An atypical 3-ketoacyl ACP synthase III required for acyl homoserine lactone synthesis in Pseudomonas syringae pv. syringae B728a

The last step of the initiation phase of fatty acid biosynthesis in most bacteria is catalyzed by the 3-ketoacyl-acyl carrier protein (ACP) synthase III (FabH). Pseudomonas syringae pv. syringae strain B728a encodes two FabH homologs, Psyr_3467 and Psyr_3830, which we designated PssFabH1 and PssFabH2, respectively. Here, we explored the roles of these two 3-ketoacyl-ACP synthase (KAS) III proteins. We found that PssFabH1 is similar to the Escherichia coli FabH in using acetyl-acetyl-coenzyme A (CoA ) as a substrate in vitro, whereas PssFabH2 uses acyl-CoAs (C4-C10) or acyl-ACPs (C6-C10). Mutant analysis showed that neither KAS III protein is essential for the de novo fatty acid synthesis and cell growth. Loss of PssFabH1 reduced the production of an acyl homoserine lactone (AHL) quorum-sensing signal, and this production was partially restored by overexpressing FabH homologs from other bacteria. AHL production was also restored by inhibiting fatty acid elongation and providing exogenous butyric acid. Deletion of PssFabH1 supports the redirection of acyl-ACP toward biosurfactant synthesis, which in turn enhances swarming motility. Our study revealed that PssFabH1 is an atypical KAS III protein that represents a new KAS III clade that functions in providing a critical fatty acid precursor, butyryl-ACP, for AHL synthesis.IMPORTANCEAcyl homoserine lactones (AHLs) are important quorum-sensing compounds in Gram-negative bacteria. Although their formation requires acylated acyl carrier proteins (ACPs), how the acylated intermediate is shunted from cellular fatty acid synthesis to AHL synthesis is not known. Here, we provide in vivo evidence that Pseudomonas syringae strain B728a uses the enzyme PssFabH1 to provide the critical fatty acid precursor butyryl-ACP for AHL synthesis. Loss of PssFabH1 reduces the diversion of butyryl-ACP to AHL, enabling the accumulation of acyl-ACP for synthesis of biosurfactants that contribute to bacterial swarming motility. We report that PssFabH1 and PssFabH2 each encode a 3-ketoacyl-acyl carrier protein synthase (KAS) III in P. syringae B728a. Whereas PssFabH2 is able to function in redirecting intermediates from β-oxidation to fatty acid synthesis, PssFabH1 is an atypical KAS III protein that represents a new KAS III clade based on its sequence, non-involvement in cell growth, and novel role in AHL synthesis.

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.

Comprehensive analysis of KCS gene family in Citrinae reveals the involvement of CsKCS2 and CsKCS11 in fruit cuticular wax synthesis at ripening

Cuticular wax covers the surface of fleshy fruit and plays a protective role in fruit development and postharvest storage, including reducing fruit water loss, resisting biotic and abiotic stress and affecting fruit glossiness. The β-ketoacyl-CoA synthase (KCS) is the rate-limiting enzyme of very long chain fatty acids (VLCFAs) synthesis, which provides precursors for the synthesis of cuticular wax. In this study, a total of 96 KCS genes were identified in six Citrinae species, including 13, 16, 21, 14, 16 and 16 KCS genes in the primitive species (Atalantia buxifolia), the wild species (Citrus ichangensis), and four cultivated species (Citrus medica, Citrus grandis, Citrus sinensis and Citrus clementina), respectively. Compared with primitive species, wild and cultivated species showed expansion of KCS gene family. Evolutionary analysis of KCS gene family indicated that uneven gain and loss of genes resulted in variable numbers of KCS genes in Citrinae, and KCS genes have undergone purifying selection. Expression profiles in C. sinensis revealed that the KCS genes had diverse expression patterns among various tissues. Furthermore, CsKCS2 and CsKCS11 were predominantly expressed in the flavedo and their expression increased sharply with ripening. Subcellular localization analysis indicated that CsKCS2 and CsKCS11 were located in the endoplasmic reticulum. Further, heterologous expression of CsKCS2 and CsKCS11 in Arabidopsis significantly increased the content of cuticular wax in leaves. Thus, CsKCS2 and CsKCS11 are involved in the accumulation of fruit cuticular wax at ripening. This work will facilitate further functional verification and understanding of the evolution of KCS genes in Citrinae.

A seed coat-specific β-ketoacyl-CoA synthase, KCS12, is critical for preserving seed physical dormancy

Physical dormancy in seeds exists widely in seed plants and plays a vital role in maintaining natural seed banks. The outermost cuticle of the seed coat forms a water-impermeable layer, which is critical for establishing seed physical dormancy. We previously set up the legume plant Medicago truncatula as an excellent model for studying seed physical dormancy, and our studies revealed that a class II KNOTTED-like homeobox, KNOX4, is a transcription factor critical for controlling hardseededness. Here we report the function of a seed coat β-ketoacyl-CoA synthase, KCS12. The expression level of KCS12 is significantly downregulated in the knox4 mutant. The KCS12 gene is predominantly expressed in the seed coat, and seed development in the M. truncatula kcs12 mutant is altered. Further investigation demonstrated that kcs12 mutant seeds lost physical dormancy and were able to absorb water without scarification treatment. Chemical analysis revealed that concentrations of C24:0 lipid polyester monomers are significantly decreased in mutant seeds, indicating that KCS12 is an enzyme that controls the production of very long chain lipid species in the seed coat. A chromatin immunoprecipitation assay demonstrated that the expression of KCS12 in the seed coat is directly regulated by the KNOX4 transcription factor. These findings define a molecular mechanism by which KNOX4 and KCS12 control formation of the seed coat and seed physical dormancy.

The 3-ketoacyl-CoA synthase WFL is involved in lateral organ development and cuticular wax synthesis in Medicago truncatula

A 3-ketoacyl-CoA synthase involved in biosynthesis of very long chain fatty acids and cuticular wax plays a vital role in aerial organ development in M. truncatula. Cuticular wax is composed of very long chain fatty acids and their derivatives. Defects in cuticular wax often result in organ fusion, but little is known about the role of cuticular wax in compound leaf and flower development in Medicago truncatula. In this study, through an extensive screen of a Tnt1 retrotransposon insertion population in M. truncatula, we identified four mutant lines, named wrinkled flower and leaf (wfl) for their phenotype. The phenotype of the wfl mutants is caused by a Tnt1 insertion in Medtr3g105550, encoding 3-ketoacyl-CoA synthase (KCS), which functions as a rate-limiting enzyme in very long chain fatty acid elongation. Reverse transcription-quantitative PCR showed that WFL was broadly expressed in aerial organs of the wild type, such as leaves, floral organs, and the shoot apical meristem, but was expressed at lower levels in roots. In situ hybridization showed a similar expression pattern, mainly detecting the WFL transcript in epidermal cells of the shoot apical meristem, leaf primordia, and floral organs. The wfl mutant leaves showed sparser epicuticular wax crystals on the surface and increased water permeability compared with wild type. Further analysis showed that in wfl leaves, the percentage of C20:0, C22:0, and C24:0 fatty acids was significantly increased, the amount of cuticular wax was markedly reduced, and wax constituents were altered compared to the wild type. The reduced formation of cuticular wax and wax composition changes on the leaf surface might lead to the developmental defects observed in the wfl mutants. These findings suggest that WFL plays a key role in cuticular wax formation and in the late stage of leaf and flower development in M. truncatula.

α-proteobacteria synthesize biotin precursor pimeloyl-ACP using BioZ 3-ketoacyl-ACP synthase and lysine catabolism

Pimelic acid, a seven carbon α,ω-dicarboxylic acid (heptanedioic acid), is known to provide seven of the ten biotin carbon atoms including all those of the valeryl side chain. Distinct pimelate synthesis pathways were recently elucidated in Escherichia coli and Bacillus subtilis where fatty acid synthesis plus dedicated biotin enzymes produce the pimelate moiety. In contrast, the α-proteobacteria which include important plant and mammalian pathogens plus plant symbionts, lack all of the known pimelate synthesis genes and instead encode bioZ genes. Here we report a pathway in which BioZ proteins catalyze a 3-ketoacyl-acyl carrier protein (ACP) synthase III-like reaction to produce pimeloyl-ACP with five of the seven pimelate carbon atoms being derived from glutaryl-CoA, an intermediate in lysine degradation. Agrobacterium tumefaciens strains either deleted for bioZ or which encode a BioZ active site mutant are biotin auxotrophs, as are strains defective in CaiB which catalyzes glutaryl-CoA synthesis from glutarate and succinyl-CoA.

Functional analysis of β-ketoacyl-CoA synthase from biofuel feedstock Thlaspi arvense reveals differences in the triacylglycerol biosynthetic pathway among Brassicaceae

Differences in FAE1 enzyme affinity for the acyl-CoA substrates, as well as the balance between the different pathways involved in their incorporation to triacylglycerol might be determinant of the different composition of the seed oil in Brassicaceae. Brassicaceae present a great heterogeneity of seed oil and fatty acid composition, accumulating Very Long Chain Fatty Acids with industrial applications. However, the molecular determinants of these differences remain elusive. We have studied the β-ketoacyl-CoA synthase from the high erucic feedstock Thlaspi arvense (Pennycress). Functional characterization of the Pennycress FAE1 enzyme was performed in two Arabidopsis backgrounds; Col-0, with less than 2.5% of erucic acid in its seed oil and the fae1-1 mutant, deficient in FAE1 activity, that did not accumulate erucic acid. Seed-specific expression of the Pennycress FAE1 gene in Col-0 resulted in a 3 to fourfold increase of erucic acid content in the seed oil. This increase was concomitant with a decrease of eicosenoic acid levels without changes in oleic ones. Interestingly, only small changes in eicosenoic and erucic acid levels occurred when the Pennycress FAE1 gene was expressed in the fae1-1 mutant, with high levels of oleic acid available for elongation, suggesting that the Pennycress FAE1 enzyme showed higher affinity for eicosenoic acid substrates, than for oleic ones in Arabidopsis. Erucic acid was incorporated to triacylglycerol in the transgenic lines without significant changes in their levels in the diacylglycerol fraction, suggesting that erucic acid was preferentially incorporated to triacylglycerol via DGAT1. Expression analysis of FAE1, AtDGAT1, AtLPCAT1 and AtPDAT1 genes in the transgenic lines further supported this conclusion. Differences in FAE1 affinity for the oleic and eicosenoic substrates among Brassicaceae, as well as their incorporation to triacylglycerol might explain the differences in composition of their seed oil.

Genome wide analysis and functional identification of MdKCS genes in apple

Apple fruit is covered by cuticle wax, which plays important roles protecting fruits from adverse environmental conditions. β-Ketoacyl-CoA synthase (KCS) is the key rate-limiting enzyme in plant wax synthesis. In this study, we identified 28 KCS gene family members from apple (Malus × domestica Borkh.) by homology analysis. Multi-sequence alignment and phylogenetic analyses revealed that the 28 MdKCS genes were divided into four subgroups, including KCS1-like, FAE1-like, FDH-like, and CER6. A chromosomal localization analysis revealed that 27 apple KCS genes were located on 11 chromosomes, while MdKCS28 was localized to the unassembled genomic scaffold. Most of the MdKCS proteins were hydrophilic proteins and they had similar secondary and tertiary structures. The prediction of cis-acting elements of the MdKCS gene promoters suggested that the MdKCS genes may be widely involved in hormone signaling and the stress response. Furthermore, the quantitative real-time polymerase chain reaction results showed that eight MdKCS genes were highly expressed in the apple pericarp, and were significantly induced by drought, abscisic acid (ABA), and NaCl treatments. We transformed the MdKCS21 gene into apple calli, and found the MdKCS21 overexpressing transgenic apple calli exhibited higher tolerance to ABA treatment. Finally, the MdKCS proteins were localized to the endoplasmic reticulum and vacuolar membrane by confocal laser microscopy. This study established a foundation to further analyze the function of KCS genes and provided candidate genes for molecular improvement of wax content in apple.

Assembling a plug-and-play production line for combinatorial biosynthesis of aromatic polyketides in Escherichia coli

Polyketides are a class of specialised metabolites synthesised by both eukaryotes and prokaryotes. These chemically and structurally diverse molecules are heavily used in the clinic and include frontline antimicrobial and anticancer drugs such as erythromycin and doxorubicin. To replenish the clinicians' diminishing arsenal of bioactive molecules, a promising strategy aims at transferring polyketide biosynthetic pathways from their native producers into the biotechnologically desirable host Escherichia coli. This approach has been successful for type I modular polyketide synthases (PKSs); however, despite more than 3 decades of research, the large and important group of type II PKSs has until now been elusive in E. coli. Here, we report on a versatile polyketide biosynthesis pipeline, based on identification of E. coli-compatible type II PKSs. We successfully express 5 ketosynthase (KS) and chain length factor (CLF) pairs-e.g., from Photorhabdus luminescens TT01, Streptomyces resistomycificus, Streptoccocus sp. GMD2S, Pseudoalteromonas luteoviolacea, and Ktedonobacter racemifer-as soluble heterodimeric recombinant proteins in E. coli for the first time. We define the anthraquinone minimal PKS components and utilise this biosynthetic system to synthesise anthraquinones, dianthrones, and benzoisochromanequinones (BIQs). Furthermore, we demonstrate the tolerance and promiscuity of the anthraquinone heterologous biosynthetic pathway in E. coli to act as genetically applicable plug-and-play scaffold, showing it to function successfully when combined with enzymes from phylogenetically distant species, endophytic fungi and plants, which resulted in 2 new-to-nature compounds, neomedicamycin and neochaetomycin. This work enables plug-and-play combinatorial biosynthesis of aromatic polyketides using bacterial type II PKSs in E. coli, providing full access to its many advantages in terms of easy and fast genetic manipulation, accessibility for high-throughput robotics, and convenient biotechnological scale-up. Using the synthetic and systems biology toolbox, this plug-and-play biosynthetic platform can serve as an engine for the production of new and diversified bioactive polyketides in an automated, rapid, and versatile fashion.

Identification of 4-methyl-5-oxo-octane-1,8-dioic acid and the derivatives as metabolites of steroidal C,D-ring degradation in Comamonas testosteroni TA441

Comamonas testosteroni TA441 degrades steroids via 9,17-dioxo-1,2,3,4,10,19-hexanorandrostan-5-oic acid, which is presumed to be further degraded by β-oxidation. In the β-oxidation process, Coenzyme A (CoA)-ester of 9-oxo-1,2,3,4,5,6,10,19-octanor-13,17-secoandrost-8(14)-ene-7,17-dioic acid is produced and converted by β-ketoacyl-CoA-transferase encoded by ORF1 and ORF2 (scdL1L2) to cleave the remaining C-ring. In this study, we isolated and identified 4-methyl-5-oxo-octane-1,8-dioic acid and 4-methyl-5-oxo-3-octene-1,8-dioic acid from the culture of the ORF3 (scdN)-null mutant as metabolites of steroid degradation (ADD and cholic acid analogues; cholic acid, chenodeoxycholic acid, deoxycholic acid, and lithocholic acid). In addition of these compounds, UHPLC/MS analysis of the culture of the scdN-null mutant revealed significant accumulation of another compound, which was detected as a dominant peak of m/z 155 ([M-CO₂]^-) accompanied by a small peak of parental ion (m/z 199 [M^-]). On the bases of experimental data, this compound was presumed to be 4-methyl-5-oxo-2-octene-1,8-dioic acid, whose CoA-ester was indicated to be converted by scdN-encoded CoA-hydratase into the CoA-ester of 3-hydroxy-4-methyl-5-oxooctan-1,7-carboxylic acid.

Differences in substrate specificity of V. cholerae FabH enzymes suggest new approaches for the development of novel antibiotics and biofuels

Vibrio cholerae, the causative pathogen of the life-threatening infection cholera, encodes two copies of β-ketoacyl-acyl carrier protein synthase III (vcFabH1 and vcFabH2). vcFabH1 and vcFabH2 are pathogenic proteins associated with fatty acid synthesis, lipid metabolism, and potential applications in biofuel production. Our biochemical assays characterize vcFabH1 as exhibiting specificity for acetyl-CoA and CoA thioesters with short acyl chains, similar to that observed for FabH homologs found in most gram-negative bacteria. vcFabH2 prefers medium chain-length acyl-CoA thioesters, particularly octanoyl-CoA, which is a pattern of specificity rarely seen in bacteria. Structural characterization of one vcFabH1 and six vcFabH2 structures determined in either apo form or in complex with acetyl-CoA/octanoyl-CoA indicate that the substrate-binding pockets of vcFabH1 and vcFabH2 are of different sizes, accounting for variations in substrate chain-length specificity. An unusual and unique feature of vcFabH2 is its C-terminal fragment that interacts with both the substrate-entrance loop and the dimer interface of the enzyme. Our discovery of the pattern of substrate specificity of both vcFabH1 and vcFabH2 can potentially aid the development of novel antibacterial agents against V. cholerae. Additionally, the distinctive substrate preference of FabH2 in V. cholerae and related facultative anaerobes conceivably make it an attractive component of genetically engineered bacteria used for commercial biofuel production.

NLR Mutations Suppressing Immune Hybrid Incompatibility and Their Effects on Disease Resistance

Genetic divergence between populations can lead to reproductive isolation. Hybrid incompatibilities (HI) represent intermediate points along a continuum toward speciation. In plants, genetic variation in disease resistance (R) genes underlies several cases of HI. The progeny of a cross between Arabidopsis (Arabidopsis thaliana) accessions Landsberg erecta (Ler, Poland) and Kashmir2 (Kas2, central Asia) exhibits immune-related HI. This incompatibility is due to a genetic interaction between a cluster of eight TNL (TOLL/INTERLEUKIN1 RECEPTOR-NUCLEOTIDE BINDING-LEU RICH REPEAT) RPP1 (RECOGNITION OF PERONOSPORA PARASITICA1)-like genes (R1-R8) from Ler and central Asian alleles of a Strubbelig-family receptor-like kinase (SRF3) from Kas2. In characterizing mutants altered in Ler/Kas2 HI, we mapped multiple mutations to the RPP1-like Ler locus. Analysis of these suppressor of Ler/Kas2 incompatibility (sulki) mutants reveals complex, additive and epistatic interactions underlying RPP1-like Ler locus activity. The effects of these mutations were measured on basal defense, global gene expression, primary metabolism, and disease resistance to a local Hyaloperonospora arabidopsidis isolate (Hpa Gw) collected from Gorzów (Gw), where the Landsberg accession originated. Gene expression sectors and metabolic hallmarks identified for HI are both dependent and independent of RPP1-like Ler members. We establish that mutations suppressing immune-related Ler/Kas2 HI do not compromise resistance to Hpa Gw. QTL mapping analysis of Hpa Gw resistance point to RPP7 as the causal locus. This work provides insight into the complex genetic architecture of the RPP1-like Ler locus and immune-related HI in Arabidopsis and into the contributions of RPP1-like genes to HI and defense.

Wax Crystal-Sparse Leaf 4, encoding a β-ketoacyl-coenzyme A synthase 6, is involved in rice cuticular wax accumulation

WSL4 encodes a KCS6 protein which is required for cuticular wax accumulation in rice. Very long chain fatty acids (VLCFAs) are essential precursors for cuticular wax biosynthesis. VLCFA biosynthesis occurs in the endoplasmic reticulum and requires the fatty acid elongase (FAE) complex. The β-ketoacyl-coenzyme A synthase (KCS) catalyzes the first step of FAE-mediated VLCFA elongation. Here we characterized the Wax Crystal-Sparse Leaf 4 (WSL4) gene involved in leaf cuticular wax accumulation in rice. The wsl4 mutant displayed a pleiotropic phenotype including dwarfism, less tiller numbers and reduced surface wax load. Map-based cloning and nucleotide sequencing results revealed that wsl4 carried a single nucleotide substitution in the second exon of a putative KCS6 gene, encoding one subunit of the FAE complex for VLCFAs. Genetic complementation confirmed that the mutation in WSL4 was responsible for the phenotype of wsl4. WSL4 was constitutively expressed in various rice tissues and localized in the endoplasmic reticulum. Both WSL4-RNAi transgenic lines and WSL4 knocked-out mutants exhibited wax-deficient phenotypes similar to the wsl4 mutant. These data indicate that WSL4 is required for cuticular wax accumulation in rice.

Functional divergence of chloroplast Cpn60α subunits during Arabidopsis embryo development

Chaperonins are a class of molecular chaperones that assist in the folding and assembly of a wide range of substrates. In plants, chloroplast chaperonins are composed of two different types of subunits, Cpn60α and Cpn60β, and duplication of Cpn60α and Cpn60β genes occurs in a high proportion of plants. However, the importance of multiple Cpn60α and Cpn60β genes in plants is poorly understood. In this study, we found that loss-of-function of CPNA2 (AtCpn60α2), a gene encoding the minor Cpn60α subunit in Arabidopsis thaliana, resulted in arrested embryo development at the globular stage, whereas the other AtCpn60α gene encoding the dominant Cpn60α subunit, CPNA1 (AtCpn60α1), mainly affected embryonic cotyledon development at the torpedo stage and thereafter. Further studies demonstrated that CPNA2 can form a functional chaperonin with CPNB2 (AtCpn60β2) and CPNB3 (AtCpn60β3), while the functional partners of CPNA1 are CPNB1 (AtCpn60β1) and CPNB2. We also revealed that the functional chaperonin containing CPNA2 could assist the folding of a specific substrate, KASI (β-ketoacyl-[acyl carrier protein] synthase I), and that the KASI protein level was remarkably reduced due to loss-of-function of CPNA2. Furthermore, the reduction in the KASI protein level was shown to be the possible cause for the arrest of cpna2 embryos. Our findings indicate that the two Cpn60α subunits in Arabidopsis play different roles during embryo development through forming distinct chaperonins with specific AtCpn60β to assist the folding of particular substrates, thus providing novel insights into functional divergence of Cpn60α subunits in plants.

Chaperonins are large oligomeric complexes that are involved in the folding and assembly of numerous proteins in various species. In contrast to other types of chaperonins, chloroplast chaperonins are characterized by the hetero-oligomeric structure composed of two unique types of subunits, Cpn60α and Cpn60β, each of which is present in two or more paralogous forms in most of higher plants. However, the functional significance underlying the wide array of subunit types and complex oligomeric arrangement remains largely unknown. Here, we investigated the role of the minor Cpn60α subunit AtCpn60α2 in Arabidopsis embryo development, and found that AtCpn60α2 is important for the transition of globular embryos to heart-shaped embryos, whereas loss of the dominant Cpn60α subunit AtCpn60α1 affects embryonic cotyledon development. Further studies demonstrated that AtCpn60α2 could form functional chaperonins with AtCpn60β2 and AtCpn60β3 to specifically assist in folding of the substrate KASI, which is important for the formation of heart-shaped embryos. Our results suggest that duplication of Cpn60α genes in higher plants can increase the potential number of chloroplast chaperonin substrates and provide chloroplast chaperonins with more roles in plant growth and development, thus revealing the relationship between duplication and functional specialization of chaperonin genes.

Molecular cloning and characterization of two β-ketoacyl-acyl carrier protein synthase I genes from Jatropha curcas L

The β-ketoacyl-acyl carrier protein synthase I (KASI) is involved in de novo fatty acid biosynthesis in many organisms. Two putative KASI genes, JcKASI-1 and JcKASI-2, were isolated from Jatropha curcas. The deduced amino acid sequences of JcKASI-1 and JcKASI-2 exhibit around 83.8% and 72.5% sequence identities with AtKASI, respectively, and both contain conserved Cys-His-Lys-His-Phe catalytic active sites. Phylogenetic analysis indicated that JcKASI-2 belongs to a clade with several KASI proteins from dicotyledonous plants. Both JcKASI genes were expressed in multiple tissues, most strongly in filling stage seeds of J. curcas. Additionally, the JcKASI-1 and JcKASI-2 proteins were both localized to the plastids. Expressing JcKASI-1 in the Arabidopsis kasI mutant rescued the mutant's phenotype and restored the fatty acid composition and oil content in seeds to wild-type, but expressing JcKASI-2 in the Arabidopsis kasI mutant resulted in only partial rescue. This implies that JcKASI-1 and JcKASI-2 exhibit partial functional redundancy and KASI genes play a universal role in regulating fatty acid biosynthesis, growth, and development in plants.

Accumulation of medium-chain, saturated fatty acyl moieties in seed oils of transgenic Camelina sativa

With its high seed oil content, the mustard family plant Camelina sativa has gained attention as a potential biofuel source. As a bioenergy crop, camelina has many advantages. It grows on marginal land with low demand for water and fertilizer, has a relatively short life cycle, and is stress tolerant. As most other crop seed oils, camelina seed triacylglycerols (TAGs) consist of mostly long, unsaturated fatty acyl moieties, which is not desirable for biofuel processing. In our efforts to produce shorter, saturated chain fatty acyl moieties in camelina seed oil for conversion to jet fuel, a 12:0-acyl-carrier thioesterase gene, UcFATB1, from California bay (Umbellularia californica Nutt.) was expressed in camelina seeds. Up to 40% of short chain laurate (C12:0) and myristate (C14:0) were present in TAGs of the seed oil of the transgenics. The total oil content and germination rate of the transgenic seeds were not affected. Analysis of positions of these two fatty acyl moieties in TAGs indicated that they were present at the sn-1 and sn-3 positions, but not sn-2, on the TAGs. Suppression of the camelina KASII genes by RNAi constructs led to higher accumulation of palmitate (C16:0), from 7.5% up to 28.5%, and further reduction of longer, unsaturated fatty acids in seed TAGs. Co-transformation of camelina with both constructs resulted in enhanced accumulation of all three medium-chain, saturated fatty acids in camelina seed oils. Our results show that a California bay gene can be successfully used to modify the oil composition in camelina seed and present a new biological alternative for jet fuel production.

Genetic enhancement of palmitic acid accumulation in cotton seed oil through RNAi down-regulation of ghKAS2 encoding β-ketoacyl-ACP synthase II (KASII)

Palmitic acid (C16:0) already makes up approximately 25% of the total fatty acids in the conventional cotton seed oil. However, further enhancements in palmitic acid content at the expense of the predominant unsaturated fatty acids would provide increased oxidative stability of cotton seed oil and also impart the high melting point required for making margarine, shortening and confectionary products free of trans fatty acids. Seed-specific RNAi-mediated down-regulation of β-ketoacyl-ACP synthase II (KASII) catalysing the elongation of palmitoyl-ACP to stearoyl-ACP has succeeded in dramatically increasing the C16 fatty acid content of cotton seed oil to well beyond its natural limits, reaching up to 65% of total fatty acids. The elevated C16 levels were comprised of predominantly palmitic acid (C16:0, 51%) and to a lesser extent palmitoleic acid (C16:1, 11%) and hexadecadienoic acid (C16:2, 3%), and were stably inherited. Despite of the dramatic alteration of fatty acid composition and a slight yet significant reduction in oil content in these high-palmitic (HP) lines, seed germination remained unaffected. Regiochemical analysis of triacylglycerols (TAG) showed that the increased levels of palmitic acid mainly occurred at the outer positions, while C16:1 and C16:2 were predominantly found in the sn-2 position in both TAG and phosphatidylcholine. Crossing the HP line with previously created high-oleic (HO) and high-stearic (HS) genotypes demonstrated that HP and HO traits could be achieved simultaneously; however, elevation of stearic acid was hindered in the presence of high level of palmitic acid.

[Effect of cis-vaccinate acid on swarming ability of Pseudomonas aeruginosa]

OBJECTIVE

To study the effect of fatty acids composition on swarming mobility in Pseudomonas aeruginosa.

METHODS

We constructed a fabF-knockout mutant of PAO1 (YFF-1) by double exchange principle, overexpressed FabF in YFF-1 mutant to recover the mobility, and compared the swarming ability of wild type, YFF-1 mutant and mutant with plasmid pUCP18Gm-fabF. The change of fatty acids composition was analyzed using gas chromatography to explain the difference of swarming ability.

RESULTS

Swarming ability disappeared in YFF-1 mutant and was recovered in YFF-1 with plasmid pUCP18Gm-fabF. Gas chromatography analysis revealed that fatty acids composition changed in YFF-1. The cis-vaccinate acid (C18:1delta11) content decreased from 33.6% to 8.9%, and the ratio of unsaturated fatty acids to saturated fatty acids (UFA: SFA) was deduced from 0.96 to 0.74. The recovery of cis-vaccinate acid content was 20.9% and UFA:SFA 1.09 after expression of fabF.

CONCLUSION

Expression level of FabF played an important role in regulating swarming ability of PAO1. The decrease of cis-vaccinate acid content and unsaturation degree of fatty acids, especially the sharp decrease of cis-vaccinate acid, may be vital causes of swarming ability disappearance in YFF-1.

Ralstonia solanacearum RSp0194 Encodes a Novel 3-Keto-Acyl Carrier Protein Synthase III

Fatty acid synthesis (FAS), a primary metabolic pathway, is essential for survival of bacteria. Ralstonia solanacearum, a β-proteobacteria member, causes a bacterial wilt affecting more than 200 plant species, including many economically important plants. However, thus far, the fatty acid biosynthesis pathway of R. solanacearum has not been well studied. In this study, we characterized two forms of 3-keto-ACP synthase III, RsFabH and RsFabW, in R. solanacearum. RsFabH, the homologue of Escherichia coli FabH, encoded by the chromosomal RSc1050 gene, catalyzes the condensation of acetyl-CoA with malonyl-ACP in the initiation steps of fatty acid biosynthesis in vitro. The RsfabH mutant lost de novo fatty acid synthetic ability, and grows in medium containing free fatty acids. RsFabW, a homologue of Pseudomonas aeruginosa PA3286, encoded by a megaplasmid gene, RSp0194, condenses acyl-CoA (C₂-CoA to C₁₀-CoA) with malonyl-ACP to produce 3-keto-acyl-ACP in vitro. Although the RsfabW mutant was viable, RsfabW was responsible for RsfabH mutant growth on medium containing free fatty acids. Our results also showed that RsFabW could condense acyl-ACP (C₄-ACP to C₈-ACP) with malonyl-ACP, to produce 3-keto-acyl-ACP in vitro, which implies that RsFabW plays a special role in fatty acid synthesis of R. solanacearum. All of these data confirm that R. solanacearum not only utilizes acetyl-CoA, but also, utilizes medium-chain acyl-CoAs or acyl-ACPs as primers to initiate fatty acid synthesis.

Transient silencing of the KASII genes is feasible in Nicotiana benthamiana for metabolic engineering of wax ester composition

The beta-ketoacyl-ACP synthase II (KASII) is an enzyme in fatty acid biosynthesis, catalyzing the elongation of 16:0-acyl carrier protein (ACP) to 18:0-ACP in plastids. Mutations in KASII genes in higher plants can lead to lethality, which makes it difficult to utilize the gene for lipid metabolic engineering. We demonstrated previously that transient expression of plastid-directed fatty acyl reductases and wax ester synthases could result in different compositions of wax esters. We hypothesized that changing the ratio between C16 (palmitoyl-compounds) and C18 (stearoyl-compounds) in the plastidic acyl-ACP pool by inhibition of KASII expression would change the yield and composition of wax esters via substrate preference of the introduced enzymes. Here, we report that transient inhibition of KASII expression by three different RNAi constructs in leaves of N. benthamiana results in almost complete inhibition of KASII expression. The transient RNAi approach led to a shift of carbon flux from a pool of C18 fatty acids to C16, which significantly increased wax ester production in AtFAR6-containing combinations. The results demonstrate that transient inhibition of KASII in vegetative tissues of higher plants enables metabolic studies towards industrial production of lipids such as wax esters with specific quality and composition.

Evolutionary conserved function of barley and Arabidopsis 3-KETOACYL-CoA SYNTHASES in providing wax signals for germination of powdery mildew fungi

For plant pathogenic fungi, such as powdery mildews, that survive only on a limited number of host plant species, it is a matter of vital importance that their spores sense that they landed on the right spot to initiate germination as quickly as possible. We investigated a barley (Hordeum vulgare) mutant with reduced epicuticular leaf waxes on which spores of adapted and nonadapted powdery mildew fungi showed reduced germination. The barley gene responsible for the mutant wax phenotype was cloned in a forward genetic screen and identified to encode a 3-KETOACYL-CoA SYNTHASE (HvKCS6), a protein participating in fatty acid elongation and required for synthesis of epicuticular waxes. Gas chromatography-mass spectrometry analysis revealed that the mutant has significantly fewer aliphatic wax constituents with a chain length above C-24. Complementation of the mutant restored wild-type wax and overcame germination penalty, indicating that wax constituents less present on the mutant are a crucial clue for spore germination. Investigation of Arabidopsis (Arabidopsis thaliana) transgenic plants with sense silencing of Arabidopsis REQUIRED FOR CUTICULAR WAX PRODUCTION1, the HvKCS6 ortholog, revealed the same germination phenotype against adapted and nonadapted powdery mildew fungi. Our findings hint to an evolutionary conserved mechanism for sensing of plant surfaces among distantly related powdery mildews that is based on KCS6-derived wax components. Perception of such a signal must have been evolved before the monocot-dicot split took place approximately 150 million years ago.

Mechanistic details for anthraniloyl transfer in PqsD: the initial step in HHQ biosynthesis

PqsD mediates the conversion of anthraniloyl-coenzyme A (ACoA) to 2-heptyl-4-hydroxyquinoline (HHQ), a precursor of the Pseudomonas quinolone signal (PQS) molecule. Due to the role of the quinolone signaling pathway of Pseudomonas aeruginosa in the expression of several virulence factors and biofilm formation, PqsD is a potential target for controlling this nosocomial pathogen, which exhibits a low susceptibility to standard antibiotics. PqsD belongs to the β-ketoacyl-ACP synthase family and is similar in structure to homologous FabH enzymes in E. coli and Mycobacterium tuberculosis. Here, we used molecular dynamics simulations to obtain the structural position of the substrate ACoA in the binding pocket of PqsD, and semiempirical molecular orbital calculations to study the reaction mechanism for the catalytic cleavage of ACoA. Our findings suggest a nucleophilic attack of the deprotonated sulfur of Cys112 at the carbonyl carbon of ACoA and a switch in the protonation pattern of His257 whereby Nδ is protonated and the proton of Nε is shifted to the sulfur of CoA during the reaction. This is in agreement with the experimentally determined decreased catalytic activity of the Cys112Ser mutant, whereas the Cys112Ala, His257Phe, and Asn287Ala mutants are all inactive. ESI mass-spectrometric measurements of the Asn287Ala mutant show that anthraniloyl remains covalently bound to Cys112, thus further supporting the inference from our computed mechanism that Asn287 does not take part in the cleavage of ACoA. Since this mutant is inactive, we suggest instead that Asn287 must play an essential role in the subsequent formation of HHQ in vitro.

Phosphorylation of KasB regulates virulence and acid-fastness in Mycobacterium tuberculosis

Mycobacterium tuberculosis bacilli display two signature features: acid-fast staining and the capacity to induce long-term latent infections in humans. However, the mechanisms governing these two important processes remain largely unknown. Ser/Thr phosphorylation has recently emerged as an important regulatory mechanism allowing mycobacteria to adapt their cell wall structure/composition in response to their environment. Herein, we evaluated whether phosphorylation of KasB, a crucial mycolic acid biosynthetic enzyme, could modulate acid-fast staining and virulence. Tandem mass spectrometry and site-directed mutagenesis revealed that phosphorylation of KasB occurred at Thr334 and Thr336 both in vitro and in mycobacteria. Isogenic strains of M. tuberculosis with either a deletion of the kasB gene or a kasB_T334D/T336D allele, mimicking constitutive phosphorylation of KasB, were constructed by specialized linkage transduction. Biochemical and structural analyses comparing these mutants to the parental strain revealed that both mutant strains had mycolic acids that were shortened by 4–6 carbon atoms and lacked trans-cyclopropanation. Together, these results suggested that in M. tuberculosis, phosphorylation profoundly decreases the condensing activity of KasB. Structural/modeling analyses reveal that Thr334 and Thr336 are located in the vicinity of the catalytic triad, which indicates that phosphorylation of these amino acids would result in loss of enzyme activity. Importantly, the kasB_T334D/T336D phosphomimetic and deletion alleles, in contrast to the kasB_T334A/T336A phosphoablative allele, completely lost acid-fast staining. Moreover, assessing the virulence of these strains indicated that the KasB phosphomimetic mutant was attenuated in both immunodeficient and immunocompetent mice following aerosol infection. This attenuation was characterized by the absence of lung pathology. Overall, these results highlight for the first time the role of Ser/Thr kinase-dependent KasB phosphorylation in regulating the later stages of mycolic acid elongation, with important consequences in terms of acid-fast staining and pathogenicity.

Acid-fast staining has been used since 1882 as the hallmark diagnostic test for detecting Mycobacterium tuberculosis, the causative agent of tuberculosis. It has been attributed to the presence of a waxy cell envelope, and primarily to its key components, mycolic acids. Here, we report a new mechanism of regulation in which phosphorylation of KasB, involved in the completion of full-length mycolic acids, leads to shortened mycolic acids and loss of acid-fast staining. Moreover, a M. tuberculosis mutant strain mimicking constitutive phosphorylation of KasB is severely attenuated for growth in both immunocompetent and immunosuppressed mice and fails to cause mortality and pathophysiological symptoms. These results emphasize the critical role of kinase-dependent phosphorylation in the pathogenesis of M. tuberculosis by controlling the mycolic acid chain length. Our study demonstrates the importance of a regulatory mechanism governing acid-fastness and virulence of M. tuberculosis.

The initiation ketosynthase (FabH) is the sole rate-limiting enzyme of the fatty acid synthase of Synechococcus sp. PCC 7002

Cyanobacteria are Gram-negative bacteria that are desirable hosts for biodiesel production, because they are photosynthetic, relatively fast growing, and can secrete products. We have reconstituted the fatty acid synthase (FAS) of the cyanobacterium Synechococcus sp. PCC 7002 and subjected it to in vitro kinetic analysis. Our data revealed that the overall rate of this metabolic pathway is exclusively limited by the FabH ketosynthase, which initiates product synthesis by condensing malonyl-ACP with acetyl-CoA to form acetoacetyl-ACP. This finding sharply contrasts with our previous findings that the Escherichia coli FAS is predominantly limited by its dehydratase (FabZ) and enoyl reductase (FabI) activities and that FabH activity is not limiting. We therefore reconstituted and analyzed a set of "hybrid" FASs. When the Synechococcus FabH was used to replace its counterpart in the reconstituted E. coli FAS, the resulting synthase was strongly limited by FabH activity. Conversely, replacement of the E. coli FabZ with its Synechococcus homolog dramatically alleviated the dependence of E. coli FAS activity on FabZ. In agreement with this finding, introduction of the E. coli FabH in the Synechococcus FAS virtually eliminated its dependence on this subunit, whereas substitution of the Synechococcus FabZ with its E. coli homolog shifted a substantial fraction of the overall flux control in the Synechococcus FAS to FabZ. Our findings demonstrate that the rate-limiting steps can differ dramatically between closely related bacterial fatty acid synthases, and that such regulatory behavior is fundamentally the property of the controlling enzyme(s).

Arabidopsis 3-ketoacyl-coenzyme a synthase9 is involved in the synthesis of tetracosanoic acids as precursors of cuticular waxes, suberins, sphingolipids, and phospholipids

Very-long-chain fatty acids (VLCFAs) with chain lengths from 20 to 34 carbons are involved in diverse biological functions such as membrane constituents, a surface barrier, and seed storage compounds. The first step in VLCFA biosynthesis is the condensation of two carbons to an acyl-coenzyme A, which is catalyzed by 3-ketoacyl-coenzyme A synthase (KCS). In this study, amino acid sequence homology and the messenger RNA expression patterns of 21 Arabidopsis (Arabidopsis thaliana) KCSs were compared. The in planta role of the KCS9 gene, showing higher expression in stem epidermal peels than in stems, was further investigated. The KCS9 gene was ubiquitously expressed in various organs and tissues, including roots, leaves, and stems, including epidermis, silique walls, sepals, the upper portion of the styles, and seed coats, but not in developing embryos. The fluorescent signals of the KCS9::enhanced yellow fluorescent protein construct were merged with those of BrFAD2::monomeric red fluorescent protein, which is an endoplasmic reticulum marker in tobacco (Nicotiana benthamiana) epidermal cells. The kcs9 knockout mutants exhibited a significant reduction in C24 VLCFAs but an accumulation of C20 and C22 VLCFAs in the analysis of membrane and surface lipids. The mutant phenotypes were rescued by the expression of KCS9 under the control of the cauliflower mosaic virus 35S promoter. Taken together, these data demonstrate that KCS9 is involved in the elongation of C22 to C24 fatty acids, which are essential precursors for the biosynthesis of cuticular waxes, aliphatic suberins, and membrane lipids, including sphingolipids and phospholipids. Finally, possible roles of unidentified KCSs are discussed by combining genetic study results and gene expression data from multiple Arabidopsis KCSs.

Deficiency in a very-long-chain fatty acid β-ketoacyl-coenzyme a synthase of tomato impairs microgametogenesis and causes floral organ fusion

Previously, it was shown that β-ketoacyl-coenzyme A synthase ECERIFERUM6 (CER6) is necessary for the biosynthesis of very-long-chain fatty acids with chain lengths beyond C₂₈ in tomato (Solanum lycopersicum) fruits and C₂₆ in Arabidopsis (Arabidopsis thaliana) leaves and the pollen coat. CER6 loss of function in Arabidopsis resulted in conditional male sterility, since pollen coat lipids are responsible for contact-mediated pollen hydration. In tomato, on the contrary, pollen hydration does not rely on pollen coat lipids. Nevertheless, mutation in SlCER6 impairs fertility and floral morphology. Here, the contribution of SlCER6 to the sexual reproduction and flower development of tomato was addressed. Cytological analysis and cross-pollination experiments revealed that the slcer6 mutant has male sterility caused by (1) hampered pollen dispersal and (2) abnormal tapetum development. SlCER6 loss of function provokes a decrease of n- and iso-alkanes with chain lengths of C₂₇ or greater and of anteiso-alkanes with chain lengths of C₂₈ or greater in flower cuticular waxes, but it has no impact on flower cuticle ultrastructure and cutin content. Expression analysis confirmed high transcription levels of SlCER6 in the anther and the petal, preferentially in sites subject to epidermal fusion. Hence, wax deficiency was proposed to be the primary reason for the flower fusion phenomenon in tomato. The SlCER6 substrate specificity was revisited. It might be involved in elongation of not only linear but also branched very-long-chain fatty acids, leading to production of the corresponding alkanes. SlCER6 implements a function in the sexual reproduction of tomato that is different from the one in Arabidopsis: SlCER6 is essential for the regulation of timely tapetum degradation and, consequently, microgametogenesis.

Natural variation in seed very long chain fatty acid content is controlled by a new isoform of KCS18 in Arabidopsis thaliana

Oil from oleaginous seeds is mainly composed of triacylglycerols. Very long chain fatty acids (VLCFAs) are major constituents of triacylglycerols in many seed oils and represent valuable feedstock for industrial purposes. To identify genetic factors governing natural variability in VLCFA biosynthesis, a quantitative trait loci (QTL) analysis using a recombinant inbred line population derived from a cross between accessions Bay-0 and Shahdara was performed in Arabidopsis thaliana. Two fatty acid chain length ratio (CLR) QTL were identified, with one major locus, CLR.2, accounting for 77% of the observed phenotypic variation. A fine mapping and candidate gene approach showed that a key enzyme of the fatty acid elongation pathway, the β-ketoacyl-CoA synthase 18 (KCS18), was responsible for the CLR.2 QTL detected between Bay-0 and Shahdara. Association genetics and heterologous expression in yeast cells identified a single point mutation associated with an alteration of KCS18 activity, uncovering the molecular bases for the modulation of VLCFA content in these two natural populations of Arabidopsis. Identification of this kcs18 mutant with altered activity opens new perspectives for the modulation of oil composition in crop plants.

Characterization of molecular mechanisms controlling fabAB transcription in Pseudomonas aeruginosa

BACKGROUND

The FabAB pathway is one of the unsaturated fatty acid (UFA) synthesis pathways for Pseudomonas aeruginosa. It was previously noted that this operon was upregulated in biofilms and repressed by exogenous UFAs. Deletion of a 30 nt fabA upstream sequence, which is conserved in P. aeruginosa, P. putida, and P. syringae, led to a significant decrease in fabA transcription, suggesting positive regulation by an unknown positive regulatory mechanism.

METHODS/PRINCIPAL FINDINGS

Here, genetic and biochemical approaches were employed to identify a potential fabAB activator. Deletion of candidate genes such as PA1611 or PA1627 was performed to determine if any of these gene products act as a fabAB activator. However, none of these genes were involved in the regulation of fabAB transcription. Use of mariner-based random mutagenesis to screen for fabA activator(s) showed that several genes encoding unknown functions, rpoN and DesA may be involved in fabA regulation, but probably via indirect mechanisms. Biochemical attempts performed did fail to isolate an activator of fabAB operon.

CONCLUSION/SIGNIFICANCE

The data suggest that fabA expression might not be regulated by protein-binding, but by a distinct mechanism such as a regulatory RNA-based mechanism.

Transcriptional activation of Brassica napus β-ketoacyl-ACP synthase II with an engineered zinc finger protein transcription factor

Targeted gene regulation via designed transcription factors has great potential for precise phenotypic modification and acceleration of novel crop trait development. Canola seed oil composition is dictated largely by the expression of genes encoding enzymes in the fatty acid biosynthetic pathway. In the present study, zinc finger proteins (ZFPs) were designed to bind DNA sequences common to two canola β-ketoacyl-ACP Synthase II (KASII) genes downstream of their transcription start site. Transcriptional activators (ZFP-TFs) were constructed by fusing these ZFP DNA-binding domains to the VP16 transcriptional activation domain. Following transformation using Agrobacterium, transgenic events expressing ZFP-TFs were generated and shown to have elevated KASII transcript levels in the leaves of transgenic T(0) plants when compared to 'selectable marker only' controls as well as of T(1) progeny plants when compared to null segregants. In addition, leaves of ZFP-TF-expressing T(1) plants contained statistically significant decreases in palmitic acid (consistent with increased KASII activity) and increased total C18. Similarly, T(2) seed displayed statistically significant decreases in palmitic acid, increased total C18 and reduced total saturated fatty acid contents. These results demonstrate that designed ZFP-TFs can be used to regulate the expression of endogenous genes to elicit specific phenotypic modifications of agronomically relevant traits in a crop species.

Extensive in vivo resilience of persistent Salmonella

Chronic infections caused by persistent pathogens represent an important health problem. Here, we establish a simple practical mouse Salmonella infection model for identifying bacterial maintenance functions that are essential for persistency. In this model, a substantial fraction of Salmonella survived even several days of treatment with a potent fluoroquinolone antibiotic indicating stringency of the model. Evaluation of twelve metabolic defects revealed dramatically different requirements for Salmonella during persistency as compared to acute infections. Disrupted synthesis of unsaturated/cyclopropane fatty acids was the only defect that resulted in rapid Salmonella clearance suggesting that this pathway might contain suitable targets for antimicrobial chemotherapy of chronic infection.

Cloning and characterization of a β-ketoacyl-acyl carrier protein synthase II from Jatropha curcas

A cDNA clone encoding a putative β-ketoacyl-acyl carrier protein (ACP) synthase II (KASII), a key enzyme in fatty acid biosynthesis, was isolated from Jatropha curcas L., a woody oil plant. The isolated cDNA clone of JcKASII contained a 1722-bp open reading frame coding for 573 amino acids with a predicted molecular mass of about 60.98 kDa and the conserved Cys(324) residues that has been proposed as the active site of KASII proteins. The deduced amino acid sequence of the cDNA clone had about 70-84% identity with the KASII from other plants. The transcript of JcKASII was detected in all tissues examined and increased during seed maturation. Expression of JcKASII in the Arabidopsis KASII mutant (fab1) could complement the fatty acid composition of the mutant. Overexpression of JcKASII cDNA under the cauliflower mosaic virus 35S promoter in Arabidopsis resulted in decreasing 16-carbon fatty acids and increasing 18-carbon fatty acids in leaves and seeds. Taken together, these results show that JcKASII could function in 18-carbon fatty acids accumulation in plant and may be useful in the genetic engineering of J. curcas.

Fatty acid elongase is required for shoot development in rice

Organisms are covered extracellularly with cuticular waxes that consist of various fatty acids. In higher plants, extracellular waxes act as indispensable barriers to protect the plants from physical and biological stresses such as drought and pathogen attacks. However, the effect of fatty acid composition on plant development under normal growth conditions is not well understood. Here we show that the ONION1 (ONI1) gene, which encodes a fatty acid elongase (β-ketoacyl CoA synthase) involved in the synthesis of very-long-chain fatty acids, is required for correct fatty acid composition and normal shoot development in rice. oni1 mutants containing a reduced amount of very-long-chain fatty acids produced very small shoots, with an aberrant outermost epidermal cell layer, and ceased to grow soon after germination. These mutants also showed abnormal expression of a KNOX family homeobox gene. ONI1 was specifically expressed in the outermost cell layer of the shoot apical meristem and developing lateral organs. These results show that fatty acid elongase is required for formation of the outermost cell layer, and this layer is indispensable for entire shoot development in rice.

Active site modification of the β-ketoacyl-ACP synthase FabF3 of Streptomyces coelicolor affects the fatty acid chain length of the CDA lipopeptides

Using site directed mutagenesis we altered an active site residue (Phe107) of the enzyme encoded by fabF3 (SCO3248) in the Streptomyces coelicolor gene cluster required for biosynthesis of the calcium dependent antibiotics (CDAs), successfully generating two novel CDA derivatives comprising truncated (C4) lipid side chains and confirming that fabF3 encodes a KAS-II homologue that is involved in determining CDA fatty acid chain length.

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.

De novo biosynthesis of fatty acids plays critical roles in the response of the photosynthetic machinery to low temperature in Arabidopsis

The Arabidopsis thaliana kas3 mutant was isolated based on the hypersensitivity of PSII to low temperature using a Chl fluorescence imaging technique. Chl content was lower in kas3 seedlings cultured at 23 degrees C than in the wild type, but PSII activity was only mildly affected. However, after the chilling treatment at 4 degrees C for 7 d, PSII activity was severely impaired in kas3. PSII was more sensitive to light at 4 degrees C in the presence of lincomycin, suggesting that the kas3 mutation accelerates at least the PSII photodamage. The kas3 mutation causes an amino acid alteration in 3-ketoacyl-ACP synthase III (KasIII), leading to the partial loss of the de novo synthesis pathway for fatty acids in plastids. Consequently, the total fatty acid level was reduced to 75% of the wild-type level in kas3 at 23 degrees C and was further reduced to 60% at 4 degrees C. The composition of fatty acids was also slightly affected in kas3 at both 4 and 23 degrees C. Consistent with the results of the electron transport analysis, the chilling treatment also destabilized PsaA and cytochrome (Cyt) f and D1 in kas3. An analysis of double mutants with pgr1 conditionally defective in Cyt b(6)f activity and with var2 defective in FtsH protease suggested that the kas3 mutation has pleiotropic effects on chloroplast function, probably impacting both the Cyt b(6)f activity and translation in chloroplasts at 23 degrees C. The full activity of KasIII is required for the biogenesis of the intact electron transport machinery in thylakoid membranes and is especially important for the process of responding to low temperature.

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.

FabH selectivity for anteiso branched-chain fatty acid precursors in low-temperature adaptation in Listeria monocytogenes

Gram-positive bacteria, including Listeria monocytogenes, adjust membrane fluidity by shortening the fatty acid chain length and increasing the proportional production of anteiso fatty acids at lower growth temperatures. The first condensation reaction in fatty acid biosynthesis is carried out by beta-ketoacyl-acyl carrier protein synthase III (FabH), which determines the type of fatty acid produced in bacteria. Here, we measured the initial rates of FabH-catalyzed condensation of malonyl-acyl carrier protein and alternate branched-chain precursor acyl-CoAs utilizing affinity-purified His-tagged L. monocytogenes FabH heterologously expressed in Escherichia coli. Listeria monocytogenes FabH showed a preference for 2-methylbutyryl-CoA, the precursor of odd-numbered anteiso fatty acids, at 30 degrees C, which was further increased at a low temperature (10 degrees C), suggesting that temperature-dependent substrate selectivity of FabH underlies the increased formation of anteiso branched-chain fatty acids during low-temperature adaptation. The increased FabH preferential condensation of 2-methylbutyryl-CoA could not be attributed to a significantly higher availability of this fatty acid precursor as acyl-CoA pool levels were reduced similarly for all fatty acid precursors at low temperatures.

Molecular cloning and characterization of a KCS gene from Cardamine graeca and its heterologous expression in Brassica oilseeds to engineer high nervonic acid oils for potential medical and industrial use

Nervonic acid 24:1 Delta15 (cis-tetracos-15-enoic acid) is a very long-chain monounsaturated fatty acid and exists in nature as an elongation product of oleic acid. There is an increasing interest in production of high nervonic acid oils for pharmaceutical, nutraceutical and industrial applications. Using a polymerase chain reaction approach, we have isolated a gene from Cardamine graeca L., which encodes a 3-ketoacyl-CoA synthase (KCS), the first component of the elongation complex involved in synthesis of nervonic acid. Expression of the Cardamine KCS in yeast resulted in biosynthesis of nervonic acid, which is not normally present in yeast cells. We transformed Arabidopsis and Brassica carinata with the Cardamine KCS under the control of the seed-specific promoter, napin. The T(3) generations of transgenic Arabidopsis and B. carinata plants expressing the Cardamine KCS showed that seed-specific expression resulted in relatively large comparative increases in nervonic acid proportions in Arabidopsis seed oil, and 15-fold increase in nervonic acid proportions in B. carinata seed oil. The highest nervonic acid level in transgenic B. carinata lines reached 44%, with only 6% of residual erucic acid. In contrast, similar transgenic expression of the Cardamine KCS in high erucic B. napus resulted in 30% nervonic acid but with 20% residual erucic acid. Experiments using the Lunaria KCS gene gave results similar to the latter. In both cases, the erucic acid content is too high for human or animal consumption. Thus, the Cardamine KCS: B. carinata high nervonic/highly reduced erucic transgenic seed oils will be the most suitable for testing in pharmaceutical/nutraceutical applications to improve human and animal health.

Escherichia coli unsaturated fatty acid synthesis: complex transcription of the fabA gene and in vivo identification of the essential reaction catalyzed by FabB

Although the unsaturated fatty acid (UFA) synthetic pathway of Escherichia coli is the prototype of such pathways, several unresolved issues have accumulated over the years. The key players are the fabA and fabB genes. Earlier studies of fabA transcription showed that the gene was transcribed from two promoters, with one being positively regulated by the FadR protein. The other weaker promoter (which could not be mapped with the technology then available) was considered constitutive because its function was independent of FadR. However, the FabR negative regulator was recently shown to represses fabA transcription. We report that the weak promoter overlaps the FadR-dependent promoter and is regulated by FabR. This promoter is strictly conserved in all E. coli and Salmonella enterica genomes sequenced to date and is thought to provide insurance against inappropriate regulation of fabA transcription by exogenous saturated fatty acids. Also, the fabAup promoter, a mutant promoter previously isolated by selection for increased FabA activity, was shown to be a promoter created de novo by a four-base deletion within the gene located immediately upstream of fabA. Demonstration of the key UFA synthetic reaction catalyzed by FabB has been elusive, although it was known to catalyze an elongation reaction. Strains lacking FabB are UFA auxotrophs indicating that the enzyme catalyzes an essential step in UFA synthesis. Using thioesterases specific for hydrolysis of short chain acyl-ACPs, the intermediates of the UFA synthetic pathway have been followed in vivo for the first time. These experiments showed that a fabB mutant strain accumulated less cis-5-dodecenoic acid than the parental wild-type strain. These data indicate that the key reaction in UFA synthesis catalyzed by FabB is elongation of the cis-3-decenoyl-ACP produced by FabA.

Rhizobium leguminosarum biovar viciae 3841, deficient in 27-hydroxyoctacosanoate-modified lipopolysaccharide, is impaired in desiccation tolerance, biofilm formation and motility

The lipopolysaccharide (LPS) of the Gram-negative legume symbiont Rhizobium leguminosarum biovar viciae 3,841 contains several unique modifications, including the addition of a 27-hydroxyoctacosanoic acid (27OHC28 : 0), also termed the very long chain fatty acid (VLCFA), attached at the 2' position of lipid A. A transposon mutant that lacks expression of two putative 3-oxo-acyl [acyl-carrier protein] synthase II genes, fabF1 and fabF2, from the VLCFA biosynthetic cluster, was isolated and characterized. MS indicated that the lipid A of the mutant lacked the VLCFA modification, and sodium deoxycholate (DOC)-PAGE of the LPS indicated further structural alterations. The mutant was characteristically sensitive to several stresses that would be experienced in the soil environment, such as desiccation and osmotic stresses. An increase in the excretion of neutral surface polysaccharides was observed in the mutant. This mutant was also altered in its attachment to solid surfaces, and was non-motile, with most of the mutant cells lacking flagella. Despite the pleiotropic effects of the mutation, these mutants were still able to nodulate legumes and fix atmospheric nitrogen. This report emphasizes that a structurally intact VLCFA-containing lipid A is critical to cellular traits that are important for survival in the rhizosphere.

Increase in nervonic acid content in transformed yeast and transgenic plants by introduction of a Lunaria annua L. 3-ketoacyl-CoA synthase (KCS) gene

Nervonic acid is a Very Long-Chain Monounsaturated Fatty Acid (VLCMFA), 24:1 Delta15 (cis-tetracos-15-enoic acid) found in the seed oils of Lunaria annua, borage, hemp, Acer (Purpleblow maple) and Tropaeolum speciosum (Flame flower). However, of these, only the "money plant" (Lunaria annua L.) has been studied and grown sparingly for future development as a niche crop and the outlook has been disappointing. Therefore, our goal was to isolate and characterize strategic new genes for high nervonic acid production in Brassica oilseed crops. To this end, we have isolated a VLCMFA-utilizing 3-Keto-Acyl-CoA Synthase (KCS; fatty acid elongase; EC 2.3.1.86) gene from Lunaria annua and functionally expressed it in yeast, with the recombinant KCS protein able to catalyze the synthesis of several VLCMFAs, including nervonic acid. Seed-specific expression of the Lunaria KCS in Arabidopsis resulted in a 30-fold increase in nervonic acid proportions in seed oils, compared to the very low quantities found in the wild-type. Similar transgenic experiments using B. carinata as the host resulted in a 7-10 fold increase in seed oil nervonic acid proportions. KCS enzyme activity assays indicated that upon using (14)C-22:1-CoA as substrate, the KCS activity from developing seeds of transgenic B. carinata was 20-30-fold higher than the low erucoyl-elongation activity exhibited by wild type control plants. There was a very good correlation between the Lun KCS transcript intensity and the resultant 22:1-CoA KCS activity in developing seed. The highest nervonic acid level in transgenic B. carinata expressing the Lunaria KCS reached 30%, compared to 2.8% in wild type plant. In addition, the erucic acid proportions in these transgenic lines were considerably lower than that found in native Lunaria oil. These results show the functional utility of the Lunaria KCS in engineering new sources of high nervonate/reduced erucic oils in the Brassicaceae.

Transcriptomic and reverse genetic analyses of branched-chain fatty acid and acyl sugar production in Solanum pennellii and Nicotiana benthamiana

Acyl sugars containing branched-chain fatty acids (BCFAs) are exuded by glandular trichomes of many species in Solanaceae, having an important defensive role against insects. From isotope-feeding studies, two modes of BCFA elongation have been proposed: (1) fatty acid synthase-mediated two-carbon elongation in the high acyl sugar-producing tomato species Solanum pennellii and Datura metel; and (2) alpha-keto acid elongation-mediated one-carbon increments in several tobacco (Nicotiana) species and a Petunia species. To investigate the molecular mechanisms underlying BCFAs and acyl sugar production in trichomes, we have taken a comparative genomic approach to identify critical enzymatic steps followed by gene silencing and metabolite analysis in S. pennellii and Nicotiana benthamiana. Our study verified the existence of distinct mechanisms of acyl sugar synthesis in Solanaceae. From microarray analyses, genes associated with alpha-keto acid elongation were found to be among the most strongly expressed in N. benthamiana trichomes only, supporting this model in tobacco species. Genes encoding components of the branched-chain keto-acid dehydrogenase complex were expressed at particularly high levels in trichomes of both species, and we show using virus-induced gene silencing that they are required for BCFA production in both cases and for acyl sugar synthesis in N. benthamiana. Functional analysis by down-regulation of specific KAS I genes and cerulenin inhibition indicated the involvement of the fatty acid synthase complex in BCFA production in S. pennellii. In summary, our study highlights both conserved and divergent mechanisms in the production of important defense compounds in Solanaceae and defines potential targets for engineering acyl sugar production in plants for improved pest tolerance.

Wax Crystal-Sparse Leaf1 encodes a beta-ketoacyl CoA synthase involved in biosynthesis of cuticular waxes on rice leaf

Cuticular waxes, forming the plant/atmosphere interface of plants colonizing the terrestrial environment, are complex mixtures of very-long chain fatty acids (VLCFAs) and their derivatives. In VLCFAs biosynthesis, beta-ketoacyl CoA synthase (E.C.2.3.1.119, KCS) is the key enzyme. Using T-DNA insertional mutagenesis, we identified a cuticle-deficient rice mutant, which displayed a pleiotropic phenotype including reduced growth, leaf fusion, sparse wax crystals, enhanced sensitivity to drought and low fertility. Further analysis indicated that T-DNA was inserted in the 5'-UTR intron of the affected gene, Wax Crystal-Sparse Leaf1 (WSL1), and abnormal transcript caused the loss-of-function of WSL1 gene. Genetic complementation experiment confirmed the function of the candidate gene. WSL1 was predicted to encode a polypeptide containing a conserved FAE1_CUT1_RppA domain typical of the KCS family proteins. Qualitative and quantitative wax composition analyses by gas chromatography-mass spectrometry (GC-MS) demonstrated a marked reduction of total cuticular wax load on wsl1 leaf blades and sheaths, and VLCFA precursors of C20-C24 decreased in both. Moreover, ubiquitous expression of the WSL1 gene gave a hint that WSL1-catalyzed elongation of VLCFAs might participate in a wide range of rice growth and development processes beyond biosynthesis of cuticular waxes.

Wax Crystal-Sparse Leaf1 encodes a beta-ketoacyl CoA synthase involved in biosynthesis of cuticular waxes on rice leaf

Cuticular waxes, forming the plant/atmosphere interface of plants colonizing the terrestrial environment, are complex mixtures of very-long chain fatty acids (VLCFAs) and their derivatives. In VLCFAs biosynthesis, beta-ketoacyl CoA synthase (E.C.2.3.1.119, KCS) is the key enzyme. Using T-DNA insertional mutagenesis, we identified a cuticle-deficient rice mutant, which displayed a pleiotropic phenotype including reduced growth, leaf fusion, sparse wax crystals, enhanced sensitivity to drought and low fertility. Further analysis indicated that T-DNA was inserted in the 5'-UTR intron of the affected gene, Wax Crystal-Sparse Leaf1 (WSL1), and abnormal transcript caused the loss-of-function of WSL1 gene. Genetic complementation experiment confirmed the function of the candidate gene. WSL1 was predicted to encode a polypeptide containing a conserved FAE1_CUT1_RppA domain typical of the KCS family proteins. Qualitative and quantitative wax composition analyses by gas chromatography-mass spectrometry (GC-MS) demonstrated a marked reduction of total cuticular wax load on wsl1 leaf blades and sheaths, and VLCFA precursors of C20-C24 decreased in both. Moreover, ubiquitous expression of the WSL1 gene gave a hint that WSL1-catalyzed elongation of VLCFAs might participate in a wide range of rice growth and development processes beyond biosynthesis of cuticular waxes.

Separate entrance and exit portals for ligand traffic in Mycobacterium tuberculosis FabH

Mycobacterium tuberculosis FabH initiates type II fatty acid synthase-catalyzed formation of the long chain (C(16)-C(22)) acyl-coenzyme A (CoA) precursors of mycolic acids, which are major constituents of the bacterial cell envelope. Crystal structures of M. tuberculosis FabH (mtFabH) show the substrate binding site to be a buried, extended L-shaped channel with only a single solvent access portal. Entrance of an acyl-CoA substrate through the solvent portal would require energetically unfavorable reptational threading of the substrate to its reactive position. Using a class of FabH inhibitors, we have tested an alternative hypothesis that FabH exists in an "open" form during substrate binding and product release, and a "closed" form in which catalysis and intermediate steps occur. This hypothesis is supported by mass spectrometric analysis of the product profile and crystal structures of complexes of mtFabH with these inhibitors.

Molecular cloning and expression analysis of a gene encoding a putative beta-ketoacyl-acyl carrier protein (ACP) synthase III (KAS III) from Jatropha curcas

A cDNA clone encoding a putative beta-ketoacyl-acyl carrier protein (ACP) synthase III (KAS III) was isolated from Jatropha curcas L., a woody oil plant. The cDNA clone (named JcKAS III) contained a 1203-bp open reading frame coding for 400 amino acids with a predicted molecular mass of about 42 kDa. The deduced amino acid sequence of the cDNA clone shares about 80% identity to KAS III from other plants, and contains a conserved Cys(176) in the active site and the amino acid motif G(355)NTSAAS(361) which is responsible for binding regulatory acyl-ACPs. Southern blotting analysis indicated that JcKAS III is a single copy gene in the J. curcas genome. Quantitative real-time PCR analysis showed that JcKAS III was expressed in all tissues examined with highest expression in roots, and that expression of JcKAS III increased as seeds developed.

Structure of 3-oxoacyl-(acyl-carrier protein) synthase II from Thermus thermophilus HB8

The beta-ketoacyl-(acyl carrier protein) synthases (beta-keto-ACP synthases; KAS) catalyse the addition of two-carbon units to the growing acyl chain during the elongation phase of fatty-acid synthesis. As key regulators of bacterial fatty-acid synthesis, they are promising targets for the development of new antibacterial agents. The crystal structure of 3-oxoacyl-ACP synthase II from Thermus thermophilus HB8 (TtKAS II) has been solved by molecular replacement and refined at 2.0 A resolution. The crystal is orthorhombic, space group P2(1)2(1)2, with unit-cell parameters a = 72.07, b = 185.57, c = 62.52 A, and contains one homodimer in the asymmetric unit. The subunits adopt the well known alpha-beta-alpha-beta-alpha thiolase fold that is common to ACP synthases. The structural and sequence similarities of TtKAS II to KAS I and KAS II enzymes of known structure from other sources support the hypothesis of comparable enzymatic activity. The dimeric state of TtKAS II is important to create each fatty-acid-binding pocket. Closer examination of KAS structures reveals that compared with other KAS structures in the apo form, the active site of TtKAS II is more accessible because of the ;open' conformation of the Phe396 side chain.

Expression and characterization of polyketide synthase module involved in the late step of cephabacin biosynthesis from Lysobacter lactamgenus

The cephabacins produced by Lysobacter lactamgenus are beta-lactam antibiotics composed of a cephem nucleus, an acetate residue, and an oligopeptide side chain. In order to understand the precise implication of the polyketide synthase (PKS) module in the biosynthesis of cephabacin, the genes for its core domains, beta-ketoacyl synthase (KS), acyltransferase (AT), and acyl carrier protein (ACP), were amplified and cloned into the pET-32b(+) expression vector. The sfp gene encoding a protein that can modify apo-ACP to its active holo-form was also amplified. The recombinant KS, AT, apo-ACP, and Sfp overproduced in the form of His6-tagged fusion proteins in E. coli BL21(DE3) were purified by nickel-affinity chromatography. Formation of stable peptidyl-S-KS was observed by in vitro acylation of the KS domain with the substrate [L-Ala-L-Ala-LAla- L-3H-Arg] tetrapeptide-S-N-acetylcysteamine, which is the evidence for the selective recognition of tetrapeptide produced by nonribosomal peptide synthetase (NRPS) in the NRPS/ PKS hybrid. In order to confirm whether malonyl CoA is the extender unit for acetylation of the peptidyl moiety, the AT domain, ACP domain, and Sfp protein were treated with 14C-malonyl-CoA. The results clearly show that the AT domain is able to recognize the extender unit and decarboxylatively acetylated for the elongation of the tetrapeptide. However, the transfer of the activated acetyl group to the ACP domain was not observed, probably attributed to the improper capability of Sfp to activate apo-ACP to the holo-ACP form.

Virus-induced silencing of Comt, pAmt and Kas genes results in a reduction of capsaicinoid accumulation in chili pepper fruits

Capsaicinoids are responsible for the pungent taste of chili pepper fruits of Capsicum species. Capsaicinoids are biosynthesized through both the phenylpropanoid and the branched-fatty acids pathways. Fragments of Comt (encoding a caffeic acid O-methyltransferase), pAmt (a putative aminotransferase), and Kas (a beta-keto-acyl-[acyl-carrier-protein] synthase) genes, that are differentially expressed in placenta tissue of pungent chili pepper, were individually inserted into a Pepper huasteco yellow veins virus (PHYVV)-derived vector to determine, by virus-induced gene silencing, irrespective of whether these genes are involved in the biosynthesis of capsaicinoids. Reduction of the respective mRNA levels as well as the presence of related siRNAs confirmed the silencing of these three genes. Morphological alterations were evident in plants inoculated with PHYVV::Comt and PHYVV::Kas constructs; however, plants inoculated with PHYVV::pAmt showed no evident alterations. On the other hand, fruit setting was normal in all cases. Biochemical analysis of placenta tissues showed that, indeed, independent silencing of all three genes led to a dramatic reduction in capsaicinoid content in the fruits demonstrating the participation of these genes in capsaicinoid biosynthesis. Using this approach it was possible to generate non-pungent chili peppers at high efficiency.