A Cerulenin Insensitive Short Chain 3-Ketoacyl-Acyl Carrier Protein Synthase in Spinacia oleracea Leaves

Diversity in fatty acid elongation enzymes: The FabB long-chain β-ketoacyl-ACP synthase I initiates fatty acid synthesis in Pseudomonas putida F1

The condensation of acetyl-CoA with malonyl-acyl carrier protein (ACP) by β-ketoacyl-ACP synthase III (KAS III, FabH) and decarboxylation of malonyl-ACP by malonyl-ACP decarboxylase are the two pathways that initiate bacterial fatty acid synthesis (FAS) in Escherichia coli. In addition to these two routes, we report that Pseudomonas putida F1 β-ketoacyl-ACP synthase I (FabB), in addition to playing a key role in fatty acid elongation, also initiates FAS in vivo. We report that although two P. putida F1 fabH genes (PpfabH1 and PpfabH2) both encode functional KAS III enzymes, neither is essential for growth. PpFabH1 is a canonical KAS III similar to E. coli FabH whereas PpFabH2 catalyzes condensation of malonyl-ACP with short- and medium-chain length acyl-CoAs. Since these two KAS III enzymes are not essential for FAS in P. putida F1, we sought the P. putida initiation enzyme and unexpectedly found that it was FabB, the elongation enzyme of the oxygen-independent unsaturated fatty acid pathway. P. putida FabB decarboxylates malonyl-ACP and condenses the acetyl-ACP product with malonyl-ACP for initiation of FAS. These data show that P. putida FabB, unlike the paradigm E. coli FabB, can catalyze the initiation reaction in FAS.

Breeding for sustainable oilseed crop yield and quality in a changing climate

As the effects of climate change continue to alter crop-growing conditions year-to-year on both prime and marginal agricultural landscapes, we must consider the effects not only on yield but also on quality. This is particularly true for oilseed crops. In this review, we explore the importance of oilseeds in general and the specific uses of major oilseed crops including soybean, sunflower, canola, peanut, and cottonseed. We review the physiology of seed oil production, from the perspective of the plant's adaptation to environmental changes. Of particular importance is the role of temperature and water availability on oil synthesis. We then discuss how this influences genetic variation, phenotype variability due to environment, and the interaction of genetics and environment to affect composition and yield of vegetable oils. The ability to predict these effects using genomics and bioinformatics is an important new frontier for breeders to maximize stability of a desired fatty acid composition for their crop over increasingly extreme agricultural environments.

Acyl-Acyl Carrier Protein Desaturases and Plant Biotic Interactions

Interactions between land plants and other organisms such as pathogens, pollinators, or symbionts usually involve a variety of specialized effectors participating in complex cross-talks between organisms. Fatty acids and their lipid derivatives play important roles in these biological interactions. While the transcriptional regulation of genes encoding acyl–acyl carrier protein (ACP) desaturases appears to be largely responsive to biotic stress, the different monounsaturated fatty acids produced by these enzymes were shown to take active part in plant biotic interactions and were assigned with specific functions intrinsically linked to the position of the carbon–carbon double bond within their acyl chain. For example, oleic acid, an omega-9 monounsaturated fatty acid produced by Δ⁹-stearoyl–ACP desaturases, participates in signal transduction pathways affecting plant immunity against pathogen infection. Myristoleic acid, an omega-5 monounsaturated fatty acid produced by Δ⁹-myristoyl–ACP desaturases, serves as a precursor for the biosynthesis of omega-5 anacardic acids that are active biocides against pests. Finally, different types of monounsaturated fatty acids synthesized in the labellum of orchids are used for the production of a variety of alkenes participating in the chemistry of sexual deception, hence favoring plant pollination by hymenopterans.

Highest accumulated microalgal lipids (polar and non-polar) for biodiesel production with advanced wastewater treatment: Role of lipidomics

Microalgal lipids consist of non-polar and polar lipids. Triacyleglyceride (TAG), a non-polar lipid, is convertible to biodiesel, whereas glycolipids and phospholipids are polar and not convertible to biodiesel owing to their high degree of unsaturation (polyunsaturated fatty acids), which makes the production process insufficient and expensive. In this review, microalgal species that contain the highest lipid content (≥40%) in the literature till 2019 are highlighted. The differentiation between non-polar and polar lipids and the limitations in the conversion of polar lipids to biodiesel are reported. The basic and advanced factors contributing to the accumulation of lipids convertible to biodiesel is discussed. Microalgal species including Scenedesmus obliquus, Ourococcus multisporus, Chlamydomonas pitschmannii, Micractinium reisseri, and Botryococcus braunii with high lipid content are potential candidates for biomass/biodiesel production and nutrient removal from wastewater. Application of lipidomics and transcriptomics to manipulate the lipid associated gene acetyl-CoA synthetase in microalgae improves the accumulative lipid content.

β-Ketoacyl-acyl Carrier Protein Synthase I (KASI) Plays Crucial Roles in the Plant Growth and Fatty Acids Synthesis in Tobacco

Fatty acids serve many functions in plants, but the effects of some key genes involved in fatty acids biosynthesis on plants growth and development are not well understood yet. To understand the functions of 3-ketoacyl-acyl-carrier protein synthase I (KASI) in tobacco, we isolated two KASI homologs, which we have designated NtKASI-1 and NtKASI-2. Expression analysis showed that these two KASI genes were transcribed constitutively in all tissues examined. Over-expression of NtKASI-1 in tobacco changed the fatty acid content in leaves, whereas over-expressed lines of NtKASI-2 exhibited distinct phenotypic features such as slightly variegated leaves and reduction of the fatty acid content in leaves, similar to the silencing plants of NtKASI-1 gene. Interestingly, the silencing of NtKASI-2 gene had no discernibly altered phenotypes compared to wild type. The double silencing plants of these two genes enhanced the phenotypic changes during vegetative and reproductive growth compared to wild type. These results uncovered that these two KASI genes had the partially functional redundancy, and that the KASI genes played a key role in regulating fatty acids synthesis and in mediating plant growth and development in tobacco.

Transcriptome Analysis Comparison of Lipid Biosynthesis in the Leaves and Developing Seeds of Brassica napus

Brassica napus seed is a lipid storage organ containing approximately 40% oil, while its leaves contain many kinds of lipids for many biological roles, but the overall amounts are less than in seeds. Thus, lipid biosynthesis in the developing seeds and the leaves is strictly regulated which results the final difference of lipids. However, there are few reports about the molecular mechanism controlling the difference in lipid biosynthesis between developing seeds and leaves. In this study, we tried to uncover this mechanism by analyzing the transcriptome data for lipid biosynthesis. The transcriptome data were de novo assembled and a total of 47,216 unigenes were obtained, which had an N50 length and median of 1271 and 755 bp, respectively. Among these unigenes, 36,368 (about 77.02%) were annotated and there were 109 up-regulated unigenes and 72 down-regulated unigenes in the developing seeds lipid synthetic pathway after comparing with leaves. In the oleic acid pathway, 23 unigenes were up-regulated and four unigenes were down-regulated. During triacylglycerol (TAG) synthesis, the key unigenes were all up-regulated, such as phosphatidate phosphatase and diacylglycerol O-acyltransferase. During palmitic acid, palmitoleic acid, stearic acid, linoleic acid and linolenic acid synthesis in leaves, the unigenes were nearly all up-regulated, which indicated that the biosynthesis of these particular fatty acids were more important in leaves. In the developing seeds, almost all the unigenes in the ABI3VP1, RKD, CPP, E2F-DP, GRF, JUMONJI, MYB-related, PHD and REM transcript factor families were up-regulated, which helped us to discern the regulation mechanism underlying lipid biosynthesis. The differential up/down-regulation of the genes and TFs involved in lipid biosynthesis in developing seeds and leaves provided direct evidence that allowed us to map the network that regulates lipid biosynthesis, and the identification of new TFs that are up-regulated in developing seeds will help us to further elucidate the lipids biosynthesis pathway in developing seeds and leaves.

Transcriptome analysis of yellow horn (Xanthoceras sorbifolia Bunge): a potential oil-rich seed tree for biodiesel in China

Background

Yellow horn (Xanthoceras sorbifolia Bunge) is an oil-rich seed shrub that grows well in cold, barren environments and has great potential for biodiesel production in China. However, the limited genetic data means that little information about the key genes involved in oil biosynthesis is available, which limits further improvement of this species. In this study, we describe sequencing and de novo transcriptome assembly to produce the first comprehensive and integrated genomic resource for yellow horn and identify the pathways and key genes related to oil accumulation. In addition, potential molecular markers were identified and compiled.

Methodology/Principal Findings

Total RNA was isolated from 30 plants from two regions, including buds, leaves, flowers and seeds. Equal quantities of RNA from these tissues were pooled to construct a cDNA library for 454 pyrosequencing. A total of 1,147,624 high-quality reads with total and average lengths of 530.6 Mb and 462 bp, respectively, were generated. These reads were assembled into 51,867 unigenes, corresponding to a total of 36.1 Mb with a mean length, N50 and median of 696, 928 and 570 bp, respectively. Of the unigenes, 17,541 (33.82%) were unmatched in any public protein databases. We identified 281 unigenes that may be involved in de novo fatty acid (FA) and triacylglycerol (TAG) biosynthesis and metabolism. Furthermore, 6,707 SSRs, 16,925 SNPs and 6,201 InDels with high-confidence were also identified in this study.

Conclusions

This transcriptome represents a new functional genomics resource and a foundation for further studies on the metabolic engineering of yellow horn to increase oil content and modify oil composition. The potential molecular markers identified in this study provide a basis for polymorphism analysis of Xanthoceras, and even Sapindaceae; they will also accelerate the process of breeding new varieties with better agronomic characteristics.

Sequence mining and transcript profiling to explore differentially expressed genes associated with lipid biosynthesis during soybean seed development

BACKGROUND

Soybean (Glycine max L.) is one of the most important oil crops in the world. It is desirable to increase oil yields from soybean, and so this has been a major goal of oilseed engineering. However, it is still uncertain how many genes and which genes are involved in lipid biosynthesis.

RESULTS

Here, we evaluated changes in gene expression over the course of seed development using Illumina (formerly Solexa) RNA-sequencing. Tissues at 15 days after flowering (DAF) served as the control, and a total of 11592, 16594, and 16255 differentially expressed unigenes were identified at 35, 55, and 65 DAF, respectively. Gene Ontology analyses detected 113 co-expressed unigenes associated with lipid biosynthesis. Of these, 15 showed significant changes in expression levels (log2fold values ≥ 1) during seed development. Pathway analysis revealed 24 co-expressed transcripts involved in lipid biosynthesis and fatty acid biosynthesis pathways. We selected 12 differentially expressed genes and analyzed their expressions using qRT-PCR. The results were consistent with those obtained from Solexa sequencing.

CONCLUSION

These results provide a comprehensive molecular biology background for research on soybean seed development, particularly with respect to the process of oil accumulation. All of the genes identified in our research have significance for breeding soybeans with increased oil contents.

Co-expression analysis identifies CRC and AP1 the regulator of Arabidopsis fatty acid biosynthesis

Fatty acids (FAs) play crucial rules in signal transduction and plant development, however, the regulation of FA metabolism is still poorly understood. To study the relevant regulatory network, fifty-eight FA biosynthesis genes including de novo synthases, desaturases and elongases were selected as "guide genes" to construct the co-expression network. Calculation of the correlation between all Arabidopsis thaliana (L.) genes with each guide gene by Arabidopsis co-expression dating mining tools (ACT) identifies 797 candidate FA-correlated genes. Gene ontology (GO) analysis of these co-expressed genes showed they are tightly correlated to photosynthesis and carbohydrate metabolism, and function in many processes. Interestingly, 63 transcription factors (TFs) were identified as candidate FA biosynthesis regulators and 8 TF families are enriched. Two TF genes, CRC and AP1, both correlating with 8 FA guide genes, were further characterized. Analyses of the ap1 and crc mutant showed the altered total FA composition of mature seeds. The contents of palmitoleic acid, stearic acid, arachidic acid and eicosadienoic acid are decreased, whereas that of oleic acid is increased in ap1 and crc seeds, which is consistent with the qRT-PCR analysis revealing the suppressed expression of the corresponding guide genes. In addition, yeast one-hybrid analysis and electrophoretic mobility shift assay (EMSA) revealed that CRC can bind to the promoter regions of KCS7 and KCS15, indicating that CRC may directly regulate FA biosynthesis.

De novo transcriptomic analysis of an oleaginous microalga: pathway description and gene discovery for production of next-generation biofuels

Background

Eustigmatos cf. polyphem is a yellow-green unicellular soil microalga belonging to the eustimatophyte with high biomass and considerable production of triacylglycerols (TAGs) for biofuels, which is thus referred to as an oleaginous microalga. The paucity of microalgae genome sequences, however, limits development of gene-based biofuel feedstock optimization studies. Here we describe the sequencing and de novo transcriptome assembly for a non-model microalgae species, E. cf. polyphem, and identify pathways and genes of importance related to biofuel production.

Results

We performed the de novo assembly of E. cf. polyphem transcriptome using Illumina paired-end sequencing technology. In a single run, we produced 29,199,432 sequencing reads corresponding to 2.33 Gb total nucleotides. These reads were assembled into 75,632 unigenes with a mean size of 503 bp and an N50 of 663 bp, ranging from 100 bp to >3,000 bp. Assembled unigenes were subjected to BLAST similarity searches and annotated with Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) orthology identifiers. These analyses identified the majority of carbohydrate, fatty acids, TAG and carotenoids biosynthesis and catabolism pathways in E. cf. polyphem.

Conclusions

Our data provides the construction of metabolic pathways involved in the biosynthesis and catabolism of carbohydrate, fatty acids, TAG and carotenoids in E. cf. polyphem and provides a foundation for the molecular genetics and functional genomics required to direct metabolic engineering efforts that seek to enhance the quantity and character of microalgae-based biofuel feedstock.

Transcriptome sequencing and annotation of the microalgae Dunaliella tertiolecta: pathway description and gene discovery for production of next-generation biofuels

Background

Biodiesel or ethanol derived from lipids or starch produced by microalgae may overcome many of the sustainability challenges previously ascribed to petroleum-based fuels and first generation plant-based biofuels. The paucity of microalgae genome sequences, however, limits gene-based biofuel feedstock optimization studies. Here we describe the sequencing and de novo transcriptome assembly for the non-model microalgae species, Dunaliella tertiolecta, and identify pathways and genes of importance related to biofuel production.

Results

Next generation DNA pyrosequencing technology applied to D. tertiolecta transcripts produced 1,363,336 high quality reads with an average length of 400 bases. Following quality and size trimming, ~ 45% of the high quality reads were assembled into 33,307 isotigs with a 31-fold coverage and 376,482 singletons. Assembled sequences and singletons were subjected to BLAST similarity searches and annotated with Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) orthology (KO) identifiers. These analyses identified the majority of lipid and starch biosynthesis and catabolism pathways in D. tertiolecta.

Conclusions

The construction of metabolic pathways involved in the biosynthesis and catabolism of fatty acids, triacylglycrols, and starch in D. tertiolecta as well as the assembled transcriptome provide a foundation for the molecular genetics and functional genomics required to direct metabolic engineering efforts that seek to enhance the quantity and character of microalgae-based biofuel feedstock.

Arabidopsis β-ketoacyl-[acyl carrier protein] synthase i is crucial for fatty acid synthesis and plays a role in chloroplast division and embryo development

Lipid metabolism plays a pivotal role in cell structure and in multiple plant developmental processes. β-Ketoacyl-[acyl carrier protein] synthase I (KASI) catalyzes the elongation of de novo fatty acid (FA) synthesis. Here, we report the functional characterization of KASI in the regulation of chloroplast division and embryo development. Phenotypic observation of an Arabidopsis thaliana T-DNA insertion mutant, kasI, revealed multiple morphological defects, including chlorotic (in netted patches) and curly leaves, reduced fertility, and semidwarfism. There are only one to five enlarged chloroplasts in the mesophyll cells of chlorotic sectors of young kasI rosette leaves, indicating suppressed chloroplast division under KASI deficiency. KASI deficiency results in a significant change in the polar lipid composition, which causes the suppressed expression of FtsZ and Min system genes, disordered Z-ring placement in the oversized chloroplast, and inhibited polymerization of FtsZ protein at mid-site of the chloroplast in kasI. In addition, KASI deficiency results in disrupted embryo development before the globular stage and dramatically reduces FA levels (~33.6% of the wild type) in seeds. These results demonstrate that de novo FA synthesis is crucial and has pleiotropic effects on plant growth. The polar lipid supply is important for chloroplast division and development, revealing a key function of FA synthesis in plastid development.

Plant lipids: metabolism, mutants, and membranes

The mechanisms that regulate plant lipid metabolism determine the dietary and industrial value of storage oils found in economically important species and may control the ability of many plants to survive exposure to temperature extremes. Many of the problems researchers have in defining the pathways, enzymes, and genes involved in plant lipid metabolism appear to be amenable to analysis by genetic approaches. Mutants with alterations in membrane lipid composition have also been used to study the structural and adaptive roles of lipids. The application of genetic engineering methods affords opportunities for researchers to apply knowledge gained about plant lipid metabolism toward enhanced use of plant oils as abundant and renewable sources of reduced carbon.

The role of beta-ketoacyl-acyl carrier protein synthase III in the condensation steps of fatty acid biosynthesis in sunflower

The beta-ketoacyl-acyl carrier protein synthase III (KAS III; EC 2.3.1.180) is a condensing enzyme catalyzing the initial step of fatty acid biosynthesis using acetyl-CoA as primer. To determine the mechanisms involved in the biosynthesis of fatty acids in sunflower (Helianthus annuus L.) developing seeds, a cDNA coding for HaKAS III (EF514400) was isolated, cloned and sequenced. Its protein sequence is as much as 72% identical to other KAS III-like ones such as those from Perilla frutescens, Jatropha curcas, Ricinus communis or Cuphea hookeriana. Phylogenetic study of the HaKAS III homologous proteins infers its origin from cyanobacterial ancestors. A genomic DNA gel blot analysis revealed that HaKAS III is a single copy gene. Expression levels of this gene, examined by Q-PCR, revealed higher levels in developing seeds storing oil than in leaves, stems, roots or seedling cotyledons. Heterologous expression of HaKAS III in Escherichia coli altered their fatty acid content and composition implying an interaction of HaKAS III with the bacterial FAS complex. Testing purified HaKAS III recombinant protein by adding to a reconstituted E. coli FAS system lacking condensation activity revealed a novel substrate specificity. In contrast to all hitherto characterized plant KAS IIIs, the activities of which are limited to the first cycles of intraplastidial fatty acid biosynthesis yielding C6 chains, HaKAS III participates in at least four cycles resulting in C10 chains.

Microalgal triacylglycerols as feedstocks for biofuel production: perspectives and advances

Microalgae represent an exceptionally diverse but highly specialized group of micro-organisms adapted to various ecological habitats. Many microalgae have the ability to produce substantial amounts (e.g. 20-50% dry cell weight) of triacylglycerols (TAG) as a storage lipid under photo-oxidative stress or other adverse environmental conditions. Fatty acids, the building blocks for TAGs and all other cellular lipids, are synthesized in the chloroplast using a single set of enzymes, of which acetyl CoA carboxylase (ACCase) is key in regulating fatty acid synthesis rates. However, the expression of genes involved in fatty acid synthesis is poorly understood in microalgae. Synthesis and sequestration of TAG into cytosolic lipid bodies appear to be a protective mechanism by which algal cells cope with stress conditions, but little is known about regulation of TAG formation at the molecular and cellular level. While the concept of using microalgae as an alternative and renewable source of lipid-rich biomass feedstock for biofuels has been explored over the past few decades, a scalable, commercially viable system has yet to emerge. Today, the production of algal oil is primarily confined to high-value specialty oils with nutritional value, rather than commodity oils for biofuel. This review provides a brief summary of the current knowledge on oleaginous algae and their fatty acid and TAG biosynthesis, algal model systems and genomic approaches to a better understanding of TAG production, and a historical perspective and path forward for microalgae-based biofuel research and commercialization.

Identification and Molecular Characterization of the β-Ketoacyl-[Acyl Carrier Protein] Synthase Component of the Arabidopsis Mitochondrial Fatty Acid Synthase

Substrate specificity of condensing enzymes is a predominant factor determining the nature of fatty acyl chains synthesized by type II fatty acid synthase (FAS) enzyme complexes composed of discrete enzymes. The gene (mtKAS) encoding the condensing enzyme, beta-ketoacyl-[acyl carrier protein] (ACP) synthase (KAS), constituent of the mitochondrial FAS was cloned from Arabidopsis thaliana, and its product was purified and characterized. The mtKAS cDNA complemented the KAS II defect in the E. coli CY244 strain mutated in both fabB and fabF encoding KAS I and KAS II, respectively, demonstrating its ability to catalyze the condensation reaction in fatty acid synthesis. In vitro assays using extracts of CY244 containing all E. coli FAS components, except that KAS I and II were replaced by mtKAS, gave C(4)-C(18) fatty acids exhibiting a bimodal distribution with peaks at C(8) and C(14)-C(16). Previously observed bimodal distributions obtained using mitochondrial extracts appear attributable to the mtKAS enzyme in the extracts. Although the mtKAS sequence is most similar to that of bacterial KAS IIs, sensitivity of mtKAS to the antibiotic cerulenin resembles that of E. coli KAS I. In the first or priming condensation reaction of de novo fatty acid synthesis, purified His-tagged mtKAS efficiently utilized malonyl-ACP, but not acetyl-CoA as primer substrate. Intracellular targeting using green fluorescent protein, Western blot, and deletion analyses identified an N-terminal signal conveying mtKAS into mitochondria. Thus, mtKAS with its broad chain length specificity accomplishes all condensation steps in mitochondrial fatty acid synthesis, whereas in plastids three KAS enzymes are required.

Different elongation pathways in the biosynthesis of acyl groups of trichome exudate sugar esters from various solanaceous plants

Two common pathways are known for elongation of aliphatic acids via acetate in biological organisms: the fatty acid synthase (FAS) and the alpha-ketoacid elongation (alphaKAE) pathways. The alphaKAE route is utilized in many biosynthetic pathways, including the tricarboxylic acid cycle, leucine biosynthesis, and in formation of coenzyme B, glucosinolates, alpha-ketoadipate, sugar-ester acyl acids, short-chain alcohols of yeast and Clostridium species, 2-amino-4-methylhex-4-enoic acid, and l-gamma-phenyl butyrine. In the FAS route, both carbons from acetyl-acyl carrier protein are retained per elongation cycle, while in the alphaKAE route only one carbon from acetyl-coenzyme A is retained. Available evidence indicates that different members of the family Solanaceae may use one or the other of these elongation mechanisms in the synthesis of acyl groups of trichome-exuded sugar esters. In both, precursors for elongation are derived from branched-chain amino acid metabolism. Here we compared radiolabeling patterns in sugar-ester acyl groups from trichomes (the specific tissue in which sugar esters are synthesized) of the tobaccos, Nicotiana benthamiana, N. gossei, N. glutinosa, of Petunia x hybrida cv. Falcon Red & White, and Datura metel, and epidermal peels of Lycopsersicon pennellii after their synthesis from [2-(14)C]-, [1-(14)C]- and [1,2-(14)C]acetate. Recovered acyl acids were purified and then degraded to determine label distribution between the carboxyl termini and the remainder of the molecules. Six- and 20-h incubations were studied, and membrane fatty acids were monitored as internal controls for FAS-mediated elongation. Results are consistent with participation of alphaKAE in synthesis of sugar-ester acyl groups of tobaccos and petunia, but apparently FAS is utilized in the formation of these groups in L. pennellii and D. metel.

An unusual seed-specific 3-ketoacyl-ACP synthase associated with the biosynthesis of petroselinic acid in coriander

Petroselinic acid (18:1 delta6) is the major component of the seed oil of Umbelliferae species such as coriander (Coriandrum sativum) as well as Araliaceae and Garryaceae species. This unusual fatty acid is synthesized in plastids by the delta4 desaturation of palmitoyl-acyl carrier protein (16:0-ACP) and subsequent elongation of delta4-hexadecenoyl (16:1 delta4)-ACP. To characterize the enzymatic nature of the elongation reaction, an in vitro assay was developed with 16:1 delta4-ACP and 16:0-ACP as substrates. Extracts from developing coriander seeds elongated 16:1 delta4-ACP in a competitive assay at rates ten-fold greater than that with 16:0-ACP. In contrast, extracts from castor seeds, which do not synthesize petroselinic acid, displayed a strong preference for the elongation of 16:0-ACP rather than 16:1 A4-ACP. In addition, the elongation of 16:1 A4-ACP and 16:0-ACP by coriander seed extracts was strongly inhibited by cerulenin at concentrations as low as 10 microM. This finding suggested that the elongation of 16:1 A4-ACP and 16:0-ACP in coriander seed is catalyzed by a 3-ketoacyl-ACP synthase (KAS) 1-type enzyme(s), rather than a KAS II-type activity that is typically associated with 16:0-ACP elongation. Consistent with this, a cDNA for a diverged form of KAS I was isolated from a cDNA library prepared from developing coriander seed. Using a variety of heterologous probing techniques, no KAS II-type cDNAs could be identified in this library. Multiple alignment of KAS amino acid sequences indicated that, although the polypeptide corresponding to the coriander cDNA is more closely related to KAS I. its active site motif deviates from those found in both KAS I and KAS II enzymes. Also suggestive of a possible role in petroselinic acid synthesis, antibodies raised to the recombinant protein recognize an abundant 45 kDa polypeptide in coriander endosperm that is not detected in coriander leaves. These antibodies also recognize a major band of similar size in developing seeds of English ivy (Hedera helix), an Araliaceae species that also accumulates petroselinic acid in a seed-specific manner.

Overexpression of 3-ketoacyl-acyl-carrier protein synthase IIIs in plants reduces the rate of lipid synthesis

A cDNA coding for 3-ketoacyl-acyl-carrier protein (ACP) synthase III (KAS III) from spinach (Spinacia oleracea; So KAS III) was used to isolate two closely related KAS III clones (Ch KAS III-1 and Ch KAS III-2) from Cuphea hookeriana. Both Ch KAS IIIs are expressed constitutively in all tissues examined. An increase in the levels of 16:0 was observed in tobacco (Nicotiana tabacum, WT-SR) leaves overexpressing So KAS III when under the control of the cauliflower mosaic virus-35S promoter and in Arabidopsis and rapeseed (Brassica napus) seeds overexpressing either of the Ch KAS IIIs driven by napin. These data indicate that this enzyme has a universal role in fatty acid biosynthesis, irrespective of the plant species from which it is derived or the tissue in which it is expressed. The transgenic rapeseed seeds also contained lower levels of oil as compared with the wild-type levels. In addition, the rate of lipid synthesis in transgenic rapeseed seeds was notably slower than that of the wild-type seeds. The results of the measurements of the levels of the acyl-ACP intermediates as well as any changes in levels of other fatty acid synthase enzymes suggest that malonyl-ACP, the carbon donor utilized by all the 3- ketoacyl-ACP synthases, is limiting in the transgenic plants. This further suggests that malonyl-coenzyme A is a potential limiting factor impacting the final oil content as well as further extension of 16:0.

The crystal structure of beta-ketoacyl-acyl carrier protein synthase II from Synechocystis sp. at 1.54 A resolution and its relationship to other condensing enzymes

Condensing enzymes, catalyzing the formation of carbon-carbon bonds in several biosynthetic pathways, have lately been recognized as potential drug targets against cancer and tuberculosis, as crucial for combinatorial biosynthesis of antibiotics and related compounds, and as determinants of plant oil composition. beta-Ketoacyl-ACP synthases (KAS) are the condensing enzymes present in the fatty acid biosynthesis pathway and are able to elongate an acyl chain bound to either co-enzyme A (CoA) or acyl carrier protein (ACP) with a two-carbon unit derived from malonyl-ACP. Several isoforms of KAS with different substrate specificity are present in most species. We have determined the crystal structure of KAS II from Synechocystis sp. PCC 6803 to 1.54 A resolution giving a detailed description of the active site geometry. In order to analyze the structure-function relationships in this class of enzymes in more detail, we have compared all presently known three-dimensional structures of condensing enzymes from different pathways. The comparison reveals that these enzymes can be divided into three structural and functional classes. This classification can be related to variations in the catalytic mechanism and the set of residues in the catalytic site, e.g. due to differences in the nature of the second substrate providing the two-carbon elongation unit. The variation in the acyl-carrier (ACP or CoA) specificity might also be connected to this classification and residues involved in ACP binding in structure class 2 can be suggested based on the comparison. Finally, the two subunits in the dimer contribute differently to formation of the substrate binding-pocket in the three structural classes.

Knockout of the regulatory site of 3-ketoacyl-ACP synthase III enhances short- and medium-chain acyl-ACP synthesis

3-ketoacyl-acyl carrier protein synthase (KAS) III catalyses the first condensing step of the fatty acid synthase (FAS) type II reaction in plants and bacteria, using acetyl CoA and malonyl-acyl carrier protein (ACP) as substrates. Enzymatic characterization of recombinant KAS III from Cuphea wrightii embryo shows that this enzyme is strongly inhibited by medium-chain acyl-ACP end products of the FAS reaction, i.e. inhibition by lauroyl-ACP was uncompetitive towards acetyl CoA and non-competitive with regard to malonyl-ACP. This indicated a distinct attachment site for regulatory acyl-ACPs. Based on alignment of primary structures of various KAS IIIs and 3-ketoacyl CoA synthases, we suspected the motif G290NTSAAS296 to be responsible for binding of regulatory acyl-ACPs. Deletion of the tetrapeptide G290NTS293 led to a change of secondary structure and complete loss of KAS III condensing activity. Exchange of asparagine291 to aspartate, alanine294 to serine and alanine295 to proline, however, produced mutant enzymes with slightly reduced condensing activity, yet with insensitivity towards acyl-ACPs. To assess the potential of unregulated KAS III as tool in oil production, we designed in vitro experiments employing FAS preparations from medium-chain fatty acid-producing Cuphea lanceolata seeds and long-chain fatty acid-producing rape seeds, each supplemented with a fivefold excess of the N291D KAS III mutant. High amounts of short-chain acyl-ACPs in the case of C. lanceolata, and of medium-chain acyl-ACPs in the case of rape seed preparations, were obtained. This approach targets regulation and offers new possibilities to derive transgenic or non-transgenic plants for production of seed oils with new qualities.

Novel inhibitors of the condensing enzymes of the type II fatty acid synthase of pea (Pisum sativum)

The type II fatty acid synthases (FASs) of higher plants (and Escherichia coli) contain three condensing enzymes called beta-ketoacyl-ACP synthases (KAS), where ACP is acyl-carrier-protein. We have used novel derivatives of the antibiotic thiolactomycin to inhibit these enzymes. Overall de novo fatty acid biosynthesis was measured using [1-(14)C]acetate substrate and chloroplast preparations from pea leaves, and [1-(14)C]laurate was used to distinguish between the effects of the inhibitors on KAS I from those on KAS II. In addition, the activities of these enzymes, together with the short-chain condensing enzyme, KAS III, were measured directly. Six analogues were tested and two, both with extended hydrocarbon side chains, were found to be more effective inhibitors than thiolactomycin. Incubations with chloroplasts and direct assay of the individual condensing enzymes showed that all three compounds inhibited the pea FAS condensing enzymes in the order KAS II > KAS I > KAS III. These results demonstrate the general activity of thiolactomycin and its derivatives against these FAS condensation reactions, and suggest that such compounds will be useful for further detailed studies of inhibition and for use as pharmaceuticals against Type II FASs of pathogens.

Reaction mechanism of recombinant 3-oxoacyl-(acyl-carrier-protein) synthase III from Cuphea wrightii embryo, a fatty acid synthase type II condensing enzyme

A unique feature of fatty acid synthase (FAS) type II of higher plants and bacteria is 3-oxoacyl-[acyl-carrier-protein (ACP)] synthase III (KAS III), which catalyses the committing condensing reaction. Working with KAS IIIs from Cuphea seeds we obtained kinetic evidence that KAS III catalysis follows a Ping-Pong mechanism and that these enzymes have substrate-binding sites for acetyl-CoA and malonyl-ACP. It was the aim of the present study to identify these binding sites and to elucidate the catalytic mechanism of recombinant Cuphea wrightii KAS III, which we expressed in Escherichia coli. We engineered mutants, which allowed us to dissect the condensing reaction into three stages, i.e. formation of acyl-enzyme, decarboxylation of malonyl-ACP, and final Claisen condensation. Incubation of recombinant enzyme with [1-(14)C]acetyl-CoA-labelled Cys(111), and the replacement of this residue by Ala and Ser resulted in loss of overall condensing activity. The Cys(111)Ser mutant, however, still was able to bind acetyl-CoA and to catalyse subsequent binding and decarboxylation of malonyl-ACP to acetyl-ACP. We replaced His(261) with Ala and Arg and found that the former lost activity, whereas the latter retained overall condensing activity, which indicated a general-base action of His(261). Double mutants Cys(111)Ser/His(261)Ala and Cys(111)Ser/His(261)Arg were not able to catalyse overall condensation, but the double mutant containing Arg induced decarboxylation of [2-(14)C]malonyl-ACP, a reaction indicating the role of His(261) in general-acid catalysis. Finally, alanine scanning revealed the involvement of Arg(150) and Arg(306) in KAS III catalysis. The results offer for the first time a detailed mechanism for a condensing reaction catalysed by a FAS type II condensing enzyme.

Biosynthesis and elongation of short- and medium-chain-length fatty acids

Short- and medium-chain-length fatty acids (FAs) are important constituents of a wide array of natural products. Branched and straight short-chain-length FAs originate from branched chain amino acid metabolism, and serve as primers for elongation in FA synthase-like reactions. However, a recent model proposes that the one-carbon extension reactions that utilize 2-oxo-3-methylbutyric acid in leucine biosynthesis also catalyze a repetitive one-carbon elongation of short-chain primers to medium-chain-length FAs. The existence of such a mechanism would require a novel form of regulation to control carbon flux between amino acid and FA biosynthesis. A critical re-analysis of the data used to support this pathway fails to support the hypothesis for FA elongation by one-carbon extension cycles of alpha-ketoacids. Therefore, we tested the hypothesis experimentally using criteria that distinguish between one- and two-carbon elongation mechanisms: (a) isotopomer patterns in terminal carbon atom pairs of branched and straight FAs resulting from differential labeling with [(13)C]ăcetate; (b)(13)C]threonine labeling patterns in odd- and even chain length FAs; and (c) differential sensitivity of elongation reactions to inhibition by cerulenin. All three criteria indicated that biosynthesis of medium-chain length FAs is mediated primarily by FA synthase-like reactions.

Structure of the complex between the antibiotic cerulenin and its target, beta-ketoacyl-acyl carrier protein synthase

In the biosynthesis of fatty acids, the beta-ketoacyl-acyl carrier protein (ACP) synthases catalyze chain elongation by the addition of two-carbon units derived from malonyl-ACP to an acyl group bound to either ACP or CoA. The enzyme is a possible drug target for treatment of certain cancers and for tuberculosis. The crystal structure of the complex of the enzyme from Escherichia coli, and the fungal mycotoxin cerulenin reveals that the inhibitor is bound in a hydrophobic pocket formed at the dimer interface. Cerulenin is covalently attached to the active site cysteine through its C2 carbon atom. The fit of the inhibitor to the active site is not optimal, and there is thus room for improvement through structure based design.

KAS IV: a 3-ketoacyl-ACP synthase from Cuphea sp. is a medium chain specific condensing enzyme

cDNA clones encoding a novel 3-ketoacyl-ACP synthase (KAS) have been isolated from Cuphea. The amino acid sequence of this enzyme is different from the previously characterized classes of KASs, designated KAS I and III, and similar to those designated as KAS II. To define the acyl chain specificity of this enzyme, we generated transgenic Brassica plants over-expressing the cDNA encoded protein in a seed specific manner. Expression of this enzyme in transgenic Brassica seeds which normally do not produce medium chain fatty acids does not result in any detectable modification of the fatty acid profile. However, co-expression of the Cuphea KAS with medium chain specific thioesterases, capable of production of either 12:0 or 8:0/10:0 fatty acids in seed oil, strongly enhances the levels of these medium chain fatty acids as compared with seed oil of plants expressing the thioesterases alone. By contrast, co-expression of the Cuphea KAS along with an 18:0/18.1-ACP thioesterase does not result in any detectable modification of the fatty acids. These data indicate that the Cuphea KAS reported here has a different acyl-chain specificity to the previously characterized KAS I, II and III. Therefore, we designate this enzyme KAS IV, a medium chain specific condensing enzyme.

Inactivation of the polyketide synthase, 6-methylsalicylic acid synthase, by the specific modification of Cys-204 of the beta-ketoacyl synthase by the fungal mycotoxin cerulenin

Cerulenin, [(2S,3R)-2,3-epoxy-4-oxo-7,10-dodecadienoylamide], a mycotoxin produced by Cephalosporium caerulens, irreversibly inactivated 6-methylsalicylic acid synthase from Penicillium patulum. A combination of radiolabelling studies with [3H]cerulenin, proteolytic and chemical digestion and N-terminal sequencing of labelled peptides indicated that the site of cerulenin modification is the highly reactive substrate-binding Cys-204 of the beta-ketoacyl synthase enzyme component. The thiol-specific inhibitor, iodoacetamide, was also shown to alkylate this residue. These findings are analogous with those observed for the reaction of cerulenin and iodoacetamide with type-I fatty acid synthases, demonstrating the close similarity between 6-methylsalicylic acid synthase and type-I fatty acid synthases.

Heterologously expressed acyl carrier protein domain of rat fatty acid synthase functions in Escherichia coli fatty acid synthase and Streptomyces coelicolor polyketide synthase systems

INTRODUCTION

Fatty acid synthases (FASs) catalyze the de novo biosynthesis of long-chain saturated fatty acids by a process common to eubacteria and eukaryotes, using either a set of monofunctional proteins (Type II FAS) or a polypeptide containing several catalytic functions (Type I FAS). To compare the features of a Type I domain with its Type II counterpart we expressed and characterized an acyl carrier protein (ACP) domain of the Type I rat FAS.

RESULTS

An ACP domain of rat FAS was defined that allows expression of a small percentage of active holo-ACP both in Escherichia coli, increasing fivefold upon co-expression with an E. coli holo-ACP synthase, and in Streptomyces coelicolor. The rat ACP domain functions with some components of the E. coli FAS, and can replace the actinorhodin polyketide synthase (PKS) ACP in S. coelicolorA3(2). Purification of the rat ACP domain from E. coli resulted in loss of its functionality. Purified apo-ACP could be converted to its holo-form upon incubation with purified E. coli holo-ACP synthase in vitro, however, suggesting that the loss of functionality was not due to a conformational change.

CONCLUSIONS

Functionality of the recombinant rat ACP was shown in distantly related and diverse enzyme systems, suggesting that Type I and Type II ACPs have a similar conformation. A procedure was described that might permit the production of rat FAS holo-ACP for structural and further biochemical characterization.

Condensing enzymes from Cuphea wrightii associated with medium chain fatty acid biosynthesis

Seed oils of most Cuphea species contain > 90% medium chain (C8-C14) fatty acids. Thioesterases with specificity for these substrates are important determinants of the medium chain phenotype. The role of condensing enzymes, however, has not been investigated. cDNA clones encoding beta-ketoacyl-acyl carrier protein (ACP) synthase (KAS) were isolated from C. wrightii, a C10/C12-producing species. Deduced amino acid sequences of four unique clones were approximately 60% identical to plant KAS I sequences and approximately 75% identical to a distinct class of KAS sequences recently identified in castor and barley. A 46 kDa protein that was observed only in developing and mature seed was detected using antiserum directed against recombinant Cuphea KAS protein. The 46 kDa protein was abundant in developing seeds of six medium chain-producing Cuphea species but barely detected in one long chain-producing species. A 48 kDa protein identified immunologically as KAS I was expressed in both medium and long chain-producing Cuphea species and was detected in all tissues tested. In in vitro assays, extracts from C. wrightii and C. viscosissima developing embryos were unable to extend fatty acid chains beyond C10 following treatment with 10 microns cerulenin, a potent inhibitor of KAS I. However, a C. viscosissima mutant, cpr-1, whose seed oils are deficient in caprate relative to wild type, was impaired in extension of C8 to C10 in this assay and Western analysis revealed a specific deficiency in 46 kDa KAS in cpr-1 embryos. These results implicate cerulenin-resistant condensing activity in production of medium chain fatty acids in Cuphea.

Decarboxylation of malonyl-(acyl carrier protein) by 3-oxoacyl-(acyl carrier protein) synthases in plant fatty acid biosynthesis

In order to identify regulatory steps in fatty acid biosynthesis, the influence of intermediate 3-oxoacyl-(acyl carrier proteins) (3-oxoacyl-ACPs) and end-product acyl-ACPs of the fatty acid synthase reaction on the condensation reaction was investigated in vitro, using total fatty acid synthase preparations and purified 3-oxoacyl-ACP synthases (KASs; EC 2.3.1.41) from Cuphea lanceolata seeds. KAS I and II in the fatty acid synthase preparations were assayed for the elongation of octanoyl- and hexadecanoyl-ACP respectively, and the accumulation of the corresponding condensation product 3-oxoacyl-ACP was studied by modulating the content of the reducing equivalentS NADH and NADPH. Complete omission of reducing equivalents resulted with either KAS in the abnormal synthesis of acetyl-ACP from malonyl-ACP by a decarboxylation reaction. Supplementation with NADPH or NADH, separately or in combination with recombinant 3-oxoacyl-ACP reductase (EC 1.1.1.100), led to a decrease in the amount of acetyl-ACP and a simultaneous increase in elongation products. This demonstrates that the accumulation of 3-oxoacyl-ACP inhibits the condensation reaction on the one hand, and induces the decarboxylation of malonyl-ACP on the other. By carrying out similar experiments with purified enzymes, this decarboxylation was attributed to the action of KAS. Our data point to a regulatory mechanism for the degradation of malonyl-ACP in plants which is activated by the accumulation of the fatty acid synthase intermediate 3-oxoacyl-ACP.

Molecular cloning of a cDNA encoding 3-ketoacyl-acyl carrier protein synthase III from leek

3-Ketoacyl-acyl carrier protein synthase III (KAS III) catalyzes the initial condensation of malonyl-acyl carrier protein (ACP) with acetyl-CoA in plant and bacterial fatty acid biosynthesis. The first cDNA clone encoding KAS III from a monocot is reported here. A cDNA clone was isolated from a leek epidermal cDNA library by screening with spinach and Arabidopsis heterologous probes from KAS III cDNA clones. When expressed in Escherichia coli, the cloned enzyme was able to catalyze the expected condensation reaction, was insensitive to cerulenin (100 microM) and cross-reacted with spinach KAS III antibody. The 1476-bp cDNA clone contained a 1206-bp open reading frame which encoded a 402-amino acid polypeptide. The deduced amino acid sequence showed significant similarities to other KAS IIIs although the leek sequence had some notable differences in regions otherwise completely conserved in dicots. Northern blot analyses indicated that KAS III transcript levels were similar in leaf epidermis and parenchyma, although developmental changes in transcript levels differed between these two tissues. In addition, leek KAS III was expressed in a manner comparable to leek oleoyl-ACP thioesterase (OTE), another enzyme of fatty acid biosynthesis, in both tissues.

Uses of biotechnology in modifying plant lipids

This review discusses fatty acid modification of oilseeds with additional emphasis on production of oxygenated derivatives. In a relatively short period, less than a decade, our understanding of the enzymes involved in plant fatty acid synthesis has increased to the point where we understand how they might be used in oilseed modification. Further, through modern molecular biological techniques, the actual genes for many of these important enzymes have been cloned. Use of genetic transformation systems has allowed us to fundamentally alter the normal biosynthetic pathways in highly specific ways, in manners that would be either difficult or impossible using traditional breeding techniques. Alteration of plant lipid biosynthesis is not restricted to using genes from the plants themselves, but interspecies transfer is possible, either from completely unrelated plant species (often of no commercial value but possessing unusual biochemical properties) or from animals, fungi, and prokaryotic organisms. In this way "designer" plants possessing altered metabolism, tailored to the interests or needs of certain industries, nutritionists, and the consumer can be created.

Modification of Plant Lipid Synthesis

Genetic engineering of new storage oils and fats has produced oil crop plants with fatty acid compositions unattainable by plant breeding alone. The combination of classical breeding methods with molecular techniques provides new ways for designing oils for food and nonfood uses. Alterations in the position and number of double bonds, variation in fatty acid chain length, and the introduction of desired functional groups have already been achieved in model systems. Short-term prospects include crops such as rapeseed or soybean engineered to have greater than 70 to 80 percent medium-chain fatty acids by content, greater than 90 percent oleic acid, and high erucic acid content, and engineered to form ricinoleic acid in seed storage tissues.

Specific inhibition of plant fatty acid elongation by a long-chain cerulenin analogue

Cerulenin analogues with 16 or 18 carbon atoms inhibit both ATP-dependent and acyl-CoA-dependent fatty acid elongations. Prior incubation of microsomes with inhibitors is necessary to obtain maximal inhibition. The analogues act on the first reaction of the elongation process catalysed by the 3-ketoacyl-CoA synthase. The 18-carbon analogue has no, or little, effect on the fatty acid synthesis, while cerulenin and its 16-carbon analogue totally inhibit this synthesis. The 18-carbon analogue appears to be a specific inhibitor of the synthesis of very long-chain fatty acids, with no effect on de novo fatty acid synthesis.

Modification of Brassica napus seed oil by expression of the Escherichia coli fabH gene, encoding 3-ketoacyl-acyl carrier protein synthase III

The Escherichia coli fabH gene encoding 3-ketoacyl-acyl carrier protein synthase III (KAS III) was isolated and the effect of overproduction of bacterial KAS III was compared in both E. coli and Brassica napus. The change in fatty acid profile of E. coli was essentially the same as that reported by Tsay et al. (J Biol Chem 267 (1992) 6807-6814), namely higher C14:0 and lower C18:1 levels. In our study, however, an arrest of cell growth was also observed. This and other evidence suggests that in E. coli the accumulation of C14:0 may not be a direct effect of the KAS III overexpression, but a general metabolic consequence of the arrest of cell division. Bacterial KAS III was expressed in a seed- and developmentally specific manner in B. napus in either cytoplasm or plastid. Significant increases in KAS III activities were observed in both these transformation groups, up to 3.7 times the endogenous KAS III activity in mature seeds. Only the expression of the plastid-targeted KAS III gene, however, affected the fatty acid profile of the storage lipids, such that decreased amounts of C18:1 and increased amounts of C18:2 and C18:3 were observed as compared to control plants. Such changes in fatty acid composition reflect changes in the regulation and control of fatty acid biosynthesis. We propose that fatty acid biosynthesis is not controlled by one rate-limiting enzyme, such as acetyl-CoA carboxylase, but rather is shared by a number of component enzymes of the fatty acid biosynthetic machinery.

A Mutant of Arabidopsis Deficient in the Elongation of Palmitic Acid

The overall fatty acid composition of leaf lipids in a mutant of Arabidopsis thaliana was characterized by an increased level of 16:0 and a concomitant decrease of 18-carbon fatty acids as a consequence of a single recessive nuclear mutation at the fab1 locus. Quantitative analysis of the fatty acid composition of individual lipids established that lipids synthesized by both the prokaryotic and eukaryotic pathways were affected by the mutation. Direct enzyme assays demonstrated that the mutant plants were deficient in the activity of 3-ketoacyl-acyl carrier protein synthase II; therefore, it is inferred that fab1 may encode this enzyme. Labeling experiments with [14C]acetate and lipase positional analysis indicated that the mutation results in a small shift in the partitioning of lipid synthesis between the prokaryotic and eukaryotic pathways. Synthesis of chloroplast lipids by the prokaryotic pathway was increased with a corresponding reduction in the eukaryotic pathway.

Acetoacyl-acyl carrier protein synthase from avocado: its purification, characterisation and clear resolution from acetyl CoA:ACP transacylase

beta-ketoacyl-ACP synthetase III (KAS III) has been purified from avocado using a six-step purification procedure. The enzyme, which is cerulenin-insensitive and thiolactomycin-sensitive, was assayed using a partial component reaction: acetyl CoA:ACP transacylase (ACAT) activity. KAS III activity is distinguished from ACAT activity on the basis that the former is highly stimulated by the addition of malonyl CoA in the presence of malonyl-CoA:ACP transacylase, and the latter is not. KAS III and ACAT activity have been separated from each other thus providing the first evidence that these two discrete activities exist in higher plants. Both of these enzymes have been implicated in the initial reactions of fatty acid synthesis. KAS III was purified 134-fold using a combination of PEG precipitation, Fast Q, ammonium sulphate precipitation, Phenyl Sepharose and ACP-affinity chromatography. The enzyme requires Triton X-100 for solubility and is highly salt sensitive. The subunit molecular mass of 37 kDa has been identified by SDS-PAGE. The results of gel filtration analysis are consistent with the native enzyme being homodimeric. The native molecular mass of KAS III is 69 kDa and that of ACAT 18.5 kDa. The enzyme has a pH optimum of 7.0-7.5, which is similar to the pH optimum of the ACAT reaction. The Km for acetyl CoA is 12.5 microM and the Km for malonyl-ACP is 14 microM. Both KAS III and ACAT are sensitive to thiolactomycin inhibition. The results are discussed with respect to the potential role of acetyl CoA:ACP transacylase in plants.

3-Ketoacyl-acyl carrier protein synthase III from spinach (Spinacia oleracea) is not similar to other condensing enzymes of fatty acid synthase

A cDNA clone encoding spinach (Spinacia oleracea) 3-ketoacyl-acyl carrier protein synthase III (KAS III), which catalyzes the initial condensing reaction in fatty acid biosynthesis, was isolated. Based on the amino acid sequence of tryptic digests of purified spinach KAS III, degenerate polymerase chain reaction (PCR) primers were designed and used to amplify a 612-bp fragment from first-strand cDNA of spinach leaf RNA. A root cDNA library was probed with the PCR fragment, and a 1920-bp clone was isolated. Its deduced amino acid sequence matched the sequences of the tryptic digests obtained from the purified KAS III. Northern analysis confirmed that it was expressed in both leaf and root. The clone contained a 1218-bp open reading frame coding for 405 amino acids. The identity of the clone was confirmed by expression in Escherichia coli BL 21 as a glutathione S-transferase fusion protein. The deduced amino acid sequence was 48 and 45% identical with the putative KAS III of Porphyra umbilicalis and KAS III of E. coli, respectively. It also had a strong local homology to the plant chalcone synthases but had little homology with other KAS isoforms from plants, bacteria, or animals.

Effect of cerulenin on the synthesis of very-long-chain fatty acids in microsomes from leek seedlings

Cerulenin inhibits the elongation of stearoyl-CoA and eicosanoyl-CoA by microsomes from leek seedlings. The inhibition depends on the cerulenin concentration and affects the biosynthesis of docosanoic and tetracosanoic acids only slightly more than that of eicosanoic acid. A 30-min preincubation of the microsomes with cerulenin allows a quantitative inhibition of the elongation at 50 microM cerulenin (50% inhibition at 15 microM cerulenin). A kinetic study of the elongating activity in the presence or in the absence of the inhibitor suggests that the inhibition is non-competitive. Analysis of the products of the reaction suggests that 3-ketoacyl-CoA synthase is the target of cerulenin. A study of the partial reactions demonstrates that the inhibition affects almost exclusively the condensation step.