acid sequence analysis of the cyanoalanine synthase gave 100% homology for the determined 12 residues with a published sequence for the mitochondrial cysteine synthase isoform. All three enzymes catalysed both the cysteine synthesis and cyanoalanine synthesis reactions, although with different efficiencies. Michaelis-Menten kinetics were observed for all three enzymes when substrate saturation experiments were performed varying O:-acetylserine, chloroalanine and cysteine. Negative co-operative kinetics were observed for cysteine synthases A and B when substrate saturation experiments were performed varying sulphide and cyanide, compared with the Michaelis-Menten kinetics observed for cyanoalanine synthase. The exception was negative co-operativity observed towards sulphide for cyanoalanine synthase with O:-acetylserine as co-substrate. The optimum sulphide concentration was dependent on the alanyl co-substrate used. The amino acid sequence similarity places these three enzymes in the same gene family, and whilst the close kinetic similarities support this, they also indicate distinct roles for the isoforms. (+info)