Herbicide sensitivity determinant of wheat plastid acetyl-CoA carboxylase is located in a 400-amino acid fragment of the carboxyltransferase domain

Biochemical and structural characterization of quizalofop-resistant wheat acetyl-CoA carboxylase

A novel nucleotide mutation in ACC1 resulting in an alanine to valine amino acid substitution in acetyl-CoA carboxylase (ACCase) at position 2004 of the Alopecurus myosuroides reference sequence (A2004V) imparts quizalofop resistance in wheat. Genotypes endowed with the homozygous mutation in one or two ACC1 homoeologs are seven- and 68-fold more resistant to quizalofop than a wildtype winter wheat in greenhouse experiments, respectively. In vitro ACCase activities in soluble protein extracts from these varieties are 3.8- and 39.4-fold more resistant to quizalofop with the homozygous mutation in either one or two genomes, relative to the wildtype. The A2004V mutation does not alter the specific activity of wheat ACCase, suggesting that this resistance trait does not affect the catalytic functions of ACCase. Modeling of wildtype and quizalofop-resistant wheat ACCase demonstrates that the A2004V amino acid substitution causes a reduction in the volume of the binding pocket that hinders quizalofop’s interaction with ACCase. Docking studies confirm that the mutation reduces the binding affinity of quizalofop. Interestingly, the models suggest that the A2004V mutation does not affect haloxyfop binding. Follow up in vivo and in vitro experiments reveal that the mutation, in fact, imparts negative cross-resistance to haloxyfop, with quizalofop-resistant varieties exhibiting higher sensitivity to haloxyfop than the wildtype winter wheat line.

Non-target-Site Resistance in Lolium spp. Globally: A Review

The Lolium genus encompasses many species that colonize a variety of disturbed and non-disturbed environments. Lolium perenne L. spp. perenne, L. perenne L. spp. multiflorum, and L. rigidum are of particular interest to weed scientists because of their ability to thrive in agricultural and non-agricultural areas. Herbicides are the main tool to control these weeds; however, Lolium spp. populations have evolved multiple- and cross-resistance to at least 14 herbicide mechanisms of action in more than 21 countries, with reports of multiple herbicide resistance to at least seven mechanisms of action in a single population. In this review, we summarize what is currently known about non-target-site resistance in Lolium spp. to acetyl CoA carboxylase, acetohydroxyacid synthase, microtubule assembly, photosystem II, 5-enolpyruvylshikimate-3-phosphate synthase, glutamine synthetase, very-long chain fatty acids, and photosystem I inhibitors. We suggest research topics that need to be addressed, as well as strategies to further our knowledge and uncover the mechanisms of non-target-site resistance in Lolium spp.

Impacts on the seagrass, Zostera nigricaulis, from the herbicide Fusilade Forte® used in the management of Spartina anglica infestations

The herbicide Fusilade Forte^® (FF) is widely applied in agricultural weed management and in the management of the invasive saltmarsh grass, Spartina anglica (ricegrass or cordgrass). FF (active ingredient fluazifop-P acid, FPA) is selective for poaceous grasses. Its primary mode of action is inhibition of the acetyl coenzyme-A carboxylase (ACCase) specific to this taxonomic group, and its secondary mode is by promotion of oxidative stress. FF is applied to S. anglica infestations in the intertidal zone, in proximity to seagrass meadows. Despite the potential for vital seagrass ecosystems to be exposed to FF, there is limited knowledge of any potential impacts. We investigated impacts of FPA on the endemic Australian seagrass, Zostera nigricaulis, measuring ACCase activity and parameters that reflect oxidative stress: photosynthetic performance, lipid peroxidation and photosynthetic pigment content. Seagrass was exposed to FF (0.01-10mgL^-1 FPA and a control) for 7d, followed by a 7-d recovery in uncontaminated seawater. An enzyme assay demonstrated that FPA ≤10mgL^-1 did not inhibit the activity of ACCase isolated from Z. nigricaulis, demonstrating that this seagrass is resistant to FF's primary mode of action. However, physiological impacts occurred following 7 days exposure to ≥0.1mgL^-1 FPA, including up to a 72% reduction in photosynthetic pigment concentration. After 7-d recovery, photosynthetic pigment content improved in treatment plants; however, treated plants exhibited higher levels of lipid peroxidation. This study demonstrates that while Z. nigricaulis is resistant to FF's primary mode of action, significant physiological impacts occur following 7 days exposure to ≥0.1mgL^-1 FPA. This study provides valuable information on the effects of FF on a non-target species that can better inform approaches to Spartina management in coastal seagrass ecosystems.

Bacterial fatty acid metabolism in modern antibiotic discovery

Bacterial fatty acid synthesis is essential for many pathogens and different from the mammalian counterpart. These features make bacterial fatty acid synthesis a desirable target for antibiotic discovery. The structural divergence of the conserved enzymes and the presence of different isozymes catalyzing the same reactions in the pathway make bacterial fatty acid synthesis a narrow spectrum target rather than the traditional broad spectrum target. Furthermore, bacterial fatty acid synthesis inhibitors are single-targeting, rather than multi-targeting like traditional monotherapeutic, broad-spectrum antibiotics. The single-targeting nature of bacterial fatty acid synthesis inhibitors makes overcoming fast-developing, target-based resistance a necessary consideration for antibiotic development. Target-based resistance can be overcome through multi-targeting inhibitors, a cocktail of single-targeting inhibitors, or by making the single targeting inhibitor sufficiently high affinity through a pathogen selective approach such that target-based mutants are still susceptible to therapeutic concentrations of drug. Many of the pathogens requiring new antibiotic treatment options encode for essential bacterial fatty acid synthesis enzymes. This review will evaluate the most promising targets in bacterial fatty acid metabolism for antibiotic therapeutics development and review the potential and challenges in advancing each of these targets to the clinic and circumventing target-based resistance. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.

Mutations in the plastidic ACCase gene endowing resistance to ACCase-inhibiting herbicide in Phalaris minor populations from India

Littleseed canarygrass (Phalaris minor Retz.) is one of the most common and troublesome weeds infesting wheat crop in India. Repeated use during the last two decades of the ACCase-inhibiting herbicide (clodinafop) to control this weed has resulted in the occurrence of resistance. Fifty-three P. minor populations were collected from wheat fields in Haryana and Punjab states of India. The dose-response assays indicated that 29 populations were resistant, 23 populations were susceptible and one population was moderately resistant to clodinafop. Sequence analysis of the CT domain of ACCase gene among resistant and susceptible populations revealed two non-synonymous mutations, Trp₂₀₂₇ to Cys and Ile₂₀₄₁ to Asn in the resistant populations. Allele-specific PCR markers were developed to differentiate between wild-type and resistant codons at positions 2027 and 2041 of ACCase in P. minor which enables molecular assays for rapid detection and resistance diagnosis for efficient weed management in wheat. This is the first report from India of a target site mutation corresponding to resistance to clodinafop in P. minor.

Resistance to acetyl-CoA carboxylase-inhibiting herbicides

Resistance to acetyl-CoA carboxylase herbicides is documented in at least 43 grass weeds and is particularly problematic in Lolium, Alopecurus and Avena species. Genetic studies have shown that resistance generally evolves independently and can be conferred by target-site mutations at ACCase codon positions 1781, 1999, 2027, 2041, 2078, 2088 and 2096. The level of resistance depends on the herbicides, recommended field rates, weed species, plant growth stages, specific amino acid changes and the number of gene copies and mutant ACCase alleles. Non-target-site resistance, or in essence metabolic resistance, is prevalent, multigenic and favoured under low-dose selection. Metabolic resistance can be specific but also broad, affecting other modes of action. Some target-site and metabolic-resistant biotypes are characterised by a fitness penalty. However, the significance for resistance regression in the absence of ACCase herbicides is yet to be determined over a practical timeframe. More recently, a fitness benefit has been reported in some populations containing the I1781L mutation in terms of vegetative and reproductive outputs and delayed germination. Several DNA-based methods have been developed to detect known ACCase resistance mutations, unlike metabolic resistance, as the genes remain elusive to date. Therefore, confirmation of resistance is still carried out via whole-plant herbicide bioassays. A growing number of monocotyledonous crops have been engineered to resist ACCase herbicides, thus increasing the options for grass weed control. While the science of ACCase herbicide resistance has progressed significantly over the past 10 years, several avenues provided in the present review remain to be explored for a better understanding of resistance to this important mode of action.

Structure, activity, and inhibition of the Carboxyltransferase β-subunit of acetyl coenzyme A carboxylase (AccD6) from Mycobacterium tuberculosis

In Mycobacterium tuberculosis, the carboxylation of acetyl coenzyme A (acetyl-CoA) to produce malonyl-CoA, a building block in long-chain fatty acid biosynthesis, is catalyzed by two enzymes working sequentially: a biotin carboxylase (AccA) and a carboxyltransferase (AccD). While the exact roles of the three different biotin carboxylases (AccA1 to -3) and the six carboxyltransferases (AccD1 to -6) in M. tuberculosis are still not clear, AccD6 in complex with AccA3 can synthesize malonyl-CoA from acetyl-CoA. A series of 10 herbicides that target plant acetyl-CoA carboxylases (ACC) were tested for inhibition of AccD6 and for whole-cell activity against M. tuberculosis. From the tested herbicides, haloxyfop, an arylophenoxypropionate, showed in vitro inhibition of M. tuberculosis AccD6, with a 50% inhibitory concentration (IC50) of 21.4 ± 1 μM. Here, we report the crystal structures of M. tuberculosis AccD6 in the apo form (3.0 Å) and in complex with haloxyfop-R (2.3 Å). The structure of M. tuberculosis AccD6 in complex with haloxyfop-R shows two molecules of the inhibitor bound on each AccD6 subunit. These results indicate the potential for developing novel therapeutics for tuberculosis based on herbicides with low human toxicity.

Pantothenic acid biosynthesis in the parasite Toxoplasma gondii: a target for chemotherapy

Toxoplasma gondii is a major food pathogen and neglected parasitic infection that causes eye disease, birth defects, and fetal abortion and plays a role as an opportunistic infection in AIDS. In this study, we investigated pantothenic acid (vitamin B5) biosynthesis in T. gondii. Genes encoding the full repertoire of enzymes for pantothenate synthesis and subsequent metabolism to coenzyme A were identified and are expressed in T. gondii. A panel of inhibitors developed to target Mycobacterium tuberculosis pantothenate synthetase were tested and found to exhibit a range of values for inhibition of T. gondii growth. Two inhibitors exhibited lower effective concentrations than the currently used toxoplasmosis drug pyrimethamine. The inhibition was specific for the pantothenate pathway, as the effect of the pantothenate synthetase inhibitors was abrogated by supplementation with pantothenate. Hence, T. gondii encodes and expresses the enzymes for pantothenate synthesis, and this pathway is essential for parasite growth. These promising findings increase our understanding of growth and metabolism in this important parasite and highlight pantothenate synthetase as a new drug target.

Micronucleus assay in human lymphocytes after exposure to alloxydim sodium herbicide in vitro

This study evaluates the cytotoxic and genotoxic potential of alloxydim sodium using micronucleus (MN) assay, in human peripheral lymphocytes. MN assay was used to investigate the genotoxic effects of alloxydim sodium in human peripheral lymphocytes treated with 250, 500, 750, 1,000 µg/ml concentrations of alloxydim sodium for 24 and 48 h. Solvent, negative and positive controls were also used in the experiments in parallel. The obtained results were evaluated in statistical analyses by using Dunnett-t test (two sided) and p < 0.05 was accepted as significant. Alloxydim sodium significantly increased the MN formation compared with the negative control, at both 750 and 1,000 µg/ml concentrations and treatment periods. We also evaluated the nuclear division index (NDI) for cytotoxicity of this pesticide in the experiment, and finally observed a significant decrease of the NDI values at all concentrations of alloxydim sodium and at both treatment periods.

Resistance to herbicides caused by single amino acid mutations in acetyl-CoA carboxylase in resistant populations of grassy weeds

Eleven spontaneous mutations of acetyl-CoA carboxylase have been identified in many herbicide-resistant populations of 42 species of grassy weeds, hampering application of aryloxyphenoxypropionate, cyclohexadione and phenylpyrazoline herbicides in agriculture. IC(50) shifts (resistance indices) caused by herbicide-resistant mutations were determined using a recombinant yeast system that allows comparison of the effects of single amino acid mutations in the same biochemical background, avoiding the complexity inherent in the in planta experiments. The effect of six mutations on the sensitivity of acetyl-CoA carboxylase to nine herbicides representing the three chemical classes was studied. A combination of partially overlapping binding sites of the three classes of herbicides and the structure of their variable parts explains cross-resistance among and between the three classes of inhibitors, as well as differences in their specificity. Some degree of resistance was detected for 51 of 54 herbicide/mutation combinations. Introduction of new herbicides targeting acetyl-CoA carboxylase will depend on their ability to overcome the high degree of cross-resistance already existing in weed populations.

Molecular bases for resistance to acetyl-coenzyme A carboxylase inhibitor in Japanese foxtail (Alopecurus japonicus)

BACKGROUND

Haloxyfop-R-methyl is a widely used herbicide to control Poaceae weeds. Alopecurus japonicus, a widespread annual grass, can no longer be controlled by haloxyfop-R-methyl after continuous use of this herbicide for several years.

RESULTS

Dose-response experiments have established that the Js-R biotype of A. japonicas has evolved resistance to aryloxyphenoxypropionates (APPs). Target-site enzyme sensitivity experiments have established that the haloxyfop (free acid) rate causing 50% inhibition of acetyl-CoA carboxylase (ACCase) activity (I(50)) for the resistant (Js-R) biotype is 11 times higher than that for the susceptible (Js-S) biotype. In many cases, resistance to ACCase-inhibiting herbicides is due to a resistant ACCase enzyme. Full-length DNA and mRNA sequences of the plastidic ACCase gene were amplified. Eight single-nucleotide differences were detected in this region. Four of the nucleotide changes were silent mutations. However, the other four nucleotide mutations caused four amino acid substitutions, replacing Arg-1734 with Gly, Met-1738 with Leu, Thr-1739 with Ser and Ile-2041 with Asn in the R biotype respectively; the substitution at position 2041 had been reported, while the other three had not.

CONCLUSION

The ACCase in the Js-R biotype was less susceptible to haloxyfop-R-methyl than that in the Js-S biotype. Moreover, the amino acid substitution of Ile-2041 with Asn might confer resistance to haloxyfop-R-methyl in A. japonicas.

Recombinant yeast screen for new inhibitors of human acetyl-CoA carboxylase 2 identifies potential drugs to treat obesity

Acetyl-CoA carboxylase (ACC) is a key enzyme of fatty acid metabolism with multiple isozymes often expressed in different eukaryotic cellular compartments. ACC-made malonyl-CoA serves as a precursor for fatty acids; it also regulates fatty acid oxidation and feeding behavior in animals. ACC provides an important target for new drugs to treat human diseases. We have developed an inexpensive nonradioactive high-throughput screening system to identify new ACC inhibitors. The screen uses yeast gene-replacement strains depending for growth on cloned human ACC1 and ACC2. In "proof of concept" experiments, growth of such strains was inhibited by compounds known to target human ACCs. The screen is sensitive and robust. Medium-size chemical libraries yielded new specific inhibitors of human ACC2. The target of the best of these inhibitors was confirmed with in vitro enzymatic assays. This compound is a new drug chemotype inhibiting human ACC2 with 2.8 muM IC(50) and having no effect on human ACC1 at 100 muM.

Single-site mutations in the carboxyltransferase domain of plastid acetyl-CoA carboxylase confer resistance to grass-specific herbicides

Grass weed populations resistant to aryloxyphenoxypropionate (APP) and cyclohexanedione herbicides that inhibit acetyl-CoA carboxylase (ACCase; EC 6.4.1.2) represent a major problem for sustainable agriculture. We investigated the molecular basis of resistance to ACCase-inhibiting herbicides for nine wild oat (Avena sterilis ssp. ludoviciana Durieu) populations from the northern grain-growing region of Australia. Five amino acid substitutions in plastid ACCase were correlated with herbicide resistance: Ile-1,781-Leu, Trp-1,999-Cys, Trp-2,027-Cys, Ile-2,041-Asn, and Asp-2,078-Gly (numbered according to the Alopecurus myosuroides plastid ACCase). An allele-specific PCR test was designed to determine the prevalence of these five mutations in wild oat populations suspected of harboring ACCase-related resistance with the result that, in most but not all cases, plant resistance was correlated with one (and only one) of the five mutations. We then showed, using a yeast gene-replacement system, that these single-site mutations also confer herbicide resistance to wheat plastid ACCase: Ile-1,781-Leu and Asp-2,078-Gly confer resistance to APPs and cyclohexanediones, Trp-2,027-Cys and Ile-2,041-Asn confer resistance to APPs, and Trp-1,999-Cys confers resistance only to fenoxaprop. These mutations are very likely to confer resistance to any grass weed species under selection imposed by the extensive agricultural use of the herbicides.

Interactions of aryloxyphenoxypropionic acids with sensitive and resistant acetyl-coenzyme a carboxylase by homology modeling and molecular dynamic simulations

Acetyl-coenzyme A carboxylase (ACCase) has been identified as one of the most important targets of herbicides. In the present study, we constructed homology models of the carboxyl-transferase (CT) domain of ACCase from sensitive and resistant foxtail and used these models as templates to study the molecular mechanism of herbicide resistance and stereochemistry-activity relationships of aryloxyphenoxypropionates (APPs). In the homology modeling structures, the dimer of the CT domain was formed by the side-to-side arrangement of the two monomers, in such a way that the N domain of one molecule is placed next to the C domain of the other. The dimeric association of sensitive foxtail CT was found to differ from that of resistant foxtail CT, and the spatial orientation of two key residues, Leu-695 and Ile-695, in these dimers also differed. The mutation of Ile to Leu may perturb the conformation of the dimeric interface, which may account for the molecular mechanism of herbicide resistance. Further docking analysis indicated that the binding model of high-active compounds is similar to that in the crystal structure of the enzyme-ligand complex. The different spatial orientations of ester groups of the isomers of APPs may explain the stereochemistry-activity relationship. Ser-698 formed a H-bonding interaction with all of the docked ligands, while Tyr-728 formed a pi-pi stacking interaction with some of the APPs. These findings may enhance our understanding of the molecular mechanism of herbicide resistance and stereochemistry-activity relationships, which may provide a new starting point for the identification of more potent inhibitors against both sensitive and resistant ACCase.

Inhibiting Bacterial Fatty Acid Synthesis

The type II fatty acid synthase consists of a series of individual enzymes, each encoded by a separate gene, that catalyze discrete steps in chain elongation. The formation of fatty acids is vital to bacteria, and each of the essential enzymes and their acyl group carriers represent a potential target for the development of novel antibacterial therapeutics. High resolution x-ray and/or NMR structures of representative members of every enzyme in the type II pathway are now available, and these structures are a valuable resource to guide antibacterial drug discovery. The role of each enzyme in regulating pathway activity and the diversity in the components of the pathway in the major human pathogens are important considerations in deciding the most suitable targets for future drug development.

The molecular bases for resistance to acetyl co-enzyme A carboxylase (ACCase) inhibiting herbicides in two target-based resistant biotypes of annual ryegrass (Lolium rigidum)

Acetyl-CoA carboxylase (ACCase) (EC.6.4.1.2) is an essential enzyme in fatty acid biosynthesis and, in world agriculture, commercial herbicides target this enzyme in plant species. In nearly all grass species the plastidic ACCase is strongly inhibited by commercial ACCase inhibiting herbicides [aryloxyphenoxypropionate (APP) and cyclohexanedione (CHD) herbicide chemicals]. Many ACCase herbicide resistant biotypes (populations) of L. rigidum have evolved, especially in Australia. In many cases, resistance to ACCase inhibiting herbicides is due to a resistant ACCase enzyme. Two ACCase herbicide resistant L. rigidum biotypes were studied to identify the molecular basis of ACCase inhibiting herbicide resistance. The carboxyl-transferase (CT) domain of the plastidic ACCase gene was amplified by PCR and sequenced. Amino acid substitutions in the CT domain were identified by comparison of sequences from resistant and susceptible plants. The amino acid residues Gln-102 (CAG codon) and Ile-127 (ATA codon) were substituted with a Glu residue (GAG codon) and Leu residue (TTA codon), respectively, in both resistant biotypes. Amino acid positions 102 and 127 within the fragment sequenced from L. rigidum corresponded to amino acid residues 1756 and 1781, respectively, in the A. myosuroides full ACCase sequence. Allele-specific PCR results further confirmed the mutations linked with resistance in these populations. The Ile-to-Leu substitution at position 1781 has been identified in other resistant grass species as endowing resistance to APP and CHD herbicides. The Gln-to-Glu substitution at position 1756 has not previously been reported and its role in herbicide resistance remains to be established.

Six amino acid substitutions in the carboxyl-transferase domain of the plastidic acetyl-CoA carboxylase gene are linked with resistance to herbicides in a Lolium rigidum population

The molecular basis of an acetyl-CoA carboxylase (ACCase) target-based resistant Lolium rigidum population (WLR 96) was studied here. The carboxyl-transferase domain of the plastidic ACCase gene from resistant individuals was amplified by PCR and sequenced. The DNA sequences were aligned and compared with a susceptible population. Six amino acid substitutions were identified in the resistant population. The substitution Ile-2041-Asn, known to confer resistance to ACCase-inhibiting herbicides aryloxyphenoxypropionate (APP) in Alopecurus myosuroides, was identified in most resistant plants but it is always linked with other amino acid substitutions. This was confirmed by a cleaved amplified polymorphism (CAP) marker and an allele-specific PCR. The sole amino acid substitution Ile-2041-Asn was not found in this population. It is likely this mutation evolved later among individuals already possessing the other substitutions. Three haplotypes were identified from the resistant population based on the six amino acid combinations, and two are linked with herbicide resistance in this population. The multiple amino acid substitutions including the Ile-2041-Asn form the molecular basis endowing a high degree of resistance to ACCase-inhibiting herbicides in this L. rigidum population.

Fatty acid biosynthesis as a target for novel antibacterials

The bacterial fatty acid synthesis pathway has significant potential as a target for the development of novel antibacterials. The pathway has been extensively studied in Escherichia coli, the crystal structures of the compounds involved are known and homologous genes are readily identified in the genomes of important pathogens. The currently used drugs triclosan and isoniazid are known to target one step in the pathway. Other experimental compounds such as thiolactomycin and cerulenin effectively inhibit other steps. These known pathway inhibitors are reviewed and the areas for potential future developments are explored.

Graminicide insensitivity correlates with herbicide-binding co-operativity on acetyl-CoA carboxylase isoforms

The sensitivity of grass species to important classes of graminicide herbicides inhibiting ACCase (acetyl-CoA carboxylase) is associated with a specific inhibition of the multifunctional ACCase located in the plastids of grasses. In contrast, the multisubunit form of ACCase found in the chloroplasts of dicotyledonous plants is insensitive and the minor cytosolic multifunctional isoforms of the enzyme in both types of plants are also less sensitive to inhibition. We have isolated, separated and characterized the multifunctional ACCase isoforms found in exceptional examples of grasses that are either inherently insensitive to these graminicides, or from biotypes showing acquired resistance to their use. Major and minor multifunctional enzymes were isolated from cell suspension cultures of Festuca rubra and the 'Notts A1'-resistant biotype of Alopecurus myosuroides, and their properties compared with those isolated from cells of wild-type sensitive A. myosuroides or from sensitive maize. Purifications of up to 300-fold were necessary to separate the two isoforms. The molecular masses (200-230 kDa) and K(m) values for all three substrates (ATP, bicarbonate and acetyl-CoA) were similar for the different ACCases, irrespective of their graminicide sensitivity. Moreover, we found no correlation between the ability of isoforms to carboxylate propionyl-CoA and their sensitivity to graminicides. However, insensitive purified forms of ACCase were characterized by herbicide-binding co-operativity, whereas, in contrast, sensitive forms of the enzymes were not. Our studies on isolated individual isoforms of ACCase from grasses support and extend previous indications that herbicide binding co-operativity is the only kinetic property that differentiates naturally or selected insensitive enzymes from the typical sensitive forms usually found in grasses.

Cyclohexanedione herbicides are inhibitors of rat heart acetyl-CoA carboxylase

Acetyl CoA carboxylase (ACC) catalyzes the carboxylation of acetyl CoA to form malonyl CoA. In skeletal muscle and heart, malonyl CoA functions to regulate lipid oxidation by inhibition of carnitine palmitoyltransferase-1, an enzyme which controls the entry of long chain fatty acids into mitochondria. We have found that several members of the cyclohexanedione class of herbicides are competitive inhibitors of rat heart ACC. These compounds constitute valuable reagents for drug development and the study of ACCbeta, a validated anti-obesity target.

Apicoplast fatty acid biosynthesis as a target for medical intervention in apicomplexan parasites

New chemotherapies for human and animal apicomplexan infections are needed as a component of future strategies to deal with these diseases. An extensive search for new treatments exploring the unique developmental physiology, metabolism and molecular structures of Apicomplexa is under way. The description of the full complement of about 5,300 Plasmodium falciparum genes and fast growing sequence databases for other Apicomplexa allow reconstruction of metabolic pathways of these parasites and thus accelerate identification and biochemical analysis of potential targets. The apicoplast de novo fatty acid biosynthetic pathway shows great potential as a target for small-molecule inhibitors in a stand-alone or combination chemotherapy. Three enzymatic activities, acetyl-CoA carboxylase, beta-ketoacyl-ACP synthase and enoyl-ACP reductase, respond to inhibitors previously identified for bacteria and plants, and deserve to be explored in depth. In this connection, screening systems have been established to seek more potent and specific antiparasitic compounds that are harmless to the host. To this end the interconnections of fatty acid biosynthesis in Apicomplexa with other metabolic and cellular processes must be investigated.

An isoleucine residue within the carboxyl-transferase domain of multidomain acetyl-coenzyme A carboxylase is a major determinant of sensitivity to aryloxyphenoxypropionate but not to cyclohexanedione inhibitors

A 3,300-bp DNA fragment encoding the carboxyl-transferase domain of the multidomain, chloroplastic acetyl-coenzyme A carboxylase (ACCase) was sequenced in aryloxyphenoxypropionate (APP)-resistant and -sensitive Alopecurus myosuroides (Huds.). No resistant plant contained an Ile-1,781-Leu substitution, previously shown to confer resistance to APPs and cyclohexanediones (CHDs). Instead, an Ile-2,041-Asn substitution was found in resistant plants. Phylogenetic analysis of the sequences revealed that Asn-2,041 ACCase alleles derived from several distinct origins. Allele-specific polymerase chain reaction associated the presence of Asn-2,041 with seedling resistance to APPs but not to CHDs. ACCase enzyme assays confirmed that Asn-2,041 ACCase activity was moderately resistant to CHDs but highly resistant to APPs. Thus, the Ile-2,041-Asn substitution, which is located outside a domain previously shown to control sensitivity to APPs and CHDs in wheat (Triticum aestivum), is a direct cause of resistance to APPs only. In known multidomain ACCases, the position corresponding to the Ile/Asn-2,041 residue in A. myosuroides is occupied by an Ile or a Val residue. In Lolium rigidum (Gaud.), we found Ile-Asn and Ile-Val substitutions. The Ile-Val change did not confer resistance to the APP clodinafop, whereas the Ile-Asn change did. The position and the particular substitution at this position are of importance for sensitivity to APPs.

Plant biotin-containing carboxylases

Biotin-containing proteins are found in all forms of life, and they catalyze carboxylation, decarboxylation, or transcarboxylation reactions that are central to metabolism. In plants, five biotin-containing proteins have been characterized. Of these, four are catalysts, namely the two structurally distinct acetyl-CoA carboxylases (heteromeric and homomeric), 3-methylcrotonyl-CoA carboxylase and geranoyl-CoA carboxylase. In addition, plants contain a noncatalytic biotin protein that accumulates in seeds and is thought to play a role in storing biotin. Acetyl-CoA carboxylases generate two pools of malonyl-CoA, one in plastids that is the precursor for de novo fatty acid biosynthesis and the other in the cytosol that is the precursor for fatty acid elongation and a large number of secondary metabolites. 3-Methylcrotonyl-CoA carboxylase catalyzes a reaction in the mitochondrial pathway for leucine catabolism. The exact metabolic function of geranoyl-CoA carboxylase is as yet unknown, but it may be involved in isoprenoid metabolism. This minireview summarizes the recent developments in our understanding of the structure, regulation, and metabolic functions of these proteins in plants.

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

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

An isoleucine to leucine mutation in acetyl-CoA carboxylase confers herbicide resistance in wild oat

Wild oat (Avena fatua L.) populations resistant to herbicides that inhibit acetyl-CoA carboxylase (ACCase; EC 6.4.1.2) represent an increasingly important weed control problem. The objective of this study was to determine the ACCase mutation responsible for herbicide resistance in a well-studied wild oat biotype (UMI). A 2039-bp region encompassing the carboxybiotin and acetyl-CoA binding domains of multifunctional plastidic ACCase was analyzed. DNA sequences representing three plastidic ACCase gene loci were isolated from both the resistant UMI and a herbicide-susceptible biotype, consistent with the hexaploid nature of wild oat. Only one nonsynonymous point mutation was found among the resistant wild oat sequences, inferring an isoleucine to leucine substitution. The position of this substitution corresponds to residue 1769 of wheat (Triticum aestivum L.) plastidic ACCase (GenBank accession No. AF029895). Analysis of an F2 population derived from a cross between a herbicide-resistant and a susceptible biotype confirmed co-segregation of herbicide resistance with the mutated ACCase. We conclude that the isoleucine to leucine mutation is responsible for herbicide resistance in UMI wild oat based on a comparison of the substitution site across species and ACCase types. While isoleucine is conserved among plastidic ACCases of herbicide-susceptible grasses, leucine is found in plastidic and cytosolic forms of multifunctional herbicide-resistant ACCase.

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

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

An isoleucine/leucine residue in the carboxyltransferase domain of acetyl-CoA carboxylase is critical for interaction with aryloxyphenoxypropionate and cyclohexanedione inhibitors

cDNA fragments encoding the carboxyltransferase domain of the multidomain plastid acetyl-CoA carboxylase (ACCase) from herbicide-resistant maize and from herbicide-sensitive and herbicide-resistant Lolium rigidum were cloned and sequenced. A Leu residue was found in ACCases from herbicide-resistant plants at a position occupied by Ile in all ACCases from sensitive grasses studied so far. Leu is present at the equivalent position in herbicide-resistant ACCases from other eukaryotes. Chimeric ACCases containing a 1000-aa fragment of two ACCase isozymes found in a herbicide-resistant maize were expressed in a yeast ACC1 null mutant to test herbicide sensitivity of the enzyme in vivo and in vitro. One of the enzymes was resistant/tolerant, and one was sensitive to haloxyfop and sethoxydim, rendering the gene-replacement yeast strains resistant and sensitive to these compounds, respectively. The sensitive enzyme has an Ile residue, and the resistant one has a Leu residue at the putative herbicide-binding site. Additionally, a single Ile to Leu replacement at an equivalent position changes the wheat plastid ACCase from sensitive to resistant. The effect of the opposite substitution, Leu to Ile, makes Toxoplasma gondii apicoplast ACCase resistant to haloxyfop and clodinafop. In this case, inhibition of the carboxyltransferase activity of ACCase (second half-reaction) of a large fragment of the Toxoplasma enzyme expressed in Escherichia coli was tested. The critical amino acid residue is located close to a highly conserved motif of the carboxyltransferase domain, which is probably a part of the enzyme active site, providing the basis for the activity of fop and dim herbicides.

The apicoplast as an antimalarial drug target

Resistance to commonly used malaria drugs is spreading and new drugs are required urgently. The recent identification of a relict chloroplast (apicoplast) in malaria and related parasites offers numerous new targets for drug therapy using well-characterized compounds. The apicoplast contains a range of metabolic pathways and housekeeping processes that differ radically to those of the host thereby presenting ideal strategies for drug therapy. Indeed, many compounds targeting these plastid pathways are antimalarial and have favourable profiles based on extensive knowledge from their use as antibacterials.

Subcellular localization of acetyl-CoA carboxylase in the apicomplexan parasite Toxoplasma gondii

Apicomplexan parasites such as Toxoplasma gondii contain a primitive plastid, the apicoplast, whose genome consists of a 35-kb circular DNA related to the plastid DNA of plants. Plants synthesize fatty acids in their plastids. The first committed step in fatty acid synthesis is catalyzed by acetyl-CoA carboxylase (ACC). This enzyme is encoded in the nucleus, synthesized in the cytosol, and transported into the plastid. In the present work, two genes encoding ACC from T. gondii were cloned and the gene structure was determined. Both ORFs encode multidomain proteins, each with an N-terminal extension, compared with the cytosolic ACCs from plants. The N-terminal extension of one isozyme, ACC1, was shown to target green fluorescent protein to the apicoplast of T. gondii. In addition, the apicoplast contains a biotinylated protein, consistent with the assertion that ACC1 is localized there. The second ACC in T. gondii appears to be cytosolic. T. gondii mitochondria also contain a biotinylated protein, probably pyruvate carboxylase. These results confirm the essential nature of the apicoplast and explain the inhibition of parasite growth in cultured cells by herbicides targeting ACC.