Cysteine biosynthesis in plants: isolation and functional identification of a cDNA encoding a serine acetyltransferase from Arabidopsis thaliana

Structure of soybean serine acetyltransferase and formation of the cysteine regulatory complex as a molecular chaperone

Serine acetyltransferase (SAT) catalyzes the limiting reaction in plant and microbial biosynthesis of cysteine. In addition to its enzymatic function, SAT forms a macromolecular complex with O-acetylserine sulfhydrylase. Formation of the cysteine regulatory complex (CRC) is a critical biochemical control feature in plant sulfur metabolism. Here we present the 1.75-3.0 Å resolution x-ray crystal structures of soybean (Glycine max) SAT (GmSAT) in apoenzyme, serine-bound, and CoA-bound forms. The GmSAT-serine and GmSAT-CoA structures provide new details on substrate interactions in the active site. The crystal structures and analysis of site-directed mutants suggest that His(169) and Asp(154) form a catalytic dyad for general base catalysis and that His(189) may stabilize the oxyanion reaction intermediate. Glu(177) helps to position Arg(203) and His(204) and the β1c-β2c loop for serine binding. A similar role for ionic interactions formed by Lys(230) is required for CoA binding. The GmSAT structures also identify Arg(253) as important for the enhanced catalytic efficiency of SAT in the CRC and suggest that movement of the residue may stabilize CoA binding in the macromolecular complex. Differences in the effect of cold on GmSAT activity in the isolated enzyme versus the enzyme in the CRC were also observed. A role for CRC formation as a molecular chaperone to maintain SAT activity in response to an environmental stress is proposed for this multienzyme complex in plants.

Mitochondrial serine acetyltransferase functions as a pacemaker of cysteine synthesis in plant cells

Cysteine (Cys) synthesis in plants is carried out by two sequential reactions catalyzed by the rate-limiting enzyme serine acetyltransferase (SAT) and excess amounts of O-acetylserine(thiol)lyase. Why these reactions occur in plastids, mitochondria, and cytosol of plants remained unclear. Expression of artificial microRNA (amiRNA) against Sat3 encoding mitochondrial SAT3 in transgenic Arabidopsis (Arabidopsis thaliana) plants demonstrates that mitochondria are the most important compartment for the synthesis of O-acetylserine (OAS), the precursor of Cys. Reduction of RNA levels, protein contents, SAT enzymatic activity, and phenotype strongly correlate in independent amiSAT3 lines and cause significantly retarded growth. The expression of the other four Sat genes in the Arabidopsis genome are not affected by amiRNA-SAT3 according to quantitative real-time polymerase chain reaction and microarray analyses. Application of radiolabeled serine to leaf pieces revealed severely reduced incorporation rates into Cys and even more so into glutathione. Accordingly, steady-state levels of OAS are 4-fold reduced. Decrease of sulfate reduction-related genes is accompanied by an accumulation of sulfate in amiSAT3 lines. These results unequivocally show that mitochondria provide the bulk of OAS in the plant cell and are the likely site of flux regulation. Together with recent data, the cytosol appears to be a major site of Cys synthesis, while plastids contribute reduced sulfur as sulfide. Thus, Cys synthesis in plants is significantly different from that in nonphotosynthetic eukaryotes at the cellular level.

Isoform-dependent feedback regulation of serine O-acetyltransferase isoenzymes involved in L-cysteine biosynthesis of Entamoeba histolytica

Serine acetyltransferase (SAT; EC 2.3.1.30) catalyzes the CoA-dependent acetylation of the side chain hydroxyl group of l-serine to form O-acetyl serine, in the first step of the L-cysteine biosynthetic pathway. Since this pathway is selectively present in a few parasitic protists and absent in mammals, it represents a reasonable target to develop new chemotherapeutics. Entamoeba histolytica apparently possesses three SAT isotypes (EhSAT1-3) showing 48-73% mutual identity, a calculated molecular mass of 34.4-37.7 kDa, and an isoelectric point of 5.70-6.63. To better understand the role of individual SAT isotypes, we determined kinetic and inhibitory parameters of recombinant SAT isotypes. While the three SAT isotypes showed comparable Km and k(cat) for L-serine and acetyl-CoA, they showed remarkable differences in their sensitivity to inhibition by L-cysteine. The Ki values for L-cysteine varied by 100-fold (4.7-460 microM) among SAT isotypes (EhSAT1<EhSAT2<EhSAT3). Consequently, these EhSAT isotypes revealed remarkable differences in activity in the presence of physiological L-serine and L-cysteine concentrations. We propose that multiple SAT isotypes with different properties may play complementary roles in the regulation of the cysteine biosynthetic pathway in E. histolytica under different conditions, e.g. during colonization of the intestine and tissue invasion.

Analysis of the Arabidopsis O-acetylserine(thiol)lyase gene family demonstrates compartment-specific differences in the regulation of cysteine synthesis

Cys synthesis in plants takes place in plastids, cytosol, and mitochondria. Why Cys synthesis is required in all compartments with autonomous protein biosynthesis and whether Cys is exchanged between them has remained enigmatic. This question was addressed using Arabidopsis thaliana T-DNA insertion lines deficient in the final step of Cys biosynthesis catalyzed by the enzyme O-acetylserine(thiol)lyase (OAS-TL). Null alleles of oastlA or oastlB alone showed that cytosolic OAS-TL A and plastid OAS-TL B were completely dispensable, although together they contributed 95% of total OAS-TL activity. An oastlAB double mutant, relying solely on mitochondrial OAS-TL C for Cys synthesis, showed 25% growth retardation. Although OAS-TL C alone was sufficient for full development, oastlC plants also showed retarded growth. Targeted affinity purification identified the major OAS-TL-like proteins. Two-dimensional gel electrophoresis and mass spectrometry showed no compensatory changes of OAS-TL isoforms in the four mutants. Steady state concentrations of Cys and glutathione and pulse-chase labeling with [35S]sulfate indicated strong perturbation of primary sulfur metabolism. These data demonstrate that Cys and also sulfide must be sufficiently exchangeable between cytosol and organelles. Despite partial redundancy, the mitochondria and not the plastids play the most important role for Cys synthesis in Arabidopsis.

Dominant-negative modification reveals the regulatory function of the multimeric cysteine synthase protein complex in transgenic tobacco

Cys synthesis in plants constitutes the entry of reduced sulfur from assimilatory sulfate reduction into metabolism. The catalyzing enzymes serine acetyltransferase (SAT) and O-acetylserine (OAS) thiol lyase (OAS-TL) reversibly form the heterooligomeric Cys synthase complex (CSC). Dominant-negative mutation of the CSC showed the crucial function for the regulation of Cys biosynthesis in vivo. An Arabidopsis thaliana SAT was overexpressed in the cytosol of transgenic tobacco (Nicotiana tabacum) plants in either enzymatically active or inactive forms that were both shown to interact efficiently with endogenous tobacco OAS-TL proteins. Active SAT expression resulted in a 40-fold increase in SAT activity and strong increases in the reaction intermediate OAS as well as Cys, glutathione, Met, and total sulfur contents. However, inactive SAT expression produced much greater enhancing effects, including 30-fold increased Cys levels, attributable, apparently, to the competition of inactive transgenic SAT with endogenous tobacco SAT for binding to OAS-TL. Expression levels of tobacco SAT and OAS-TL remained unaffected. Flux control coefficients suggested that the accumulation of OAS and Cys in both types of transgenic plants was accomplished by different mechanisms. These data provide evidence that the CSC and its subcellular compartmentation play a crucial role in the control of Cys biosynthesis, a unique function for a plant metabolic protein complex.

Synthesis of the sulfur amino acids: cysteine and methionine

This review will assess new features reported for the molecular and biochemical aspects of cysteine and methionine biosynthesis in Arabidopsis thaliana with regards to early published data from other taxa including crop plants and bacteria (Escherichia coli as a model). By contrast to bacteria and fungi, plant cells present a complex organization, in which the sulfur network takes place in multiple sites. Particularly, the impact of sulfur amino-acid biosynthesis compartmentalization will be addressed in respect to localization of sulfur reduction. To this end, the review will focus on regulation of sulfate reduction by synthesis of cysteine through the cysteine synthase complex and the synthesis of methionine and its derivatives. Finally, regulatory aspects of sulfur amino-acid biosynthesis will be explored with regards to interlacing processes such as photosynthesis, carbon and nitrogen assimilation.

Characterization and expression analysis of a serine acetyltransferase gene family involved in a key step of the sulfur assimilation pathway in Arabidopsis

Ser acetyltransferase (SATase; EC 2.3.1.30) catalyzes the formation of O-acetyl-Ser from L-Ser and acetyl-CoA, leading to synthesis of Cys. According to its position at the decisive junction of the pathways of sulfur assimilation and amino acid metabolism, SATases are subject to regulatory mechanisms to control the flux of Cys synthesis. In Arabidopsis (Arabidopsis thaliana) there are five genes encoding SATase-like proteins. Two isoforms, Serat3;1 and Serat3;2, were characterized with respect to their enzymatic properties, feedback inhibition by L-Cys, and subcellular localization. Functional identity of Serat3;1 and Serat3;2 was established by complementation of a SATase-deficient mutant of Escherichia coli. Cytosolic localization of Serat3;1 and Serat3;2 was confirmed by using fusion construct with the green fluorescent protein. Recombinant Serat3;1 was not inhibited by L-Cys, while Serat3;2 was a strongly feedback-inhibited isoform. Quantification of expression patterns indicated that Serat2;1 is the dominant form expressed in most tissues examined, followed by Serat1;1 and Serat2;2. Although Serat3;1 and Serat3;2 were expressed weakly in most tissues, Serat3;2 expression was significantly induced under sulfur deficiency and cadmium stress as well as during generative developmental stages, implying that Serat3;1 and Serat3;2 have specific roles when plants are subjected to distinct conditions. Transgenic Arabidopsis plants expressing the green fluorescent protein under the control of the five promoters indicated that, in all Serat genes, the expression was predominantly localized in the vascular system, notably in the phloem. These results demonstrate that Arabidopsis employs a complex array of compartment-specific SATase isoforms with distinct enzymatic properties and expression patterns to ensure the provision of Cys in response to developmental and environmental changes.

Use of biomolecular interaction analysis to elucidate the regulatory mechanism of the cysteine synthase complex from Arabidopsis thaliana

Real time biomolecular interaction analysis based on surface plasmon resonance has been proven useful for studying protein-protein interaction but has not been extended so far to investigate enzyme-enzyme interactions, especially as pertaining to regulation of metabolic activity. We have applied BIAcore technology to study the regulation of enzyme-enzyme interaction during mitochondrial cysteine biosynthesis in Arabidopsis thaliana. The association of the two enzyme subunits in the hetero-oligomeric cysteine synthase complex was investigated with respect to the reaction intermediate and putative effector O-acetylserine. We have determined an equilibrium dissociation constant of the cysteine synthase complex (K(D) = 25 +/- 4 x 10(-9) m), based on a reliable A + B <--> AB model of interaction. Analysis of dissociation kinetics in the presence of O-acetylserine revealed a half-maximal dissociation rate at 77 +/- 4 microm O-acetylserine and strong positive cooperativity for complex dissociation. The equilibrium of interaction was determined using an enzyme activity-based approach and yielded a K(m) value of 58 +/- 7 microm O-acetylserine. Both effector concentrations are in the range of intracellular O-acetylserine fluctuations and support a functional model that integrates effector-driven cysteine synthase complex dissociation as a regulatory switch for the biosynthetic pathway. The results show that BIAcore technology can be applied to obtain quantitative kinetic data of a hetero-oligomeric protein complex with enzymatic and regulatory function.

The cysteine synthase complex from plants. Mitochondrial serine acetyltransferase from Arabidopsis thaliana carries a bifunctional domain for catalysis and protein-protein interaction

Serine acetyltransferase (SAT) catalyzes the rate-limiting step of cysteine biosynthesis in bacteria and plants and functions in association with O-acetylserine (thiol) lyase (OAS-TL) in the cysteine synthase complex. Very little is known about the structure and catalysis of SATs except that they share a characteristic C-terminal hexapeptide-repeat domain with a number of enzymatically unrelated acyltransferases. Computational modeling of this domain was performed for the mitochondrial SAT isoform from Arabidopsis thaliana, based on crystal structures of bacterial acyltransferases. The results indicate a left-handed parallel beta-helix consisting of beta-sheets alternating with turns, resulting in a prism-like structure. This model was challenged by site-directed mutagenesis and tested for a suspected dual function of this domain in catalysis and hetero-oligomerization. The bifunctionality of the SAT C-terminus in transferase activity and interaction with OAS-TL is demonstrated and discussed with respect to the putative role of the cysteine synthase complex in regulation of cysteine biosynthesis.

Genomic and functional characterization of the oas gene family encoding O-acetylserine (thiol) lyases, enzymes catalyzing the final step in cysteine biosynthesis in Arabidopsis thaliana

The final step of cysteine biosynthesis in plants is catalyzed by O-acetylserine (thiol) lyase (OAS-TL), which occurs as several isoforms found in the cytosol, the plastids and the mitochondria. Genomic DNA blot hybridization and isolation of genomic clones indicate single copy genes (oasA1, oasA2, oasB and oasC) that encode the activities of OAS-TL A, B and C found in separate subcellular compartments in the model plant Arabidopsis thaliana. Sequence analysis reveals that the newly discovered oasA2 gene represents a pseudogene that is still transcribed, but is not functionally translated. The comparison of gene structures suggests that oasA1/oasA2 and oasB/oasC are closely related and may be derived from a common ancestor by subsequent duplications. OAS-TL A, B and C were overexpressed in an Escherichia coli mutant lacking cysteine synthesis and exhibited bifunctional OAS-TL and beta-cyanoalanine synthase (CAS) activities. However, all three proteins represent true OAS-TLs according to kinetic analysis and are unlikely to function in cyanide detoxification or secondary metabolism. In addition, it was demonstrated that the mitochondrial OAS-TL C exhibits in vivo protein-protein interaction capabilities with respect to cysteine synthase complex formation similar to cytosolic OAS-TL A and plastid OAS-TL B. Multiple database accessions for each of the A. thaliana OAS-TL isoforms can thus be attributed to a specified number of oas genes to which functionally defined gene products are assigned, and which are responsible for compartment-specific cysteine synthesis.

Determination of the sites required for the allosteric inhibition of serine acetyltransferase by L-cysteine in plants

Serine acetyltransferase (SATase; EC 2.3.1.30) catalyzes the formation of O-acetylserine from L-Ser and acetyl-CoA in plants and bacteria. In plants, two types of SATase have been described. One is allosterically inhibited by L-Cys, and the second is not sensitive to L-Cys inhibition. However, the allosteric site in SATase has not been identified. To understand better the mechanism of L-Cys inhibition of plant SATases, we constructed several chimeric SATase enzymes from watermelon SATase (WaSATase) (sensitive type) and Arabidopsis SAT-p (insensitive type). These enzymes were expressed in Escherichia coli, and inhibition of the mutated SATase activity by L-Cys was analyzed. Mutated WaSATase, in which Met280 was changed to Ile, was no longer inhibited by L-Cys. Analysis of the inhibition the chimeric enzymes indicated that the C-terminal region of WaSATase from Pro276 to Phe285, in which five amino acids are different from those of SAT-p, was responsible for the determination of the sensitivity to L-Cys. In particular, Gly277 in the C-terminal region of WaSATase was primarily responsible for the L-Cys inhibition. The N-terminal half of the protein, which does not contain the catalytic domain, was also important for the sensitivity to L-Cys. These results indicate that the sensitivity of SATase to L-Cys is due to the N-terminal and C-terminal regions rather than to the catalytic domain.

Overproduction of L-cysteine and L-cystine by expression of genes for feedback inhibition-insensitive serine acetyltransferase from Arabidopsis thaliana in Escherichia coli

Two cDNAs encoding feedback inhibition-insensitive serine acetyltransferases of Arabidopsis thaliana were expressed in the chromosomal serine acetyltransferase-deficient and L-cysteine non-utilizing Escherichia coli strain JM39-8. The transformants produced 1600 to 1700 mg l(-1) of L-cysteine and L-cystine from glucose. The amount of these amino acids produced per cell was 30 to 60% higher than that of an E. coli strain carrying mutant serine acetyltransferase less sensitive to feedback inhibition.

Increased resistance to oxidative stress in transgenic tobacco plants overexpressing bacterial serine acetyltransferase

Plant expression cassettes containing the Escherichia coli cysE gene alleles (encoding SAT) were constructed. After the Agrobacterium-mediated transformation of tobacco, we identified stable transformed plants containing several-fold higher SAT activity in comparison to the control plant. Determination of non-protein thiol contents indicated two- to threefold higher cysteine and glutathione levels in some of these transgenic plants. The maximal elevation of the cysteine level was about fourfold while that of GSH was about twofold higher than in the controls. The most striking physiological consequence of the modification of sulfur metabolite levels in the transgenic plants, however, was their several-fold increased resistance to oxidative stress generated by exogenous hydrogen peroxide.

Isoform-dependent differences in feedback regulation and subcellular localization of serine acetyltransferase involved in cysteine biosynthesis from Arabidopsis thaliana

Serine acetyltransferase (SATase; EC 2.3.1.30), which catalyzes the formation of O-acetyl-L-serine (OAS) from acetyl-CoA and L-serine, plays a regulatory role in the biosynthesis of cysteine by its property of feedback inhibition by cysteine in bacteria and certain plants. Three cDNA clones encoding SATase isoforms (SAT-c, SAT-p, and SAT-m) have been isolated from Arabidopsis thaliana. However, the significance of the feedback regulation has not yet been clear in these different isoforms of SATase from A. thaliana. We constructed the overexpression vectors for cDNAs encoding three SATase isoforms of A. thaliana and analyzed the inhibition of SATase activity by cysteine using the recombinant SATase proteins. In the case of SAT-c, the activity was feedback-inhibited by a low concentration of cysteine (the concentration that inhibits 50% activity; IC50 = 1.8 microM). By contrast, SAT-p and SAT-m were feedback inhibition-insensitive isozymes. We also determined the subcellular localization of three SATase isozymes by the transient expression of fusion proteins of each SATase N-terminal region with jellyfish green fluorescent protein (GFP) in 4-week-old Arabidopsis leaves. The SAT-c-GFP fusion protein was stayed in cytosol, whereas SAT-p-GFP and SAT-m-GFP fusion proteins were localized in chloroplasts and in mitochondria, respectively. These results suggest that these three SATase isoforms, which are localized in the different organelles, are subjected to different feedback regulation, presumably so as to play the particular roles for the production of OAS and cysteine in Arabidopsis cells. Regulatory circuit of cysteine biosynthesis in the plant cells is discussed.

Characterization of a chloroplast isoform of serine acetyltransferase from the thermo-acidiphilic red alga Cyanidioschyzon merolae

We isolated a gene for serine acetyltransferase (SAT), a key enzyme in sulfate assimilation, from the primitive red alga Cyanidioschyzon merolae, an inhabitant of sulfurous hot springs, and designated this gene cmSAT. The N-terminal region of the cmSAT protein has characteristics of a chloroplast targeting peptide. cmSAT protein fused with a 6x histidine tag complemented a SAT deficient Escherichia coli mutant. The protein was purified with its SAT activity, which was inhibited by cysteine, using the high affinity of the histidine tag in an Ni-NTA column. The Km values for acetyl-CoA and l-serine were 0.3 and 0.1 mM, respectively. Southern blotting indicated the existence of other SAT isoforms in C. merolae. A 2.4 kb transcript was always detected when growth was synchronized under a 12-h light/dark cycle. Under these conditions, a 31-kDa protein was always detected on immunoblots, indicating processing of the cmSAT protein and constitutive expression of cmSAT. A 45-kDa protein, thought to be the unprocessed cmSAT protein, was detected in the dark period, from M phase to early G1 phase. No significant change in the level of protein expression was detected under continuous darkness or in a sulfate-deficient medium. Using immunoelectron microscopy, the cmSAT protein was primarily detected in the stroma and a few were detected in the cytoplasm, which indicate that cmSAT protein is transported to and functions in a chloroplast.

An embryo-defective mutant of arabidopsis disrupted in the final step of biotin synthesis

Auxotrophic mutants have played an important role in the genetic dissection of biosynthetic pathways in microorganisms. Equivalent mutants have been more difficult to identify in plants. The bio1 auxotroph of Arabidopsis thaliana was shown previously to be defective in the synthesis of the biotin precursor 7, 8-diaminopelargonic acid. A second biotin auxotroph of A. thaliana has now been identified. Arrested embryos from this bio2 mutant are defective in the final step of biotin synthesis, the conversion of dethiobiotin to biotin. This enzymatic reaction, catalyzed by the bioB product (biotin synthase) in Escherichia coli, has been studied extensively in plants and bacteria because it involves the unusual addition of sulfur to form a thiophene ring. Three lines of evidence indicate that bio2 is defective in biotin synthase production: mutant embryos are rescued by biotin but not dethiobiotin, the mutant allele maps to the same chromosomal location as the cloned biotin synthase gene, and gel-blot hybridizations and polymerase chain reaction amplifications revealed that homozygous mutant plants contain a deletion spanning the entire BIO2-coding region. Here we describe how the isolation and characterization of this null allele have provided valuable insights into biotin synthesis, auxotrophy, and gene redundancy in plants.

The biosynthesis and metabolism of the aspartate derived amino acids in higher plants

The essential amino acids lysine, threonine, methionine and isoleucine are synthesised in higher plants via a common pathway starting with aspartate. The regulation of the pathway is discussed in detail, and the properties of the key enzymes described. Recent data obtained from studies of regulation at the gene level and information derived from mutant and transgenic plants are also discussed. The herbicide target enzyme acetohydroxyacid synthase involved in the synthesis of the branched chain amino acids is reviewed.

Regulation of sulfur assimilation in higher plants: a sulfate transporter induced in sulfate-starved roots plays a central role in Arabidopsis thaliana

Proton/sulfate cotransporters in the plasma membranes are responsible for uptake of the environmental sulfate used in the sulfate assimilation pathway in plants. Here we report the cloning and characterization of an Arabidopsis thaliana gene, AST68, a new member of the sulfate transporter gene family in higher plants. Sequence analysis of cDNA and genomic clones of AST68 revealed that the AST68 gene is composed of 10 exons encoding a 677-aa polypeptide (74.1 kDa) that is able to functionally complement a Saccharomyces cerevisiae mutant lacking a sulfate transporter gene. Southern hybridization and restriction fragment length polymorphism mapping confirmed that AST68 is a single-copy gene that maps to the top arm of chromosome 5. Northern hybridization analysis of sulfate-starved plants indicated that the steady-state mRNA abundance of AST68 increased specifically in roots up to 9-fold by sulfate starvation. In situ hybridization experiments revealed that AST68 transcripts were accumulated in the central cylinder of sulfate-starved roots, but not in the xylem, endodermis, cortex, and epidermis. Among all the structural genes for sulfate assimilation, sulfate transporter (AST68), APS reductase (APR1), and serine acetyltransferase (SAT1) were inducible by sulfate starvation in A. thaliana. The sulfate transporter (AST68) exhibited the most intensive and specific response in roots, indicating that AST68 plays a central role in the regulation of sulfate assimilation in plants.

Genomic structure and expression analyses of serine acetyltransferase gene in Citrullus vulgaris (watermelon)

The genomic clones of Sat gene encoding serine acetyltransferase (SATase), a key enzyme in cysteine biosynthesis in plants, were isolated from the genomic library of Citrullus vulgaris (watermelon). The determination of nucleotide sequence of 5.7 kilobase pair (kbp) length revealed the presence of two introns of 1939 basepair (bp) and 515 bp length in the gene. The transcription start point was determined by primer extension experiments. Southern blot analysis indicated the presence of a single copy of the Sat gene and a couple of additional related sequences in the genome of C. vulgaris. The expression of Sat was analyzed in watermelon plants growth under sulfur- and/or nitrogen-starved conditions and in the presence of pyrazole, O-acetylserine and N-acetylserine. Only slight increment (ca. 1.5-2-fold) of Sat gene expression was observed upon sulfur starvation for 48 h. Interestingly, the addition of pyrazole, which is a precursor of beta-pyrazolealanine (beta-PA) synthesized by SATase and cysteine/beta-PA synthase, enhanced the expression of Sat by ca. 2-fold.

Isolation of a gene encoding cysteine synthase from Flavobacterium K3-15

The cysteine synthase gene (cysK) from Flavobacterium K3-15 was cloned and sequenced. The gene exhibits 30-50% identity to known cysteine synthases on both the DNA and the amino acid levels. The pyridoxal phosphate binding site of the enzyme is part of a conserved motif comprising seven amino acids (SIKDRIA). The lys31 residue of the flavobacterial enzyme is conserved in all known cysteine synthases. The cysK gene from Flavobacterium K3-15 was heterologously expressed and the gene product identified by immunoblotting and determination of the enzyme activity.

Cloning and characterization of an Arabidopsis thaliana cDNA clone encoding an organellar isoform of serine acetyltransferase

We have cloned an Arabidopsis thaliana cDNA encoding serine acetyltransferase (EC 2.3.1.30) by functional complementation of the Escherichia coli cysE mutant JM15. The cDNA clone Sat-1 conferred serine acetyltransferase activity (with apparent Km for the two substrates acetyl CoA and L-serine of 0.043 and 3.47 mmol/dm3 respectively) on the cysE mutant. The 1515 bp full-length cDNA encodes a deduced protein of 391 amino acids which includes a putative chloroplastic targeting presequence. Northern analysis revealed a single message of 1.5 kb, while Southern hybridisation suggests a small multigene family of related sequences.