Recombinant carboxyltransferase responsive to redox of pea plastidic acetyl-CoA carboxylase

Linum usitatissimum AccD Enhances Seed Fatty Acid Accumulation and Tolerance to Environmental Stresses during Seed Germination in Arabidopsis thaliana

Flax (Linum usitatissimum L.), as an important oil-producing crop, is widely distributed throughout the world, and its seeds are rich in polyunsaturated fatty acids (FAs). Previous studies have revealed that Arabidopsis thaliana ACETYL-CoA CARBOXYLASE (AtACCase) is vital for FA biosynthesis. However, the functions of L. usitatissimum AccD (LuAccD) on FA accumulation and seed germination remain unclear. In the present study, we cloned the LuAccD coding sequence from the flax cultivar 'Longya 10', identified conserved protein domains, and performed a phylogenetic analysis to elucidate its relationship with homologs from a range of plant species. Ectopic expression of LuAccD in A. thaliana wild-type background enhanced seed FA accumulation without altering seed morphological characteristics, including seed size, 1000-seed weight, and seed coat color. Consistently, the expression of key genes involved in FA biosynthesis was greatly up-regulated in the developing seeds of LuAccD overexpression lines. Additionally, we demonstrated that LuAccD acts as a positive regulator of salt and mannitol tolerance during seed germination in A. thaliana. These results provide important insights into the functions of LuAccD, which facilitates the oil quantity and abiotic stress tolerance of oil-producing crops through genetic manipulation.

Complete Chloroplast Genome of Corethrodendron fruticosum (Papilionoideae: Fabaceae): Comparative and Phylogenetic Analysis

Corethrodendron fruticosum is an endemic forage grasses in China with high ecological value. In this study, the complete chloroplast genome of C. fruticosum was sequenced using Illumina paired-end sequencing. The C. fruticosum chloroplast genome was 123,100 bp and comprised 105 genes, including 74 protein-coding genes, 4 rRNA-coding genes, and 27 tRNA-coding genes. The genome had a GC content of 34.53%, with 50 repetitive sequences and 63 simple repeat repetitive sequences that did not contain reverse repeats. The simple repeats included 45 single-nucleotide repeats, which accounted for the highest proportion and primarily comprised A/T repeats. A comparative analysis of C. fruticosum, C. multijugum, and four Hedysarum species revealed that the six genomes were highly conserved, with differentials primarily located in the conserved non-coding regions. Moreover, the accD and clpP genes in the coding regions exhibited high nucleotide variability. Accordingly, these genes may serve as molecular markers for the classification and phylogenetic analysis of Corethrodendron species. Phylogenetic analysis further revealed that C. fruticosum and C. multijugum appeared in different clades than the four Hedysarum species. The newly sequenced chloroplast genome provides further insights into the phylogenetic position of C. fruticosum, which is useful for the classification and identification of Corethrodendron.

Retrograde and anterograde signaling in the crosstalk between chloroplast and nucleus

The chloroplast is a complex cellular organelle that not only performs photosynthesis but also synthesizes amino acids, lipids, and phytohormones. Nuclear and chloroplast genetic activity are closely coordinated through signaling chains from the nucleus to chloroplast, referred to as anterograde signaling, and from chloroplast to the nucleus, named retrograde signaling. The chloroplast can act as an environmental sensor and communicates with other cell compartments during its biogenesis and in response to stress, notably with the nucleus through retrograde signaling to regulate nuclear gene expression in response to developmental cues and stresses that affect photosynthesis and growth. Although several components involved in the generation and transmission of plastid-derived retrograde signals and in the regulation of the responsive nuclear genes have been identified, the plastid retrograde signaling network is still poorly understood. Here, we review the current knowledge on multiple plastid retrograde signaling pathways, and on potential plastid signaling molecules. We also discuss the retrograde signaling-dependent regulation of nuclear gene expression within the frame of a multilayered network of transcription factors.

Detection of significant SNP associated with production and oil quality traits in interspecific oil palm hybrids using RARSeq

A RARSeq based Association mapping study was performed in a population of 104 Elaeis oleifera x E. guineensis hybrids of five origins with the aim of finding functional markers associated to six productive and 19 oil quality traits. For this purpose mRNA of each genotype was isolated and double stranded cDNA was synthesized. Following digestion with two restriction enzymes and adapter ligation, a size selected pool of barcoded amplicons was produced and sequenced using Illumina MiSeq. The obtained sequences were processed with a "snakemake" pipeline, filtered and missing values were imputed. For all traits except two significant effects of the origin was observed. Genetic diversity analyses revealed high variability within origins and an excess of heterozygosity in the population. Two GLM models with Q matrix or PCA matrix as covariates and two MLM models incorporating in addition a Kinship matrix were tested for genotype-phenotype associations using GAPIT software. Using unadjusted p values (< 0.01) 78 potential associations were detected involving 25 SNP and 20 traits. When applying FDR multiple testing with p <  0.05, 25 significant associations remained involving eight SNP and six quality traits. Four SNP were located in genes with a potential relevant biological meaning.

Application of preparative disk gel electrophoresis for antigen purification from inclusion bodies

Specific antibodies are a reliable tool to examine protein expression patterns and to determine the protein localizations within cells. Generally, recombinant proteins are used as antigens for specific antibody production. However, recombinant proteins from mammals and plants are often overexpressed as insoluble inclusion bodies in Escherichia coli. Solubilization of these inclusion bodies is desirable because soluble antigens are more suitable for injection into animals to be immunized. Furthermore, highly purified proteins are also required for specific antibody production. Plastidic acetyl-CoA carboxylase (ACCase: EC 6.4.1.2) from Arabidopsis thaliana, which catalyzes the formation of malonyl-CoA from acetyl-CoA in chloroplasts, formed inclusion bodies when the recombinant protein was overexpressed in E. coli. To obtain the purified protein to use as an antigen, we applied preparative disk gel electrophoresis for protein purification from inclusion bodies. This method is suitable for antigen preparation from inclusion bodies because the purified protein is recovered as a soluble fraction in electrode running buffer containing 0.1% sodium dodecyl sulfate that can be directly injected into immune animals, and it can be used for large-scale antigen preparation (several tens of milligrams).

Targeting of a polytopic membrane protein to the inner envelope membrane of chloroplasts in vivo involves multiple transmembrane segments

The inner envelope membrane (IEM) of the chloroplast plays crucial roles in forming an osmotic barrier and controlling metabolite exchange between the organelle and the cytosol. The IEM therefore harbours a number of membrane proteins and requires the import and integration of these nuclear-encoded proteins for its biogenesis. Recent studies have demonstrated that the transmembrane segment of single-spanning IEM proteins plays key roles in determining their IEM localization. However, few studies have focused on the molecular mechanisms by which polytopic membrane proteins are targeted to the IEM. In this study, we investigated the targeting mechanism of polytopic IEM proteins using the protein Cor413im1 as a model substrate. Cor413im1 does not utilize a soluble intermediate for its targeting to the IEM. Furthermore, we show that the putative fifth transmembrane segment of Cor413im1 is necessary for its targeting to the IEM. The C-terminal portion containing this transmembrane segment is also able to deliver Cor413im1 protein to the IEM. However, the fifth transmembrane segment of Cor413im1 itself is insufficient to target a fusion protein to the IEM. These data suggest that the targeting of polytopic membrane proteins to the chloroplast IEM in vivo involves multiple transmembrane segments and that chloroplasts have evolved a unique mechanism for the integration of polytopic proteins to the IEM.

Genes encoding the biotin carboxylase subunit of acetyl-CoA carboxylase from Brassica napus and parental species: cloning, expression patterns, and evolution

Comparative genomics is a useful tool to investigate gene and genome evolution. Biotin carboxylase (BC), an important subunit of heteromeric acetyl-CoA carboxylase (ACCase) that is a rate-limiting enzyme in fatty acid biosynthesis in dicots, catalyzes ATP, biotin carboxyl carrier protein, and CO2 to form carboxybiotin carboxyl carrier protein. In this study, we cloned four genes encoding BC from Brassica napus L. (namely BnaC.BC.a, BnaC.BC.b, BnaA.BC.a, and BnaA.BC.b), and two were cloned from each of the two parental species Brassica rapa L. (BraA.BC.a and BraA.BC.b) and Brassica oleracea L. (BolC.BC.a and BolC.BC.b). Sequence analyses revealed that in B. napus the genes BnaC.BC.a and BnaC.BC.b were from the C genome of B. oleracea, whereas BnaA.BC.a and BnaA.BC.b were from the A genome of B. rapa. Comparative and cluster analysis indicated that these genes were divided into two major groups, BnaC.BC.a, BnaA.BC.a, BraA.BC.a, and BolC.BC.a in group-1 and BnaC.BC.b, BnaA.BC.b, BraA.BC.b, and BolC.BC.b in group-2. The divergence of group-1 and group-2 genes occurred in their common ancestor 13-17 million years ago (MYA), soon after the divergence of Arabidopsis and Brassica (15-20 MYA). This time of divergence is identical to the previously reported triplicated time of paralogous subgenomes of diploid Brassica species and the divergence date of group-1 and group-2 genes of α-carboxyltransferase, another subunit of heteromeric ACCase, in Brassica. Reverse transcription PCR revealed that the expression level of group-1 and group-2 genes varied in different organs, and the expression patterns of the two groups of genes were similar in different organs, except in flower. However, two paralogs of group-2 BC genes from B. napus could express differently in mature plants tested by generating BnaA.BC.b and BnaC.BC.b promoter-β-glucuronidase (GUS) fusions. The amino acid sequences of proteins encoded by these genes were highly conserved, except the sequence encoding predicted plastid transit peptides. The plastid transit peptides on the BC precursors of Brassica (71-72 amino acid residues) were predicted based on AtBC protein, compared, and confirmed by fusion with green fluorescent protein. Our results will be helpful in elucidating the evolution and the regulation of ACCase in the genus Brassica.

Genes encoding the alpha-carboxyltransferase subunit of acetyl-CoA carboxylase from Brassica napus and parental species: cloning, expression patterns, and evolution

Heteromeric acetyl coenzyme A carboxylase (ACCase), a rate-limiting enzyme in fatty acid biosynthesis in dicots, is a multi-enzyme complex consisting of biotin carboxylase, biotin carboxyl carrier protein, and carboxyltransferase (alpha-CT and beta-CT). In the present study, four genes encoding alpha-CT were cloned from Brassica napus, and two were cloned from each of the two parental species, B. rapa and B. oleracea. Comparative and cluster analyses indicated that these genes were divided into two major groups. The major divergence between group-1 and group-2 occurred in the second intron. Group-2 alpha-CT genes represented the ancestral form in the genus Brassica. The divergence of group-1 and group-2 genes occurred in their common ancestor 12.96-17.78 million years ago (MYA), soon after the divergence of Arabidopsis thaliana and Brassica (15-20 MYA). This time of divergence is identical to that reported for the paralogous subgenomes of diploid Brassica species (13-17 MYA). Real-time reverse transcription PCR revealed that the expression patterns of the two groups of genes were similar in different organs, except in leaves. To better understand the regulation and evolution of alpha-CT genes, promoter regions from two sets of orthologous gene copies from B. napus, B. rapa, and B. oleracea were cloned and compared. The function of the promoter of gene Bnalpha-CT-1-1 in group-1 and gene Bnalpha-CT-2-1 in group-2 was examined by assaying beta-glucuronidase activity in transgenic A. thaliana. Our results will be helpful in elucidating the evolution and regulation of ACCase in oilseed rape.

Coordination of plastid protein import and nuclear gene expression by plastid-to-nucleus retrograde signaling

Expression of nuclear-encoded plastid proteins and import of those proteins into plastids are indispensable for plastid biogenesis. One possible cellular mechanism that coordinates these two essential processes is retrograde signaling from plastids to the nucleus. However, the molecular details of how this signaling occurs remain elusive. Using the plastid protein import2 mutant of Arabidopsis (Arabidopsis thaliana), which lacks the atToc159 protein import receptor, we demonstrate that the expression of photosynthesis-related nuclear genes is tightly coordinated with their import into plastids. Down-regulation of photosynthesis-related nuclear genes is also observed in mutants lacking other components of the plastid protein import apparatus. Genetic studies indicate that the coordination of plastid protein import and nuclear gene expression is independent of proposed plastid signaling pathways such as the accumulation of Mg-protoporphyrin IX and the activity of ABA INSENSITIVE4 (ABI4). Instead, it may involve GUN1 and the transcription factor AtGLK. The expression level of AtGLK1 is tightly correlated with the expression of photosynthesis-related nuclear genes in mutants defective in plastid protein import. Furthermore, the activity of GUN1 appears to down-regulate the expression of AtGLK1 when plastids are dysfunctional. Based on these data, we suggest that defects in plastid protein import generate a signal that represses photosynthesis-related nuclear genes through repression of AtGLK1 expression but not through activation of ABI4.

A comparative genomics approach identifies a PPR-DYW protein that is essential for C-to-U editing of the Arabidopsis chloroplast accD transcript

Several nuclear-encoded proteins containing pentatricopeptide repeat (PPR) motifs have previously been identified to be trans-factors essential for particular chloroplast RNA editing events through analysis of mutants affected in chloroplast biogenesis or function. Other PPR genes are known to encode proteins involved in other aspects of organelle RNA metabolism. A function has not been assigned to most members of the large plant PPR gene family. Arabidopsis and rice each contain over 400 PPR genes, of which about a fifth exhibit a C-terminal DYW domain. We describe here a comparative genomics approach that will facilitate identification of the role of RNA-binding proteins in organelle RNA metabolism. We have implemented this strategy to identify an Arabidopsis nuclear-encoded gene RARE1 that is required for editing of the chloroplast accD transcript. RARE1 carries 15 PPR motifs, an E/E+ and a DYW domain, whereas previously reported editing factors CRR4, CRR21, and CLB19 lack a DYW domain. The accD gene encodes the beta carboxyltransferase subunit of acetyl coA carboxylase, which catalyzes the first step in fatty acid biosynthesis in chloroplasts. Despite a lack of accD C794 editing and lack of restoration of an evolutionarily conserved leucine residue in the beta carboxyltransferase protein, rare1 mutants are unexpectedly robust and reproduce under growth room conditions. Previously the serine-to-leucine alteration caused by editing was deemed essential in the light of the finding that a recombinantly expressed "unedited" form of the pea acetyl coA carboxylase was catalytically inactive.

The ferredoxin/thioredoxin system of oxygenic photosynthesis

Forty years ago, ferredoxin (Fdx) was shown to activate fructose 1,6-bisphosphatase in illuminated chloroplast preparations, thereby laying the foundation for the field now known as "redox biology." Enzyme activation was later shown to require the ubiquitous protein thioredoxin (Trx), reduced photosynthetically by Fdx via an enzyme then unknown-ferredoxin:thioredoxin reductase (FTR). These proteins, Fdx, FTR, and Trx, constitute a regulatory ensemble, the "Fdx/Trx system." The redox biology field has since grown beyond all expectations and now embraces a spectrum of processes throughout biology. Progress has been notable with plants that possess not only the plastid Fdx/Trx system, but also the earlier known NADP/Trx system in the cytosol, endoplasmic reticulum, and mitochondria. Plants contain at least 19 types of Trx (nine in chloroplasts). In this review, we focus on the structure and mechanism of action of members of the photosynthetic Fdx/Trx system and on biochemical processes linked to Trx. We also summarize recent evidence that extends the Fdx/Trx system to amyloplasts-heterotrophic plastids functional in the biosynthesis of starch and other cell components. The review highlights the plant as a model system to uncover principles of redox biology that apply to other organisms.

Arabidopsis Cor15am is a chloroplast stromal protein that has cryoprotective activity and forms oligomers

Many plants acquire increased freezing tolerance when they are exposed to nonfreezing temperatures of a certain duration. This process is known as cold acclimation and allows plants to protect themselves from freezing injury. A wide variety of polypeptides are induced during cold acclimation, among which is one encoded by COR15A in Arabidopsis (Arabidopsis thaliana). Previous studies showed that the COR15A gene encodes a small, plastid-targeted polypeptide that is processed to a mature form called Cor15am. In this study, we examined the biochemical properties and activities of Cor15am in more detail. We provide evidence that Cor15am localizes almost exclusively to the chloroplast stroma. In addition, the cold-regulated accumulation of Cor15am is affected by chloroplast functionality. Both gel-filtration chromatography and protein cross-linking reveal that Cor15am forms oligomers in the stroma of chloroplasts. Although Cor15am accumulates in response to low temperature, cold acclimation is not a prerequisite for oligomerization of Cor15am. Structural analysis suggests that Cor15am is composed of both ordered and random structures, and can stay soluble with small structural change after boiling and freeze-thaw treatments. Recombinant Cor15am exhibits in vitro cryoprotection of a freeze-labile enzyme, l-lactate dehydrogenase. Furthermore, Cor15am is capable of associating with l-lactate dehydrogenase in vitro and with potential stromal substrates in vivo. On the basis of these results, we propose that Arabidopsis Cor15am is a cryoprotective protein that forms oligomers in the chloroplast stroma, and that direct association of Cor15am with its substrates is part of its cryoprotective mechanism.

Engineering cytoplasmic male sterility via the chloroplast genome by expression of {beta}-ketothiolase

While investigating expression of the polydroxybutyrate pathway in transgenic chloroplasts, we addressed the specific role of beta-ketothiolase. Therefore, we expressed the phaA gene via the chloroplast genome. Prior attempts to express the phaA gene in transgenic plants were unsuccessful. We studied the effect of light regulation of the phaA gene using the psbA promoter and 5' untranslated region, and evaluated expression under different photoperiods. Stable transgene integration into the chloroplast genome and homoplasmy were confirmed by Southern analysis. The phaA gene was efficiently transcribed in all tissue types examined, including leaves, flowers, and anthers. Coomassie-stained gel and western blots confirmed hyperexpression of beta-ketothiolase in leaves and anthers, with proportionately high levels of enzyme activity. The transgenic lines were normal except for the male-sterile phenotype, lacking pollen. Scanning electron microscopy revealed a collapsed morphology of the pollen grains. Floral developmental studies revealed that transgenic lines showed an accelerated pattern of anther development, affecting their maturation, and resulted in aberrant tissue patterns. Abnormal thickening of the outer wall, enlarged endothecium, and vacuolation affected pollen grains and resulted in the irregular shape or collapsed phenotype. Reversibility of the male-sterile phenotype was observed under continuous illumination, resulting in viable pollen and copious amount of seeds. This study results in the first engineered cytoplasmic male-sterility system in plants, offers a new tool for transgene containment for both nuclear and organelle genomes, and provides an expedient mechanism for F(1) hybrid seed production.

Thioredoxin affinity chromatography: a useful method for further understanding the thioredoxin network

Thioredoxin affinity chromatography can be used to recognize the target proteins of thioredoxin or thioredoxin-related proteins in whole cells or certain cellular compartments. In the last couple of years, many potential target proteins have been identified from various organelles and organisms by this method. Based on the information on the target proteins provided by these studies, the complete thioredoxin-related redox networks can now be efficiently described.

Plant acetyl-CoA carboxylase: structure, biosynthesis, regulation, and gene manipulation for plant breeding

Acetyl-CoA carboxylase (ACCase) catalyzes the first committed step of fatty acid synthesis, the carboxylation of acetyl-CoA to malonyl-CoA. Two physically distinct types of enzymes are found in nature. Heteromeric ACCase composed of four subunits is usually found in prokaryotes, and homomeric ACCase composed of a single large polypeptide is found in eukaryotes. Most plants have both forms, the heteromeric form in plastids, in which de novo fatty acids are synthesized, and the homomeric form in cytosol. This review focuses on the structure and regulation of plant heteromeric ACCase and its manipulation for plant breeding.

New thioredoxin targets in the unicellular photosynthetic eukaryote Chlamydomonas reinhardtii

Proteomics were used to identify the proteins from the eukaryotic unicellular green alga Chlamydomonas reinhardtii that can be reduced by thioredoxin. These proteins were retained specifically on a thioredoxin affinity column made of a monocysteinic thioredoxin mutant able to form mixed disulfides with its targets. Of a total of 55 identified targets, 29 had been found previously in higher plants or Synechocystis, but 26 were new targets. Biochemical tests were performed on three of them, showing a thioredoxin-dependent activation of isocitrate lyase and isopropylmalate dehydrogenase and a thioredoxin-dependent deactivation of catalase that is redox insensitive in Arabidopsis. In addition, we identified a Ran protein, a previously uncharacterized nuclear target in a photosynthetic organism. The metabolic and evolutionary implications of these findings are discussed.

Expression of cytosolic and plastid acetyl-coenzyme A carboxylase genes in young wheat plants

Expression of cytosolic and plastid acetyl-coenzyme A carboxylase (ACCase) gene families at the mRNA level was analyzed in developing wheat (Triticum aestivum) plants. The major plastid ACCase mRNA level is high in the middle part of the plant and low in roots and leaf blades. An alternative plastid ACCase transcript initiated at a different promoter and using an alternative 5' splice site for the first intron accumulates to its highest level in roots. Cytosolic ACCase mRNA also consists of two species, one of which is present at approximately a constant level, whereas the other accumulates to a high level in the lower sheath section. It is likely that different promoters are also responsible for the two forms of cytosolic ACCase mRNA. The abundances of cytosolic and plastid ACCase mRNAs in the sheath section of the plant are similar. ACCase protein level is significantly lower in the leaf blades, in parallel with changes in the total ACCase mRNA level. Homoeologous ACCase genes show the same expression patterns and similar mRNA levels, suggesting that none of the genes was silenced or acquired new tissue specificity after polyploidization.

Redox signaling in the chloroplast: the ferredoxin/thioredoxin system

Chloroplasts have developed a light-dependent system for the control of the activities of key enzymes involved in assimilatory (photosynthetic) and dissimilatory pathways, which allows a switch between these opposing pathways to prevent futile cycling. This regulatory system, known as the ferredoxin/thioredoxin system, consists of several proteins constituting a redox cascade that transmits the light signal perceived by chlorophyll to selected target proteins, thereby influencing their activity. A central component of the redox cascade is a novel enzyme, the ferredoxin:thioredoxin reductase, which is capable of reducing a disulfide bridge with the help of an iron-sulfur cluster. Recent developments on the elucidation of the structures of several implicated proteins and on the mechanism of signal transfer have greatly improved our understanding of this regulatory mechanism. This review describes the components of the redox cascade, the principal target proteins, and the mechanism of action of the light-signal transfer.

Chloroplast transformation with modified accD operon increases acetyl-CoA carboxylase and causes extension of leaf longevity and increase in seed yield in tobacco

Acetyl-CoA carboxylase (ACCase) in plastids is a key enzyme regulating the rate of de novo fatty acid biosynthesis in plants. Plastidic ACCase is composed of three nuclear-encoded subunits and one plastid-encoded accD subunit. To boost ACCase levels, we examined whether overexpression of accD elevates ACCase production. Using homologous recombination, we replaced the promoter of the accD operon in the tobacco plastid genome with a plastid rRNA-operon (rrn) promoter that directs enhanced expression in photosynthetic and non-photosynthetic organs, and successfully raised the total ACCase levels in plastids. This result suggests that the level of the accD subunit is a determinant of ACCase levels, and that enzyme levels are in part controlled post-transcriptionally at the level of subunit assembly. The resultant transformants grew normally and the fatty acid content was significantly increased in leaves, but not significantly in seeds. However, the transformants displayed extended leaf longevity and a twofold increase of seed yield over the control value, which eventually almost doubled the fatty acid production per plant of the transformants relative to control and wild-type plants. These findings offer a potential method for raising plant productivity and oil production.

Multi-subunit acetyl-CoA carboxylases

Acetyl-CoA carboxylase (ACC) catalyses the first committed step of fatty acid synthesis, the carboxylation of acetyl-CoA to malonyl-CoA. Two physically distinct types of enzymes are found in nature. Bacterial and most plant chloroplasts contain a multi-subunit ACC (MS-ACC) enzyme that is readily dissociated into its component proteins. Mammals, fungi, and plant cytosols contain the second type of ACC, a single large multifunctional polypeptide. This review will focus on the structures, regulation, and enzymatic mechanisms of the bacterial and plant MS-ACCs.

The carboxyltransferase activity of the apicoplast acetyl-CoA carboxylase of Toxoplasma gondii is the target of aryloxyphenoxypropionate inhibitors

Inhibition of growth of the apicomplexan parasite Toxoplasma gondii by aryloxyphenoxypropionate herbicides has been correlated with the inhibition of its acetyl-CoA carboxylase (ACC) by these compounds. Here, full-length and C-terminal fragments of T. gondii apicoplast ACC as well as C-terminal fragments of the cytosolic ACC were expressed in Escherichia coli. The recombinant proteins that were soluble showed the expected enzymatic activities. Yeast gene-replacement strains depending for growth on the expressed T. gondii ACC were derived by complementation of a yeast ACC1 null mutation. In vitro and in vivo tests with aryloxyphenoxypropionates showed that the carboxyltransferase domain of the apicoplast T. gondii ACC is the target for this class of inhibitors. The cytosolic T. gondii ACC is resistant to aryloxyphenoxypropionates. Both T. gondii isozymes are resistant to cyclohexanediones, another class of inhibitors targeting the ACC of grass plastids.

Carbon flux and fatty acid synthesis in plants

The de novo synthesis of fatty acids in plants occurs in the plastids through the activity of fatty acid synthetase. The synthesis of the malonyl-coenzyme A that is required for acyl-chain elongation requires the import of metabolites from the cytosol and their subsequent metabolism. Early studies had implicated acetate as the carbon source for plastidial fatty acid synthesis but more recent experiments have provided data that argue against this. A range of cytosolic metabolites including glucose 6-phosphate, malate, phosphoenolpyruvate and pyruvate support high rates of fatty acid synthesis by isolated plastids, the relative utilisation of which depends upon the plant species and the organ from which the plastids are isolated. The import of these metabolites occurs via specific transporters on the plastid envelope and recent advances in the understanding of the role of these transporters are discussed. Chloroplasts are able to generate the reducing power and ATP required for fatty acid synthesis by capture of light energy in the reactions of photosynthetic electron transport. Regulation of chloroplast fatty acid synthesis is mediated by the response of acetyl-CoA carboxylase to the redox state of the plastid, which ensures that the carbon metabolism is linked to the energy status. The regulation of fatty acid synthesis in plastids of heterotrophic cells is much less well understood and is of particular interest in the tissues that accumulate large amounts of the storage oil, triacylglycerol. In these heterotrophic cells the plastids import ATP and oxidise imported carbon sources to produce the required reducing power. The sequencing of the genome of Arabidopsis thaliana has now enabled a number of aspects of plant fatty acid synthesis to be re-addressed, particularly those areas in which in vitro biochemical analysis had provided equivocal answers. Examples of such aspects and future opportunities for our understanding of plant fatty acid synthesis are presented and discussed.

Thiol-disulfide exchange between nuclear-encoded and chloroplast-encoded subunits of pea acetyl-CoA carboxylase

Fatty acid synthesis in pea chloroplasts is regulated by light/dark. The regulatory enzyme acetyl-CoA carboxylase is modulated by light/dark, presumably under redox regulation. Acetyl-CoA carboxylase is a multienzyme complex composed of biotin carboxylase and carboxyltransferase (CT). To demonstrate the redox regulation of CT, composed of the nuclear-encoded alpha and the chloroplast-encoded beta subunits, we identified the cysteine residues involved in such regulation. We expressed the recombinant CT in Escherichia coli and found that the partly deleted CT was, like the full-length CT, sensitive to a redox state. Site-directed mutagenesis of the deleted CT showed that replacement by alanine of the cysteine residue 267 in the alpha polypeptide or 442 in the beta polypeptide resulted in redox-insensitive CT and broke the intermolecular disulfide bond between the alpha and beta polypeptides. Similar results were confirmed in the full-length CT. These results indicate that the two cysteines in recombinant CT are involved in redox regulation by intermolecular disulfide-dithiol exchange between the alpha and beta subunits. Immunoblots of extract from plants incubated in the light or dark supported that such a disulfide-dithiol exchange is relevant in vivo. A covalent bond between a nuclear-encoded polypeptide and a chloroplast-encoded polypeptide probably regulates the enzyme activity in response to light.

Effect of iron on activity of soybean multi-subunit acetyl-coenzyme A carboxylase

Multi-subunit acetyl-coenzyme A carboxylase (MS-ACCase; EC 6.4.1.2) isolated from soybean chloroplasts is a labile enzyme that loses activity during purification. We found that incubating the chloroplast stromal fraction under anaerobic conditions or in the presence of 5 mM FeSO4 stimulated ACCase (acetyl-CoA-->malonyl-CoA) and carboxyltransferase (malonyl-CoA-->acetyl-CoA) activity. Fe-stimulation of activity was associated with 59Fe binding to a stromal protein fraction. ACCase and carboxyltransferase activities measured in the stromal protein fraction containing bound 59Fe were 2-fold and 6-fold greater, respectively, than the control (stromal fraction not pretreated with FeSO4). Superose 6 gel filtration chromatography indicated 59Fe comigrated with stromal protein of approximately 180 kDa that exhibited carboxyltransferase activity, but lacked ACCase activity. Anion exchange (Mono-Q) chromatography of the Superose 6 fraction yielded a protein peak that was enriched in carboxyltransferase activity and contained protein-bound 59Fe. Denaturing gels of the Mono-Q fraction indicated that the 180-kDa protein was composed of a 56-kDa subunit that was bound by an antibody raised against a synthetic beta-carboxyltransferase (beta-CTase) peptide. Incubation of the Mono-Q carboxyltransferase fraction with increasing concentrations of iron at a fixed substrate concentration resulted in increased initial velocities that fit well to a single rectangular three parameter hyperbola (v=vo+Vmax[FeSO4]/Km+[FeSO4]) consistent with iron functioning as a bound activator of catalysis. UV/Vis spectroscopy of the partially purified fraction before and after iron incubation yielded spectra consistent with a protein-bound metal cluster. These results suggest that the beta-CTase subunit of MS-ACCase in soybean chloroplasts is an iron-containing enzyme, which may in part explain its labile nature.

Chloroplast RNA editing required for functional acetyl-CoA carboxylase in plants

RNA editing is an important post-transcriptional process in chloroplasts and is thought to be functionally significant. Here we show a requirement of RNA editing for a functional enzyme. In peas, acetyl-CoA carboxylase (ACCase), a key enzyme of fatty acid synthesis, is composed of biotin carboxylase with the biotin carboxyl carrier protein and carboxyltransferase (CT). CT is composed of the nuclear-encoded alpha polypeptide and the chloroplast-encoded beta polypeptide in peas. One nucleotide of the beta polypeptide mRNA, which is edited in pea chloroplasts, converts the serine codon to the leucine codon. We show that this RNA editing is required for functional CT by comparing the unedited and edited recombinant enzymes. In plants not having a leucine codon at the same position, editing was shown to take place so as to create the leucine codon, indicating that editing is needed for in vivo CT activity and therefore for ACCase. To our knowledge, ACCase is an essential enzyme, suggesting that the chloroplast RNA editing is necessary for these plants.

Expression and characterization of recombinant human-derived Pneumocystis carinii dihydrofolate reductase

Dihydrofolate reductase (DHFR) is the target of trimethoprim (TMP), which has been widely used in combination with sulfa drugs for treatment and prophylaxis of Pneumocystis carinii pneumonia. While the rat-derived P. carinii DHFR has been well characterized, kinetic studies of human-derived P. carinii DHFR, which differs from rat-derived P. carinii DHFR by 38% in amino acid sequence, have not been reported to date. Here we report on the expression and kinetic characterization of the recombinant human-derived P. carinii DHFR. The 618-bp coding sequence of the human-derived P. carinii DHFR gene was expressed in Escherichia coli. As determined by sodium dodecyl sulfate-polyacrylamide gel eletrophoresis, the purified enzyme had a molecular mass of 25 kDa, consistent with that predicted from the DNA sequence. Kinetic analysis showed that the K(m) values for dihydrofolate and NADPH were 2.7 +/- 0.3 and 14.0 +/- 4.3 microM, respectively, which are similar to those reported for rat-derived P. carinii DHFR. Inhibition studies revealed that both TMP and pyrimethamine were poor inhibitors of human-derived P. carinii DHFR, with K(i) values of 0.28 +/- 0.08 and 0.065 +/- 0.005 microM, respectively, while trimetrexate and methotrexate were potent inhibitors, with K(i) values of 0.23 +/- 0.03 and 0.016 +/- 0.004 nM, respectively. The availability of purified recombinant enzyme in large quantities should facilitate the identification of antifolate inhibitors with greater potency and higher selectivity for human-derived P. carinii DHFR.