O-acetylserine(thio)lyase (OAS-TL) molecular expression in Pancratium maritimum L. (Amaryllidaceae) under salt stress

Different levels of salt stress affected the OAS-TL expression levels in Pancratium maritimum organs (bulb, leaf and root). A detailed method has been described for the identification of the conserved domain of the OAS-TL cDNA in sea daffodil given the scarce data available for the Amaryllidaceae family. Pancratium maritimum or sea daffodil (Amaryllidaceae) is a bulbous geophyte growing on coastal sands. In this study, we investigated the involvement of cysteine synthesis for salt tolerance through the expression of the enzyme O-acetylserine(thio)lyase (OAS-TL) during the stress response to NaCl treatments in P. maritimum. Quantitative real-time PCR was used in different organs (bulb, leaf and root).

Impact of overexpression of cytosolic isoform of O-acetylserine sulfhydrylase on soybean nodulation and nodule metabolome

Nitrogen-fixing nodules, which are also major sites of sulfur assimilation, contribute significantly to the sulfur needs of whole soybean plants. Nodules are the predominant sites for cysteine accumulation and the activity of O-acetylserine(thiol)lyase (OASS) is central to the sulfur assimilation process in plants. Here, we examined the impact of overexpressing OASS on soybean nodulation and nodule metabolome. Overexpression of OASS did not affect the nodule number, but negatively impacted plant growth. HPLC measurement of antioxidant metabolites demonstrated that levels of cysteine, glutathione, and homoglutathione nearly doubled in OASS overexpressing nodules when compared to control nodules. Metabolite profiling by LC-MS and GC-MS demonstrated that several metabolites related to serine, aspartate, glutamate, and branched-chain amino acid pathways were significantly elevated in OASS overexpressing nodules. Striking differences were also observed in the flavonoid levels between the OASS overexpressing and control soybean nodules. Our results suggest that OASS overexpressing plants compensate for the increase in carbon requirement for sulfur assimilation by reducing the biosynthesis of some amino acids, and by replenishing the TCA cycle through fatty acid hydrolysis. These data may indicate that in OASS overexpressing soybean nodules there is a moderate decease in the supply of energy metabolites to the nodule, which is then compensated by the degradation of cellular components to meet the needs of the nodule energy metabolism.

Genetic, metabolomic and transcriptomic analyses of the de novo L-cysteine biosynthetic pathway in the enteric protozoan parasite Entamoeba histolytica

The de novo L-cysteine biosynthetic pathway is critical for the growth, antioxidative stress defenses, and pathogenesis of bacterial and protozoan pathogens, such as Salmonella typhimurium and Entamoeba histolytica. This pathway involves two key enzymes, serine acetyltransferase (SAT) and cysteine synthase (CS), which are absent in mammals and therefore represent rational drug targets. The human parasite E. histolytica possesses three SAT and CS isozymes; however, the specific roles of individual isoforms and significance of such apparent redundancy remains unclear. In the present study, we generated E. histolytica cell lines in which CS and SAT expression was knocked down by transcriptional gene silencing. The strain in which CS1, 2 and 3 were simultaneously silenced and the SAT3 gene-silenced strain showed impaired growth when cultured in a cysteine lacking BI-S-33 medium, whereas silencing of SAT1 and SAT2 had no effects on growth. Combined transcriptomic and metabolomic analyses revealed that, CS and SAT3 are involved in S-methylcysteine/cysteine synthesis. Furthermore, silencing of the CS1-3 or SAT3 caused upregulation of various iron-sulfur flavoprotein genes. Taken together, these results provide the first direct evidence of the biological importance of SAT3 and CS isoforms in E. histolytica and justify the exploitation of these enzymes as potential drug targets.

Transgenic soybean plants overexpressing O-acetylserine sulfhydrylase accumulate enhanced levels of cysteine and Bowman-Birk protease inhibitor in seeds

Soybeans provide an excellent source of protein in animal feed. Soybean protein quality can be enhanced by increasing the concentration of sulfur-containing amino acids. Previous attempts to increase the concentration of sulfur-containing amino acids through the expression of heterologous proteins have met with limited success. Here, we report a successful strategy to increase the cysteine content of soybean seed through the overexpression of a key sulfur assimilatory enzyme. We have generated several transgenic soybean plants that overexpress a cytosolic isoform of O-acetylserine sulfhydrylase (OASS). These transgenic soybean plants exhibit a four- to tenfold increase in OASS activity when compared with non-transformed wild-type. The OASS activity in the transgenic soybeans was significantly higher at all the stages of seed development. Unlike the non-transformed soybean plants, there was no marked decrease in the OASS activity even at later stages of seed development. Overexpression of cytosolic OASS resulted in a 58-74% increase in protein-bound cysteine levels compared with non-transformed wild-type soybean seeds. A 22-32% increase in the free cysteine levels was also observed in transgenic soybeans overexpressing OASS. Furthermore, these transgenic soybean plants showed a marked increase in the accumulation of Bowman-Birk protease inhibitor, a cysteine-rich protein. The overall increase in soybean total cysteine content (both free and protein-bound) satisfies the recommended levels required for the optimal growth of monogastric animals.

Transgenic soybean plants overexpressing O-acetylserine sulfhydrylase accumulate enhanced levels of cysteine and Bowman-Birk protease inhibitor in seeds

Soybeans provide an excellent source of protein in animal feed. Soybean protein quality can be enhanced by increasing the concentration of sulfur-containing amino acids. Previous attempts to increase the concentration of sulfur-containing amino acids through the expression of heterologous proteins have met with limited success. Here, we report a successful strategy to increase the cysteine content of soybean seed through the overexpression of a key sulfur assimilatory enzyme. We have generated several transgenic soybean plants that overexpress a cytosolic isoform of O-acetylserine sulfhydrylase (OASS). These transgenic soybean plants exhibit a four- to tenfold increase in OASS activity when compared with non-transformed wild-type. The OASS activity in the transgenic soybeans was significantly higher at all the stages of seed development. Unlike the non-transformed soybean plants, there was no marked decrease in the OASS activity even at later stages of seed development. Overexpression of cytosolic OASS resulted in a 58-74% increase in protein-bound cysteine levels compared with non-transformed wild-type soybean seeds. A 22-32% increase in the free cysteine levels was also observed in transgenic soybeans overexpressing OASS. Furthermore, these transgenic soybean plants showed a marked increase in the accumulation of Bowman-Birk protease inhibitor, a cysteine-rich protein. The overall increase in soybean total cysteine content (both free and protein-bound) satisfies the recommended levels required for the optimal growth of monogastric animals.

Overexpression of a soybean O-acetylserine (thiol) lyase-encoding gene GmOASTL4 in tobacco increases cysteine levels and enhances tolerance to cadmium stress

O-Acetylserine (thiol) lyase (OASTL, EC 2.5.1.47) catalyzes the final step in cysteine biosynthesis. The soybean GmOASTL4 gene was overexpressed in tobacco. Transgenic plants showed markedly increased accumulation of transcripts and higher cysteine content compared with the wild-type. Upon exposure to cadmium stress, OASTL activity and cysteine levels increased significantly in transgenic plants. Cadmium accumulation and the activity of both superoxide dismutase and catalase enzymes were enhanced in transformants. These results demonstrate that overexpression of GmOASTL4 in tobacco can enhance cysteine levels and increase tolerance to cadmium stress.

Cloning, overexpression, purification, and characterization of O-acetylserine sulfhydrylase-B from Escherichia coli

O-Acetylserine sulfhydrylase-B (OASS-B, EC 2.5.1.47) is one of the two isozymes produced by Escherichia coli that catalyze the synthesis of L-cysteine from O-acetyl-L-serine and sulfide. The cysM gene encoding OASS-B was cloned and the enzyme was overexpressed in E. coli using pUC19 with a lacUV5 promoter. The enzyme was purified to homogeneity, as evidenced by SDS-PAGE. Approximately 300 mg of purified OASS-B was obtained from 1600 mL of culture broth with a purification yield of 60% or higher. The purified OASS-B was characterized and its properties compared with OASS-A. OASS-B did not form a complex with E. coli serine acetyltransferase (SAT, EC 2.3.1.30) and showed a wide range of substrate specificity in nonproteinaceous amino acid synthesis.

Overproduction of SAT and/or OASTL in transgenic plants: a survey of effects

The last steps of cysteine biosynthesis are catalysed by a bi-enzyme complex composed of serine acetyltransferase (SAT) and cysteine synthase, also called O-acetyl-serine (thiol) lyase (OASTL). SAT is responsible for the production of O-acetyl-serine (OAS) from serine and acetyl-coenzyme A, while OASTL catalyses the formation of cysteine from OAS and hydrogen sulphide. Several distinct nuclear genes for SAT and OASTL enzymes exist in plants. Products of these genes are targeted into at least three cellular compartments: cytosol, chloroplasts, and mitochondria. The SAT and OASTL enzymes are strongly evolutionary conserved, both structurally and functionally. Therefore, isoenzymes from various cellular compartments can be substituted, not only by their plant counterparts from the other cellular compartments but also by their bacterial homologues. During the last decade transgenic plants overproducing SAT, OASTL or both enzymes simultaneously were obtained independently by several research groups. These manipulations led not only to the elevated levels of the respective products, namely OAS and cysteine, but also to increased amounts of glutathione and changes in the levels of other metabolites and enzymatic activities. In several cases, the transgenic plants were also shown to be less susceptible to applied abiotic stresses. In this review, all published and some unpublished results from this laboratory related to heterologous overproduction of SAT and OASTL in transgenic plants are discussed and summarized.

Cysteine synthase overexpression in tobacco confers tolerance to sulfur-containing environmental pollutants

Cysteine (Cys) synthase [O-acetyl-L-Ser(thiol)-lyase, EC 4.2.99.8; CSase] is responsible for the final step in biosynthesis of Cys. Transgenic tobacco (Nicotiana tabacum; F(1)) plants with enhanced CSase activities in the cytosol and in the chloroplasts were generated by cross-fertilization of two transformants expressing cytosolic CSase or chloroplastic CSase. The F(1) transgenic plants were highly tolerant to toxic sulfur dioxide and sulfite. Upon fumigation with 0.1 microL L(-1) sulfur dioxide, the Cys and glutathione contents in leaves of F(1) plants were increased significantly, but not in leaves of non-transformed control plants. Furthermore, the leaves of F(1) plants exhibited the increased resistance to paraquat, a herbicide generating active oxygen species.

Increase in the stability of serine acetyltransferase from Escherichia coli against cold inactivation and proteolysis by forming a bienzyme complex

Cysteine synthetase from Escherichia coli is a bienzyme complex composed of serine acetyltransferase (SAT) and O-acetylserine sulfhydrylase-A (OASS). The effects of the complex formation on the stability of SAT against cold inactivation and proteolysis were investigated. SAT was reversibly inactivated on cooling to 0 degrees C. Ultracentrifugal analysis showed that SAT (a hexamer) was dissociated mostly into two trimers on cooling to 0 degrees C in the absence of OASS, while in the presence of OASS one trimer of the SAT subunits formed a complex with one dimer of OASS subunits. In the presence of OASS, not only the cold inactivation rate was reduced but also the reactivation rate was increased. Furthermore, SAT became stable against proteolytic attack by alpha-chymotrypsin and V8 protease by forming the complex with OASS. On the other hand, SAT was degraded by trypsin in the same manner both in the presence and in the absence of OASS. The different tendency in the stability against proteolysis with the different proteases was discussed with respect to the substrate specificity of the proteases and amino acid sequence of the C-terminal region of SAT that interacts with OASS.

Occurrence of transsulfuration in synthesis of L-homocysteine in an extremely thermophilic bacterium, Thermus thermophilus HB8

A cell extract of an extremely thermophilic bacterium, Thermus thermophilus HB8, cultured in a synthetic medium catalyzed cystathionine gamma-synthesis with O-acetyl-L-homoserine and L-cysteine as substrates but not beta-synthesis with DL-homocysteine and L-serine (or O-acetyl-L-serine). The amounts of synthesized enzymes metabolizing sulfur-containing amino acids were estimated by determining their catalytic activities in cell extracts. The syntheses of cystathionine beta-lyase (EC 4.4.1.8) and O-acetyl-L-serine sulfhydrylase (EC 4.2.99.8) were markedly repressed by L-methionine supplemented to the medium. L-Cysteine and glutathione, both at 0.5 mM, added to the medium as the sole sulfur source repressed the synthesis of O-acetylserine sulfhydrylase by 55 and 73%, respectively, confirming that this enzyme functions as a cysteine synthase. Methionine employed at 1 to 5 mM in the same way derepressed the synthesis of O-acetylserine sulfhydrylase 2.1- to 2.5-fold. A method for assaying a low concentration of sulfide (0.01 to 0.05 mM) liberated from homocysteine by determining cysteine synthesized with it in the presence of excess amounts of O-acetylserine and a purified preparation of the sulfhydrylase was established. The extract of cells catalyzed the homocysteine gamma-lyase reaction, with a specific activity of 5 to 7 nmol/min/mg of protein, but not the methionine gamma-lyase reaction. These results suggested that cysteine was also synthesized under the conditions employed by the catalysis of O-acetylserine sulfhydrylase using sulfur of homocysteine derived from methionine. Methionine inhibited O-acetylserine sulfhydrylase markedly. The effects of sulfur sources added to the medium on the synthesis of O-acetylhomoserine sulfhydrylase and the inhibition of the enzyme activity by methionine were mostly understood by assuming that the organism has two proteins having O-acetylhomoserine sulfhydrylase activity, one of which is cystathionine gamma-synthase. Although it has been reported that homocysteine is directly synthesized in T. thermophilus HB27 by the catalysis of O-acetylhomoserine sulfhydrylase on the basis of genetic studies (T. Kosuge, D. Gao, and T. Hoshino, J. Biosci. Bioeng. 90:271-279, 2000), the results obtained in this study for the behaviors of related enzymes indicate that sulfur is first incorporated into cysteine and then transferred to homocysteine via cystathionine in T. thermophilus HB8.

Subcellular localization of spinach cysteine synthase isoforms and regulation of their gene expression by nitrogen and sulfur

Subcellular localization and regulation of the spinach (Spinacia oleracea) cysteine synthase (O-acetyl-L-serine[thiol]-lyase, EC 4.2.99.8) isoforms (CysA, CysB, and CysC) were determined in transgenic tobacco (Nicotiana tabacum) and in spinach cell cultures. The 5' regions of CysB and CysC encoding the chloroplastic (CysB-TP) and the putative mitochondrial (CysC-TP) transit peptide (TP) sequences were fused to a bacterial beta-glucuronidase gene (gus) and expressed in tobacco under the control of the cauliflower mosaic virus 35S promoter. Subcellular fractionation of transgenic tobacco showed transportation of beta-glucuronidase proteins to chloroplasts by CysB-TP and to mitochondria by CysC-TP, respectively, indicating that both presequences were sufficient to act specifically as chloroplastic and mitochondrial TPs in vivo. The mRNA expression patterns of CysA (cytoplasmic form), CysB, and CysC genes under nitrogen- and sulfur-starved conditions were characterized in spinach cell cultures. In sulfur-starved cells, only slight differences (approximately 1.2- to 1.5-fold) in the mRNA levels of CysA and CysB were observed during the short-term (0-24 h) cultivation periods compared with cells grown in Murashige-Skoog medium. However, under nitrogen and nitrogen/sulfur double-deficient stress conditions, mRNA levels of CysC increased up to 500% of the original level within 72 h.

The mechanism of the auxotrophy for sulfur-containing amino acids imposed upon Escherichia coli by superoxide

Defects in both of the genes coding for the cytosolic superoxide dismutases (SODs) of Escherichia coli impose an oxygen-dependent nutritional requirement for cysteine. This is now seen to be a bradytrophy, rather than an absolute auxotrophy, since lack of Cys merely imposed a growth lag and escape from this growth lag did not involve genetic reversion. This Cys bradytrophy was not seen in the SOD-competent parental strain, and it was relieved by a cell-permeant mimic of SOD activity; hence, it was due to O2-.. It was also relieved by an osmolyte, such as sucrose; hence, it appears due to leakage from the cell of some component needed for Cys biosynthesis. Medium conditioned by the aerobic growth of the SOD-defective strain relieved the growth lag. Bioassays with Cys mutants suggested that the conditioned medium contained SO3-3 or its equivalent, and sulfite per se was able to eliminate the growth lag. However, some component of the conditioned medium reacted with added sulfite and interfered with attempts to assay for it colorimetrically. These results suggest that the cell envelope of the SOD-defective strain was weakened, directly or indirectly, by O2 and then leaked sulfite. This prevents cysteine biosynthesis until sulfite accumulates in the medium.

Role of the Saccharomyces cerevisiae general regulatory factor CP1 in methionine biosynthetic gene transcription

Saccharomyces cerevisiae general regulatory factor CP1 (encoded by the gene CEP1) is required for optimal chromosome segregation and methionine prototrophy. MET16-CYC1-lacZ reporter constructs were used to show that MET16 5'-flanking DNA contains a CP1-dependent upstream activation sequence (UAS). Activity of the UAS required an intact CP1-binding site, and the effects of cis-acting mutations on CP1 binding and UAS activity correlated. In most respects, MET16-CYC1-lacZ reporter gene expression mirrored that of chromosomal MET16; however, the endogenous gene was found to be activated in response to amino acid starvation (general control). The latter mechanism was both GCN4 and CP1 dependent. MET25 was also found to be activated by GCN4, albeit weakly. More importantly, MET25 transcription was strongly CP1 dependent in gcn4 backgrounds. The modulation of MET gene expression by GCN4 can explain discrepancies in the literature regarding CP1 dependence of MET gene transcription. Lastly, micrococcal nuclease digestion and indirect end labeling were used to analyze the chromatin structure of the MET16 locus in wild-type and cep1 cells. The results indicated that CP1 plays no major role in configuring chromatin structure in this region, although localized CP1-specific differences in nuclease sensitivity were detected.

Isolation and characterization of cDNA that encodes a putative mitochondrion-localizing isoform of cysteine synthase (O-acetylserine(thiol)-lyase) from Spinacia oleracea

The cDNA clones that encode a putative mitochondrion-localizing isoform of cysteine synthase (O-acetyl-L-serine(thiol)-lyase, O-acetyl-L-serine acetate-lyase (adding hydrogen sulfide), EC 4.2.99.8), which is denoted as cysteine synthase C, were isolated from spinach (Spinacia oleracea L.). The cDNA encodes a polypeptide of 368 amino acids containing a putative transit peptide of 30-40 amino acids at the N terminus. This leader peptide sequence exhibited several structural features common to other mitochondrion-targeting transit peptides. Homology was also detected between the putative transit peptide sequence of cysteine synthase C and other mitochondrion-targeting leader sequences. A deduced amino acid sequence of cysteine synthase C exhibited a homology of 61% with cytoplasmic isoform A and 63% with chloroplastic isoform B. A bacterial expression vector of the cDNA clone could genetically complement an Escherichia coli auxotroph lacking cysteine synthase loci and could produce the functionally active and immunoreactive cysteine synthase in E. coli. DNA blot hybridization analysis showed the presence of one or two copies of cysC gene in the genome of spinach. RNA blot hybridization analysis indicated that the expression level of cysC gene was lower than those of cysA and cysB and that the mode of cysC expression was constitutive in green and etiolated seedlings of spinach. The molecular evolutionary study of cysteine synthase proteins from plants and bacteria suggested that a common ancestor cysteine synthase gene has evolved into five cysteine synthase gene families, plant isoform A family, plant isoform B family, plant isoform C family, bacterial cysK family, and bacterial cysM family.

Modulation of cysteine biosynthesis in chloroplasts of transgenic tobacco overexpressing cysteine synthase [O-acetylserine(thiol)-lyase]

Cysteine synthase [O-acetyl-L-serine(thiol)-lyase, EC 4.2.99.8] (CSase), which is responsible for the terminal step of cysteine biosynthesis, catalyzes the formation of L-cysteine from O-acetyl-L-serine (OAS) and hydrogen sulfide. Three T-DNA vectors carrying a spinach (Spinacia oleracea) cytoplasmic CSase A cDNA (K. Saito, N. Miura, M. Yamazaki, H. Horano, I. Murakoshi [1992] Proc Natl Acad Sci USA 89: 8078-8082) were constructed as follows: pCSK3F, cDNA driven by the cauliflower mosaic virus (CaMV) 35S RNA promoter with a sense orientation; pCSK3R, cDNA driven by the CaMV 355 promoter with an antisense orientation; pCSK4F, cDNA fused with the sequence for chloroplast-targeting transit peptide of pea ribulose-1,5-biphosphate carboxylase small subunit driven by the CaMV 35S promoter with a sense orientation. These chimeric genes were transferred into tobacco (Nicotiana tabacum) with Agrobacterium-mediated transformation, and self-fertilized progeny were obtained. CSase activities in cell-free extracts of pCSK3F and pCSK4F transformants were 2- to 3-fold higher than those of control and pCSK3R plants. CSase activities in chloroplasts of pCSK4F transformants were severalfold higher than those of control and pCSK3F plants, indicating that the foreign CSase protein is transported and accumulated in a functionally active form in chloroplasts of pCSK4F plants. Isolated chloroplasts of a pCSK4F transformant had a more pronounced ability to form cysteine in response to addition of OAS and sulfur compounds than those of a control plant. In particular, feeding of OAS and sulfite resulted in enhanced cysteine formation, which required photoreduction of sulfite in chloroplasts. The enhanced cysteine formation in a pCSK4F plant responding to sulfite was also observed in leaf discs. In addition, these leaf discs were partially resistant to sulfite toxicity, possibly due to metabolic detoxification of sulfite by fixing into cysteine. These results suggested that overaccumulated foreign CSase in chloroplasts could modulate biosynthetic flow of cysteine in response to sulfur stress.

Isolation and characterization of two cDNAs encoding for compartment specific isoforms of O-acetylserine (thiol) lyase from Arabidopsis thaliana

cDNAs encoding for two isoforms of O-acetylserine (thiol) lyase (OAS-TL), which catalyzes the synthesis of cysteine, have been isolated from Arabidopsis thaliana. Secondary structure together with expression patterns derived during photomorphogenesis indicate cellular localizations in the cytosol and plastids, thus allowing a direct comparison of compartment-specific forms within one species. The cytosolic OAS-TL complemented an E. coli auxotrophic mutant lacking cysteine synthesis. Both isoforms are represented by small gene families. They are expressed under all conditions investigated and were observed to increase in expression in plants grown with limited sulfate supply.

Evidence for identity of beta-pyrazolealanine synthase with cysteine synthase in watermelon: formation of beta-pyrazole-alanine by cloned cysteine synthase in vitro and in vivo

The responsibility of cysteine synthase (EC 4.2.99.8) from watermelon (Citrullus vulgaris) for the formation of beta-(pyrazole-1-yl)-L-alanine, a non-protein amino acid specifically accumulated in Curcubitaceae plants, was confirmed in vitro and in vivo by the cloned cDNA on expression vectors, pCCS11 and pCEN1. The cDNA sequence derived from pCCS11, an expression vector driven by the lacZ promoter, was placed under the transcriptional control of strong T7 promoter of pET3d to yield an over-expression vector, pCEN1, in Escherichia coli. The concentration of the exogenous cysteine synthase protein was increased up to approximately 10% of the total soluble protein of E. coli cells by the expression of cDNA on pCEN1. beta-(Pyrazole-1-yl)-L-alanine was formed in vitro from O-acetyl-L-serine and pyrazole by the action of cysteine synthase expressed in E. coli carrying pCCS11 or pCEN1. To confirm the responsibility of cysteine synthase for the formation of beta-(pyrazole-1-yl)-L-alanine in vivo, the feeding experiments of pyrazole and serine or O-acetyl-L-serine were carried out using the transformed E. coli culture. beta-(Pyrazole-1-yl)-L-alanine was produced in vivo by feeding the substrates to the culture of E. coli carrying pCEN1. These results provide the confirming evidence that the cloned cysteine synthase of watermelon catalyzes the formation of beta-(pyrazole-1-yl)-L-alanine, indicating that beta-pyrazolealanine synthase is identical with cysteine synthase in Cucurbitaceae plants.

Introduction and expression of the bacterial genes cysE and cysK in eukaryotic cells

The coding sequences of the cysE and cysK genes from Escherichia coli, which encode the enzymes of the cysteine biosynthetic pathway, namely, serine acetyltransferase (EC 2.3.1.30) and O-acetylserine sulfhydrylase (or cysteine synthase [EC 4.2.99.8]), were modified for expression in eukaryotic cells and introduced into murine L cells. A number of fusion genes comprising the cysE or cysK coding sequences joined to the promoter of the ovine metallothionein-Ia (MT-Ia) gene and various portions of the ovine growth hormone (GH) gene were prepared. Significant differences in the level of transcription were observed, depending on the amount and arrangement of the GH gene sequences used, the highest levels being obtained with the constructs MTCE10 and MTCK7, which contained only the GH 3' untranslated gene sequences. These two constructs were fused to produce the gene MTCEK1. In this single DNA sequence, each bacterial gene is under independent MT-Ia promoter control. Expression of the cysK sequence in this construct (MT-Ia promoter-cysE-3' GH sequence-MT-Ia promoter-cysK-3' GH sequence) was elevated compared with expression of the cysK gene in MTCK7. However, expression of the cysE sequence in MTCEK1 was only 40% of that of the cysE gene cloned into MTCE10. The double-promoter configuration, which enhances the expression of the second gene in MTCEK1, is proposed as a model for the modification of bacterial genes in general.

O-acetylserine(thiol)lyase from spinach (Spinacia oleracea L.) leaf: cDNA cloning, characterization, and overexpression in Escherichia coli of the chloroplast isoform

The last enzymatic step for L-cysteine biosynthesis is catalyzed by O-acetylserine(thiol)lyase (OASTL, EC 4.2.99.8) which synthesizes L-cysteine from O-acetylserine and "sulfide." We have isolated and characterized a full-length cDNA (1432 bp) from a lambda gt11 library of spinach leaf encoding the complete precursor of the chloroplast isoform. The 1149-nucleotide open reading frame coding for O-acetylserine(thiol)lyase was in the direction opposite that of the lambda gt11 beta-galactosidase gene. The derived amino acid sequence indicates that the protein precursor consists of 383 amino acid residues including a N-terminal presequence peptide of 52 residues. The amino acid sequence of mature spinach chloroplast O-acetylserine(thiol)lyase shows 40 and 57% homology with its bacterial counterparts. Sequence comparison with several pyridoxal 5'-phosphate-containing proteins reveals the presence of a lysine residue assumed to be involved in cofactor binding. A synthetic cDNA was constructed, coding for the entire 331-amino-acid mature O-acetylserine(thiol)lyase and for an initiating methionine. A high level of expression of the active mature chloroplast isoform was achieved in an Escherichia coli strain carrying the T7 RNA polymerase system (F. W. Studier, A. H. Rosenberg, J. J. Dunn, and J. W. Dubendorff, 1990, in Methods in Enzymology, D. V. Goeddel, Ed., Vol. 185, pp. 60-89, Academic Press, San Diego, CA). Addition of pyridoxine to the bacterial growth medium enhanced the enzyme activity due to the recombinant protein. The extent of production is 25-fold higher than in chloroplast from spinach leaves and the recombinant protein presents the relative molecular mass and immunological properties of the natural enzyme from spinach leaf chloroplast. This work, together with our previous biochemical studies, are in accordance with a prokaryotic type enzyme for L-cysteine biosynthesis in higher plant chloroplasts. Southern blot analysis indicated that O-acetylserine(thiol)lyase is encoded by multiple genes in the spinach leaf genomic DNA.

Mutations affecting the sulphur assimilation pathway in Aspergillus nidulans: their effect on sulphur amino acid metabolism

Several sul-reg mutants of Aspergillus nidulans isolated as constitutive for arylsulphatase were studied with respect to the regulation of enzymes involved in cysteine and homocysteine synthesis and to the pool of sulphur amino acids. All mutants examined showed a decreased concentration of glutathione as compared with the wild type, and all mutants, with one exception, had a decreased total pool of sulphur amino acids. The results suggest that the mutants are leaky in the sulphate assimilation pathway. They show derepression of cysteine synthase, homocysteine synthase, cystathionine beta-synthase and gamma-cystathionase. In spite of having derepressed homocysteine synthase, the enzyme which constitutes an alternative pathway for homocysteine synthesis, the sul-reg mutations do not suppress lesions in genes required for the main homocysteine-synthesizing pathway. This indicates that the derepression of homocysteine synthase is not in itself sufficient for physiological functioning of this enzyme, but seems to depend also on the effectiveness of cysteine synthesis and sulphide formation.

Delayed inducibility of sulphite reductase in cysM mutants of Salmonella typhimurium under anaerobic conditions

Salmonella typhimurium cysM mutants grow at a normal rate under aerobic conditions, but only after a lag period under anaerobic conditions. No difference in the induction of two cysteine biosynthetic enzymes--sulphite reductase and O-acetyl-L-serine sulphydrylase--in wild-type, cysK and cysM strains was observed under aerobic conditions. Under anaerobic conditions, however, the cysM strain differed from the others in showing a long delay in the induction of sulphite reductase. These observations are consistent with the assumption that the observed growth delay of the cysM mutant under anaerobic conditions is the result of abnormalities in the regulation of sulphite reductase.

Regulation of s-amino acids biosynthesis in Saccharomycopsis lipolytica

ATP-sulfurylase, cysteine synthase, homocysteine synthase, arylsulfatase and beta-cystathionase in Saccharomycopsis lipolytica are repressed on the addition of methionine, homocysteine or cysteine to the growth medium. The use of appropriate mutants enabled us to demonstrate that the synthesis of these enzymes is regulated by the system involving at least two low-molecular weight effectors--most likely cysteine and methionine (or their close derivatives).