Amino acid biosynthetic enzymes as targets of herbicide action

Cytotoxicity and hormonal activity of glyphosate-based herbicides

Glyphosate-based herbicides (GBHs) are the most widely used pesticides for weed control. In parallel with the renewal of the active ingredient, polyethoxylated POE(15) containing GBHs were banned in the EU in 2016. Since then, co-formulants were changed and numerous GBHs are marketed with different excipients declared as inert substances. In our study, we focused to determine acute and chronic cytotoxicity (by Aliivibrio fischeri assay) and direct hormonal activity (estrogenic and androgenic effects measured by Saccharomyces cerevisiae BLYES/BLYAS strains, respectively) of glyphosate, AMPA, polyethoxylated POE(15) and 13 GBHs from which 11 formulations do not contain polyethoxylated POE(15). Among the pure substances, neither glyphosate nor AMPA had any effects, while polyethoxylated POE(15) exhibited pronounced toxicity and was also estrogenic but not androgenic. Regarding the acute and chronic cytotoxicity and hormonal activity of GBHs, dilution percentages calculated from EC50 values were in the most cases by one or two order of magnitude lower than the minimum recommended dilution for agricultural and household use. Relation could not be observed between the biological effects and type of glyphosate-salts; hence toxicity could be linked to the co-formulants, which are not even declared in 3 GBHs. Toxicological evaluation must focus on these substances and free accessibility of GBHs should be reconsidered.

Metal Ions Play an Essential Catalytic Role in the Mechanism of Ketol-Acid Reductoisomerase

Ketol-acid reductoisomerase (KARI) is a Mg(2+) -dependent enzyme in the branched-chain amino acid biosynthesis pathway. It catalyses a complex two-part reaction: an alkyl migration followed by a NADPH-dependent reduction. Both reactions occur within the one active site, but in particular, the mechanism of the isomerisation step is poorly understood. Here, using a combination of kinetic, thermodynamic and spectroscopic techniques, the reaction mechanisms of both Escherichia coli and rice KARI have been investigated. We propose a conserved mechanism of catalysis, whereby a hydroxide, bridging the two Mg(2+) ions in the active site, initiates the reaction by abstracting a proton from the C2 alcohol group of the substrate. While the μ-hydroxide-bridged dimetallic centre is pre-assembled in the bacterial enzyme, in plant KARI substrate binding leads to a reduction of the metal-metal distance with the concomitant formation of a hydroxide bridge. Only Mg(2+) is capable of promoting the isomerisation reaction, likely to be due to non-competent substrate binding in the presence of other metal ions.

Safety assessment of a modified acetolactate synthase protein (GM-HRA) used as a selectable marker in genetically modified soybeans

Acetolactate synthase (ALS) enzymes have been isolated from numerous organisms including soybeans (Glycine max; GM-ALS) and catalyze the first common step in biosynthesis of branched chain amino acids. Expression of an ALS protein (GM-HRA) with two amino acid changes relative to native GM-ALS protein in genetically modified soybeans confers tolerance to herbicidal active ingredients and can be used as a selectable transformation marker. The safety assessment of the GM-HRA protein is discussed. Bioinformatics comparison of the amino acid sequence did not identify similarities to known allergenic or toxic proteins. In vitro studies demonstrated rapid degradation in simulated gastric fluid (<30s) and intestinal fluid (<1min). The enzymatic activity was completely inactivated at 50 degrees C for 15 min demonstrating heat lability. The protein expressed in planta is not glycosylated and genetically modified soybeans expressing the GM-HRA protein produced similar protein/allergen profiles as its non-transgenic parental isoline. No adverse effects were observed in mice following acute oral exposure at a dose of at least 436 mg/kg of body weight or in a 28-day repeated dose dietary toxicity study at doses up to 1247 mg/kg of body weight/day. The results demonstrate GM-HRA protein safety when used in agricultural biotechnology.

Subchronic feeding study of high oleic acid soybeans (Event DP-3Ø5423-1) in Sprague-Dawley rats

DP-3Ø5423-1 (305423) is a genetically-modified (GM) soybean that was produced by biolistic insertion of a gm-fad2-1 gene fragment and the gm-hra gene into the germline of soybean seeds. The gm-fad2-1 gene fragment cosuppresses expression of the endogenous FAD2-1 gene encoding the seed-specific omega-6 fatty acid desaturase resulting in higher concentrations of oleic acid (18:1) relative to linoleic acid (18:2). The gm-hra gene encoding a modified acetolactate synthase (ALS) enzyme was used as a selectable marker. In the current study, processed fractions (meal, hulls, and oil) from 305423 soybeans, non-GM soybeans with a similar genetic background (near isoline control) and three commercially-available non-GM varieties were used to formulate diets that were nutritionally comparable to PMI Certified Rodent LabDiet 5002. Diets were fed to young adult Crl:CD(SD) rats (12/sex/group) for approximately 90 days. Compared with rats fed the non-GM control diet, no biologically relevant differences were observed in rats fed the 305423 diet with respect to body weight/gain, food consumption/efficiency, mortality, clinical signs of toxicity, or ophthalmological observations. No test diet-related effects were observed on neurobehavioral assessments, organ weights, or clinical or anatomic pathology. These results demonstrated that 305423 soybeans are as safe and wholesome as non-GM soybeans.

Building of an experimental cline with Arabidopsis thaliana to estimate herbicide fitness cost

Various management strategies aim at maintaining pesticide resistance frequency under a threshold value by taking advantage of the benefit of the fitness penalty (the cost) expressed by the resistance allele outside the treated area or during the pesticide selection "off years." One method to estimate a fitness cost is to analyze the resistance allele frequency along transects across treated and untreated areas. On the basis of the shape of the cline, this method gives the relative contributions of both gene flow and the fitness difference between genotypes in the treated and untreated areas. Taking advantage of the properties of such migration-selection balance, an artificial cline was built up to optimize the conditions where the fitness cost of two herbicide-resistant mutants (acetolactate synthase and auxin-induced target genes) in the model species Arabidopsis thaliana could be more accurately measured. The analysis of the microevolutionary dynamics in these experimental populations indicated mean fitness costs of approximately 15 and 92% for the csr1-1 and axr2-1 resistances, respectively. In addition, negative frequency dependence for the fitness cost was also detected for the axr2-1 resistance. The advantages and disadvantages of the cline approach are discussed in regard to other methods of cost estimation. This comparison highlights the powerful ability of an experimental cline to measure low fitness costs and detect sensibility to frequency-dependent variations.

Herbicidal inhibitors of amino acid biosynthesis and herbicide-tolerant crops

Acetohydroxyacid synthase (AHAS) inhibitors interfere with branched-chain amino acid biosynthesis by inhibiting AHAS. Glyphosate affects aromatic amino acid biosynthesis by inhibiting 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). Glufosinate inhibits glutamine synthetase and blocks biosynthesis of glutamine. AHAS gene variants that confer tolerance to AHAS inhibitors have been discovered in plants through selection or mutagenesis. Imidazolinone-tolerant crops have been commercialized based on these AHAS gene variants. A modified maize EPSPS gene and CP4-EPSPS gene from Agrobacterium sp. have been used to transform plants for target-based tolerance to glyphosate. A gox gene isolated from Ochrobactrum anthropi has also been employed to encode glyphosate oxidoreductase to detoxify glyphosate in plants. Glyphosate-tolerant crops with EPSPS transgene alone or both EPSPS and gox transgenes have been commercialized. Similarly, bar and pat genes isolated from Streptomyces hygroscopicus and S. viridochromogenes, respectively, have been inserted into plants to encode phosphinothricin N-acetyltransferase to detoxify glufosinate. Glufosinate-tolerant crops have been commercialized using one of these two transgenes.

Early transcriptional response of Saccharomyces cerevisiae to stress imposed by the herbicide 2,4-dichlorophenoxyacetic acid

The global gene transcription pattern of the eukaryotic experimental model Saccharomyces cerevisiae in response to sudden aggression with the widely used herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) was analysed. Under acute stress, 14% of the yeast transcripts suffered a greater than twofold change. The yeastract database was used to predict the transcription factors mediating the response registered in this microarray analysis. Most of the up-regulated genes in response to 2,4-D are known targets of Msn2p, Msn4p, Yap1p, Pdr1p, Pdr3p, Stp1p, Stp2p and Rpn4p. The major regulator of ribosomal protein genes, Sfp1p, is known to control 60% of the down-regulated genes, in particular many involved in the transcriptional and translational machinery and in cell division. The yeast response to the herbicide includes the increased expression of genes involved in the oxidative stress response, the recovery or degradation of damaged proteins, cell wall remodelling and multiple drug resistance. Although the protective role of TPO1 and PDR5 genes was confirmed, the majority of the responsive genes encoding multidrug resistance do not confer resistance to 2,4-D. The increased expression of genes involved in alternative carbon and nitrogen source metabolism, fatty acid beta-oxidation and autophagy was also registered, suggesting that acute herbicide stress leads to nutrient limitation.

A proteome analysis of the yeast response to the herbicide 2,4-dichlorophenoxyacetic acid

The intensive use of herbicides may give rise to a number of toxicological problems in non-target organisms and has led to the emergence of resistant weeds. To gain insights into the mechanisms of adaptation to the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D), we have identified variations in protein expression level in the eukaryotic experimental model Saccharomyces cerevisiae exposed to herbicide aggression, based on two-dimensional gel electrophoresis. We show results suggesting that during the adaptation period preceding the resumption of inhibited exponential growth under herbicide stress, the antioxidant enzyme Ahp1p and the heat shock proteins Hsp12p and Ssb2p (or Ssb1p) are present in higher amounts. The increased level of other enzymes involved in protein (Cdc48p) and mRNA (Dcp1p) degradation, in carbohydrate metabolism (Eno1p, Eno2p and Glk1p) and in vacuolar H(+)-ATPase (V-ATPase) function (Vma1p and Vma2p, two subunits of the peripheral catalytic sector) was also registered. V-ATPase is involved in the homeostasis of intracellular pH and in the compartmentalization of amino acids and other metabolites in the vacuole. The increased expression of amino acid biosynthetic enzymes (Arg1p, Aro3p, Aro8p, Gdh1p, His4p, Ilv3p and Met6p), also suggested by comparative analysis of the proteome, was correlated with the reduction of amino acid concentration registered in both the vacuole and the cytosol of 2,4-D-stressed cells, possibly due to the disturbance of vacuolar and plasma membrane functions by the lipophilic acid herbicide.

Dominance variation across six herbicides of the Arabidopsis thaliana csr1-1 and csr1-2 resistance alleles

Dominance of a resistance trait can be defined as a measure of the relative position of the phenotype of the heterozygote RS compared with the phenotype of the two corresponding homozygotes, SS and RR. This parameter has been shown to have primary importance in the dynamics of pesticide resistance evolution. Literature on insecticide resistance suggests that dominance levels in the presence of insecticide vary greatly from completely recessive to completely dominant. With insecticides, both the chemical applied and the dosages used have been demonstrated to affect the dominance. By contrast, almost all herbicide resistances have been found to be inherited as partially to totally dominant traits. This discrepancy between weeds and insects may partly result from the methodologies applied to measure the dominance, ie a single dose for herbicide versus several doses for insecticide. Using two well-known resistances (csr1-1 and csr1-2) to acetolactate synthase (ALS) inhibitors in Arabidopsis thaliana (L) Heynh (mouse-ear cress), we used several herbicide doses to determine the dominance level to six ALS-inhibiting herbicides. The dominance level in the presence of herbicide varied from completely dominant to completely recessive, depending on the resistance allele and the herbicide tested. The dominance of the csr1-1 and csr1-2 resistance alleles ranged from 0 (completely recessive) to 1.1 (dominant) and from 0 to 0.3 (partially dominant), respectively. The recessivity of some resistance alleles in the presence of herbicide could lead to the development of improved resistance management in order to delay or avoid herbicide resistance evolution, especially in the control of outcrossing weed species.

The dominance of the herbicide resistance cost in several Arabidopsis thaliana mutant lines

Resistance evolution depends upon the balance between advantage and disadvantage (cost) conferred in treated and untreated areas. By analyzing morphological characters and simple fitness components, the cost associated with each of eight herbicide resistance alleles (acetolactate synthase, cellulose synthase, and auxin-induced target genes) was studied in the model plant Arabidopsis thaliana. The use of allele-specific PCR to discriminate between heterozygous and homozygous plants was used to provide insights into the dominance of the resistance cost, a parameter rarely described. Morphological characters appear more sensitive than fitness (seed production) because 6 vs. 4 differences between resistant and sensitive homozygous plants were detected, respectively. Dominance levels for the fitness cost ranged from recessivity (csr1-1, ixr1-2, and axr1-3) to dominance (axr2-1) to underdominance (aux1-7). Furthermore, the dominance level of the herbicide resistance trait did not predict the dominance level of the cost of resistance. The relationship of our results to theoretical predictions of dominance and the consequences of fitness cost and its dominance in resistance management are discussed.

Interfering with different steps of protein synthesis explored by transcriptional profiling of Escherichia coli K-12

Escherichia coli responses to four inhibitors that interfere with translation were monitored at the transcriptional level. A DNA microarray method provided a comprehensive view of changes in mRNA levels after exposure to these agents. Real-time reverse transcriptase PCRanalysis served to verify observations made with microarrays, and a chromosomal grpE::lux operon fusion was employed to specifically monitor the heat shock response. 4-Azaleucine, a competitive inhibitor of leucyl-tRNA synthetase, surprisingly triggered the heat shock response. Administration of mupirocin, an inhibitor of isoleucyl-tRNA synthetase activity, resulted in changes reminiscent of the stringent response. Treatment with kasugamycin and puromycin (targeting ribosomal subunit association as well as its peptidyl-transferase activity) caused accumulation of mRNAs from ribosomal protein operons. Abundant biosynthetic transcripts were often significantly diminished after treatment with any of these agents. Exposure of a relA strain to mupirocin resulted in accumulation of ribosomal protein operon transcripts. However, the relA strain's response to the other inhibitors was quite similar to that of the wild-type strain.

Global expression profiling of yeast treated with an inhibitor of amino acid biosynthesis, sulfometuron methyl

The expression pattern of 1,529 yeast genes in response to sulfometuron methyl (SM) was analyzed by DNA microarray technology. SM, a potent herbicide, inhibits acetolactate synthase, a branched-chain amino acid biosynthetic enzyme. Exposure of yeast cells to 0.2 microg/ml SM resulted in 40% growth inhibition, a Gcn4p-mediated induction of genes involved in amino acid and cofactor biosynthesis, and starvation response. The accumulation of intermediates led to the induction of stress response genes and the repression of genes involved in carbohydrate metabolism, nucleotide biosynthesis, and sulfur assimilation. Extended exposure to SM led to a relaxation of the initial response and induction of sugar transporter and ergosterol biosynthetic genes, as well as repression of histone and lipid metabolic genes. Exposure to 5 microg/ml SM resulted in >98% growth inhibition and stimulated a similar initial expression change, but with no relaxation after extended exposure. Instead, more stress response and DNA damage repair genes become induced, suggesting a serious cellular consequence. Other salient features of metabolic regulation, such as the coordinated expression of cofactor biosynthetic genes with amino acid biosynthetic ones, were evident from our data. A potential link between SM sensitivity and ergosterol metabolism was uncovered by expression profiling and confirmed by genetic analysis.

Regulation of aspartate-derived amino-acid metabolism in Zygosaccharomyces rouxii compared to Saccharomyces cerevisiae

To elucidate the growth inhibitory effect of threonine, the regulation of the aspartate-derived amino-acid metabolism in Zygosaccharomyces rouxii, an important yeast for the flavor development in soy sauce, was investigated. It was shown that threonine inhibited the growth of Z. rouxii by blocking the methionine synthesis. It seemed that threonine blocked this synthesis by inhibiting the conversion of aspartate. In addition, it was shown that the growth of Z. rouxii, unlike that of Saccharomyces cerevisiae, was not inhibited by the herbicide sulfometuron methyl (SMM). From enzyme assays, it was concluded that the acetohydroxy acid synthase in Z. rouxii, unlike that in S. cerevisiae, was not sensitive to SMM. Furthermore, the enzyme assays demonstrated that the activity of threonine deaminase in Z. rouxii, like in S. cerevisiae, was strongly inhibited by isoleucine and stimulated by valine. From this work, it is clear that the aspartate-derived amino-acid metabolism in Z. rouxii only partly resembles that in S. cerevisiae.

Rational herbicide design by inhibition of tryptophan biosynthesis

Compounds designed to mimic the tryptophan synthase alpha subunit reactive intermediate were found to be potent inhibitors of the enzyme. These compounds are herbicidal and the herbicidal mode of action was demonstrated to be due to disruption of tryptophan biosynthesis.

Computer-Aided Reaction Design. Development of a New Facile Procedure to Synthesize 2-Mercapto-3-alkoxycarboxylate on the Basis of ab Initio Molecular Orbital Calculations

This paper describes a new facial procedure to substitute a tosyloxy group in 2-(tosyloxy)alkanoate with SH(-) to yield 2-mercaptoalkanoate on the basis of ab initio MO calculations. Combination of substrate and solvent effects can control both reactivity and selectivity of reaction for 2-(tosyloxy)-3-alkoxycarboxylic acid which gave 2-mercapto-3-alkoxycarboxylic acid in good yield while its ethyl ester gave alpha,beta-unsaturated carboxylate ester as a main product. The difference of carboxylate moiety in the substrate causes remarkable change in reactivity and selectivity. To clarify origin of the difference, ab initio MO calculations in the gas phase and in DMF have been carried out. The solvent effect was considered at RHF/6-31+G with the IPCM-SCRF model. It was confirmed that the substrate with an ester fragment prefers the E1cB to the S(N)2 mechanism. In the transition state of the S(N)2 mechanism with a carboxylate ion fragment, the nucleophile SH(-) locates far from the reaction center due to the electrostatic repulsion between the COO(-) fragment and SH(-). This repulsion causes high activation barrier in the gas phase while polar solvent can reduce the barrier height. Therefore, reaction conditions can control reactivity of carboxylic acid. On the basis of analysis of the MO calculations, subsequent experiments were designed for a new dianion system to synthesize 2-pyrimidinylthio carboxylic acid from 2-tosyloxy carboxylate. We succeeded in developing a new facile method that the two reactions for thioether carried out in a one-pot procedure in excellent yield.

Constricted flux through the branched-chain amino acid biosynthetic enzyme acetolactate synthase triggers elevated expression of genes regulated by rpoS and internal acidification

The first common enzyme of isoleucine and valine biosynthesis, acetolactate synthase (ALS), is specifically inhibited by the herbicide sulfometuron methyl (SM). To further understand the physiological consequences of flux alterations at this point in metabolism, Escherichia coli genes whose expression was induced by partial inhibition of ALS were sought. Plasmid-based fusions of random E. coli DNA fragments to Photorhabdus luminescens luxCDABE were screened for bioluminescent increases in actively growing liquid cultures slowed 25% by the addition of SM. From more than 8,000 transformants, 12 unique SM-inducible promoter-lux fusions were identified. The lux reporter genes were joined to seven uncharacterized open reading frames, f253a, f415, frvX, o513, o521, yciG, and yohF, and five known genes, inaA, IdcC, osmY, poxB, and sohA. Inactivation of the rpoS-encoded sigma factor, sigmaS, reduced basal expression levels of six of these fusions 10- to 200-fold. These six genes defined four new members of the sigmaS regulon, f253a, IdcC, yciG, and yohF, and included two known members, osmY and poxB. Furthermore, the weak acid salicylate, which causes cytoplasmic acidification, also induced increased bioluminescence from seven SM-inducible promoter-lux fusion-containing strains, namely, those with fusions of the sigmaS-controlled genes and inaA. The pattern of gene expression changes suggested that restricted ALS activity may result in intracellular acidification and induction of the sigmaS-dependent stress response.

Aromatic amino acid biosynthesis in the yeast Saccharomyces cerevisiae: a model system for the regulation of a eukaryotic biosynthetic pathway

This review focuses on the gene-enzyme relationships and the regulation of different levels of the aromatic amino acid biosynthetic pathway in a simple eukaryotic system, the unicellular yeast Saccharomyces cerevisiae. Most reactions of this branched pathway are common to all organisms which are able to synthesize tryptophan, phenylalanine, and tyrosine. The current knowledge about the two main control mechanisms of the yeast aromatic amino acid biosynthesis is reviewed. (i) At the transcriptional level, most structural genes are regulated by the transcriptional activator GCN4, the regulator of the general amino acid control network, which couples transcriptional derepression to amino acid starvation of numerous structural genes in multiple amino acid biosynthetic pathways. (ii) At the enzyme level, the carbon flow is controlled mainly by modulating the enzyme activities at the first step of the pathway and at the branch points by feedback action of the three aromatic amino acid end products. Implications of these findings for the relationship of S. cerevisiae to prokaryotic as well as to higher eukaryotic organisms and for general regulatory mechanisms occurring in a living cell such as initiation of transcription, enzyme regulation, and the regulation of a metabolic branch point are discussed.

Evaluation of the isobologram method for the assessment of mixtures of chemicals. Combination effect studies with pesticides in algal biotests

The aim of this paper is an evaluation of isobolograms, a method proposed for the assessment of combined effects of chemicals. In order to examine potentials and shortcomings of this approach for ecotoxicological purposes, algal biotests with selected pesticidal compounds were performed. Additivity, as defined by the model, is demonstrated for the combination of atrazine and metribuzin for different combination ratios, response levels, and parameters. Subadditivity is shown for amitrole and glufosinate-ammonium. The results and inherent biometrical features are discussed in terms of criteria considered suitable for comparative evaluation of biometrical models for the assessment of mixtures of chemicals.

Oxalyl hydroxamates as reaction-intermediate analogues for ketol-acid reductoisomerase

N-Hydroxy-N-isopropyloxamate (IpOHA) is an exceptionally potent inhibitor of the Escherichia coli ketol-acid reductoisomerase. In the presence of Mg2+ or Mn2+, IpOHA inhibits the enzyme in a time-dependent manner, forming a nearly irreversible complex. Nucleotide, which is essential for catalysis, greatly enhances the binding of IpOHA by the reductoisomerase, with NADPH (normally present during the enzyme's rearrangement step, i.e., conversion of a beta-keto acid into an alpha-keto acid, in either the forward or reverse physiological reactions) being more effective than NADP. In the presence of Mg2+ and NADPH, IpOHA appears to bind to the enzyme in a two-step mechanism, with an initial inhibition constant of 160 nM and a maximum rate of formation of the tight, slowly reversible complex of 0.57 min-1 (values that give an association rate of IpOHA, at low concentration, of 5.9 X 10(4) M-1 s-1). The rate of exchange of [14C]IpOHA from an enzyme-[14C]IpOHA-Mg2(+)-NADPH complex with exogenous, unlabeled IpOHA has a half-time of 6 days (150 h). This dissociation rate (1.3 X 10(-6) s-1) and the association rate determined by inactivation kinetics define an overall dissociation constant of 22 pM. By contrast, in the presence of Mn2+ and NADPH, the corresponding association and dissociation rates for IpOHA are 8.2 X 10(4) M-1 s-1 and 3.2 X 10(-6) s-1 (half-time = 2.5 days), respectively, which define an overall dissociation constant of 38 pM. In the presence of NADP or in the absence of nucleotide (both in the presence of Mg2+), the enzyme-IpOHA complex is far more labile, with dissociation half-times of 28 and 2 h, respectively. In the absence of Mg2+ or Mn2+, IpOHA does not exhibit time-dependent inhibition of the reductoisomerase.(ABSTRACT TRUNCATED AT 250 WORDS)

Dihydrodipicolinate synthase ofnicotiana sylvestris, a chloroplast-localized enzyme of the lysine pathway

The first enzyme of the lysine-biosynthesis pathway, dihydrodipicolinate synthase (DHDPS; EC 4.2.1.52) has been purified and characterized inNicotiana sylvestris Speggazini et Comes. A purification scheme was developed for the native DHDPS that subsequently led to the purification to homogeneity of its subunits using two-dimensional gel electrophoresis. Subsequent elution of the purified polypeptide has opened the way for the production of rabbit polyclonal anti-DHDPS sera. The molecular weight of the enzyme was determined to be 164000 daltons (Da) by an electrophoretic method. By labeling with [(14)C]pyruvate, the enzyme was shown to be composed of four identical subunits of 38500 Da. Pyruvate acts as a stabilizing agent and contributes to the preservation of the tetrameric structure of the enzyme. The enzyme ofN. sylvestris is strongly inhibited by lysine with anI 0.5 of 15 μM; S-(2-aminoethyl)L-cysteine and γ-hydroxylysine, two lysine analogs, were found to be only weak inhibitors. An analog of pyruvate, 2-oxobutyrate, competitively inhibited the enzyme and was found to act at the level of the pyruvate-binding site. Dihydrodipicolinate synthase was localized in the chloroplast and identified as a soluble stromal enzyme by enzymatic and immunological methods. Its properties are compared with those known for other plant and bacterial DHDPS enzymes.

The herbicidally active experimental compound Hoe 704 is a potent inhibitor of the enzyme acetolactate reductoisomerase

Growth inhibition of plants and bacteria by the experimental herbicide Hoe 704 (2-methylphosphinoyl-2-hydroxyacetic acid) was alleviated by the addition of the branched-chain amino acids to growth media. Hoe 704 caused a massive accumulation of acetoin and acetolactate, indicating its direct interference with the branched-chain amino acid biosynthetic pathway. The second enzyme of this pathway, acetolactate reductoisomerase (EC 1.1.1.86), was found to be subject to strong inhibition by Hoe 704. The inhibition was time-dependent and competitive with the enzyme's substrate, acetolactate. This report establishes acetolactate reductoisomerase as a new target for a herbicidal compound.

Pleiotropic effects of poxA regulatory mutations of Escherichia coli and Salmonella typhimurium, mutations conferring sulfometuron methyl and alpha-ketobutyrate hypersensitivity

A transposon Tn10 insertion into the Salmonella typhimurium poxA gene was identified among a set of mutations conferring sulfometuron methyl (SM) hypersensitivity. This Tn10 insertion mapped to 95 min on the S. typhimurium chromosome, a location analogous to that of poxA in the Escherichia coli genome. Like the E. coli poxA mutant, this mutant had reduced pyruvate oxidase activity, reduced cross-reacting material to antiserum to purified E. coli pyruvate oxidase, and reduced growth rates. In addition, the following phenotypes were identified for the E. coli and S. typhimurium poxA mutants: hypersensitivity to SM and alpha-ketobutyrate (AKB), deficiency in AKB metabolism, reduced activity of acetolactate synthase, and hypersensitivity to a wide range of bacterial growth inhibitors, including antibiotics, amino acid analogs, and dyes. An E. coli mutant defective in poxB, the structural gene encoding pyruvate oxidase, did not have these phenotypes; therefore, they are not solely a consequence of a pyruvate oxidase deficiency. Comparisons were made with mutant alleles of two other genes that are located near poxA and confer related phenotypes. The S. typhimurium poxA mutant differed both genetically and phenotypically from an miaA mutant. E. coli abs mutants had somewhat reduced pyruvate oxidase activity but had normal AKB metabolism. The relationship of the pleiotropic phenotypes of the poxA mutants to their SM hypersensitivity is discussed.

Involvement of ack-pta operon products in alpha-ketobutyrate metabolism by Salmonella typhimurium

The herbicide sulfometuron methyl inhibits acetolactate synthase II of Salmonella typhimurium, resulting in toxic accumulation of alpha-ketobutyrate. Four mutants, containing Tn10 insertions in the acetate kinase (ack) or phosphotransacetylase (pta) genes, were found among a collection of mutants hypersensitive to sulfometuron methyl. The genetic map location of these four Tn10 insertions at 46 min was identical to that of ack and pta point mutants. The insertion and point mutants shared the following phenotypes: resistance to fluoroacetate, sensitivity to alizarin yellow, inability to utilize inositol as a sole carbon source, and hypersensitivity to sulfometuron methyl. Three of the four Tn10 insertion mutants were deficient in phosphotransacetylase but not in acetate kinase activities, indicating insertion of Tn10 in the pta gene. The fourth mutant contained an insertion in the ack gene and was deficient in both acetate kinase and phosphotransacetylase activities. This polarity is consistent with cotranscription of ack and pta. All ack and pta mutants tested were defective in alpha-ketobutyrate turnover. Acetate kinase and phosphotransacetylase are proposed to be part of a pathway for alpha-ketobutyrate metabolism. Propionyl-CoA, an intermediate of that pathway, and propionate, the product of the pathway, accumulated upon inhibition of acetolactate synthase.

Toxic accumulation of alpha-ketobutyrate caused by inhibition of the branched-chain amino acid biosynthetic enzyme acetolactate synthase in Salmonella typhimurium

Biochemical and genetic analyses of the bacterium Salmonella typhimurium suggest that accumulation of alpha-ketobutyrate partially mediates the herbicidal activity of acetolactate synthase inhibitors. Growth inhibition of wild-type bacteria by the herbicide sulfometuron methyl was prevented by supplementing the medium with isoleucine, an allosteric inhibitor of threonine deaminase-catalyzed synthesis of alpha-ketobutyrate. In contrast, isoleucine did not rescue the growth of a mutant containing a threonine deaminase unresponsive to isoleucine. Moreover, the hypersensitivity of seven Tn10 insertion mutants to growth inhibition by sulfometuron methyl and alpha-ketobutyrate correlated with their inability to convert alpha-ketobutyrate to less noxious metabolites. We propose that alpha-ketobutyrate accumulation is an important component of sulfonylurea and imidazolinone herbicide action.