Effect of accessory proteins P19 and P20 on cytolytic activity of Cyt1Aa from Bacillus thuringiensis subsp. israelensis in Escherichia coli

The gene coding for the accessory protein P19 of Bacillus thuringiensis subsp. israelensis was expressed in Escherichia coli and its product was characterized. To investigate its putative role in delta-endotoxin crystallization as a P20-like polypeptide, each of the two encoding genes, p20 and p19, was cloned for inducible expression coordinatively with cyt1Aa. The latter is known to kill its transgenic host. P20 but not P19 stabilized Cyt1Aa and protected the host cells from its lethal effect. Neither GroEL nor GroES, expressed in trans, affected Cyt1Aa as did P20. The function of P20 is thus more specific than that of the chaperones, but that of P19 remains enigmatic. The correct sequence of p19, confirmed in all five isolates of B. thuringiensis subsp. israelensis, does not explain the slow electrophoretic mobility of its 179 amino acids product.

Binding and activation of thiamin diphosphate in acetohydroxyacid synthase

Acetohydroxyacid synthases (AHASs) are biosynthetic thiamin diphosphate- (ThDP) and FAD-dependent enzymes. They are homologous to pyruvate oxidase and other members of a family of ThDP-dependent enzymes which catalyze reactions in which the first step is decarboxylation of a 2-ketoacid. AHAS catalyzes the condensation of the 2-carbon moiety, derived from the decarboxylation of pyruvate, with a second 2-ketoacid, to form acetolactate or acetohydroxybutyrate. A structural model for AHAS isozyme II (AHAS II) from Escherichia coli has been constructed on the basis of its homology with pyruvate oxidase from Lactobacillus plantarum (LpPOX). We describe here experiments which further test the model, and test whether the binding and activation of ThDP in AHAS involve the same structural elements and mechanism identified for homologous enzymes. Interaction of a conserved glutamate with the N1' of the ThDP aminopyrimidine moiety is involved in activation of the cofactor for proton exchange in several ThDP-dependent enzymes. In accord with this, the analogue N3'-pyridyl thiamin diphosphate does not support AHAS activity. Mutagenesis of Glu47, the putative conserved glutamate, decreases the rate of proton exchange at C-2 of bound ThDP by nearly 2 orders of magnitude and decreases the turnover rate for the mutants by about 10-fold. Mutant E47A also has altered substrate specificity, pH dependence, and other changes in properties. Mutagenesis of Asp428, presumed on the basis of the model to be the crucial carboxylate ligand to Mg(2+) in the "ThDP motif", leads to a decrease in the affinity of AHAS II for Mg(2+). While mutant D428N shows ThDP affinity close to that of the wild-type on saturation with Mg(2+), D428E has a decreased affinity for ThDP. These mutations also lead to dependence of the enzyme on K(+). These experiments demonstrate that AHAS binds and activates ThDP in the same way as do pyruvate decarboxylase, transketolase, and other ThDP-dependent enzymes. The biosynthetic activity of AHAS also involves many other factors beyond the binding and deprotonation of ThDP; changes in the ligands to ThDP can have interesting and unexpected effects on the reaction.

Comparative sensitivity to UV-B radiation of two Bacillus thuringiensis subspecies and other Bacillus sp

Susceptibility of Bacillus thuringiensis spores and toxins to the UV-B range (280--330 nm) of the solar spectrum reaching Earth's surface may be responsible for its inactivation and low persistence in nature. Spores of the mosquito larvicidal B. thuringiensis subsp. israelensis were significantly more resistant to UV-B than spores of the lepidopteran-active subsp. kurstaki. Spores of subsp. israelensis were as resistant to UV-B as spores of B. subtilis and more resistant than spores of the closely related B. cereus and another mosquito larvicidal species B. sphaericus. Sensitivity of B. thuringiensis subsp. israelensis spores to UV-B radiation depended upon their culture age; 24-h cultures, approaching maximal larvicidal activity, were still sensitive. Maximal resistance to UV-B was achieved only at 48 h.

Acetohydroxyacid synthase: a proposed structure for regulatory subunits supported by evidence from mutagenesis

Valine inhibition of acetohydroxyacid synthase (AHAS) plays an important role in regulation of biosynthesis of branched-chain amino acids in bacteria. Bacterial AHASs are composed of separate catalytic and regulatory subunits; while the catalytic subunits appear to be homologous with several other thiamin diphosphate-dependent enzymes, there has been no model for the structure of the small, regulatory subunits (SSUs). AHAS III is one of three isozymes in Escherichia coli. Its large subunit (encoded by ilvI) by itself has 3-5 % activity of the holoenzyme and is not sensitive to inhibition by valine. The SSU (encoded by ilvH) associates with the large subunit and is required for full catalytic activity and valine sensitivity. The isolated SSU binds valine. The properties of several mutant SSUs shed light on the relation between their structure and regulatory function. Three mutant SSUs were obtained from spontaneous Val(R) bacterial mutants and three more were designed on the basis of an alignment of SSU sequences from valine-sensitive and resistant isozymes, or consideration of the molecular model developed here. Mutant SSUs N11A, G14D, N29H and A36V, when reconstituted with wild-type large subunit, lead to a holoenzyme with drastically reduced valine sensitivity, but with a specific activity similar to that of the wild-type. The isolated G14D and N29H subunits do not bind valine. Mutant Q59L leads to a valine-sensitive holoenzyme and isolated Q59L binds valine. T34I has an intermediate valine sensitivity. The effects of mutations on the affinity of the large subunits for SSUs also vary. D. Fischer's hybrid fold prediction method suggested a fold similarity between the N terminus of the ilvH product and the C-terminal regulatory domain of 3-phosphoglycerate dehydrogenase. On the basis of this prediction, together with the properties of the mutants, a model for the structure of the AHAS SSUs and the location of the valine-binding sites can be proposed.

PCR analysis of cry7 genes in Bacillus thuringiensis by the five conserved blocks of toxins

An alternative PCR analysis to screen for cry7 genes is proposed, based on the five conserved blocks of amino acids of Bacillus thuringiensis toxins and their encoding DNA sequences. A complete set of five primers was constructed, four direct and one reverse, yielding four specific amplicons. Modified profiles can identify new cry genes.

Determination of the dissociation constant of valine from acetohydroxy acid synthase by equilibrium partition in an aqueous two-phase system

An aqueous polyethylene glycol/salt two-phase system was used to estimate the dissociation constant, K(dis), of the Escherichia coli isoenzyme AHAS III regulatory subunit, ilvH protein, from the feedback inhibitor valine. The amounts of the bound and free radioactive valine in the system were determined. A Scatchard plot of the data revealed a 1:1 valine-protein binding ratio and K(dis) of 133+/-14 microM. The protein did not bind leucine, and the ilvH protein isolated from a valine resistant mutant showed no valine binding. This method is very simple, rapid and requires only a small amounts of protein compared to the presently used equilibrium dialysis method.

Purification of acetohydroxy acid synthase by separation in an aqueous two-phase system

Extraction in a polyethylene glycol (PEG)-phosphate aqueous two-phase system was considered as a primary step in purification of the acetohydroxy acid synthase III large catalytic subunit from an E. coli extract. Extraction optimization was achieved by varying the system parameters. Two systems with the following weight compositions were chosen for purification: PEG-2000 (16%)-phosphate (6%) and PEG-4000 (14%)-phosphate (5.5%)-KCl (8%), both at pH 7.0 and 1 mg total protein per 1 g system. Significant purification was achieved by a single extraction step with 70% recovery of the enzyme. After an additional ion-exchange chromatography step, pure enzyme was obtained in a 50% overall yield.

Characterization of acetohydroxy acid synthase activity in the archaeon Haloferax volcanii

Whereas the biochemistry of acetohydroxy acid synthase has been extensively studied in bacteria and eukaryotes, relatively little is known about the enzyme in archaea, the third kingdom of life. The present study biochemically characterizes acetohydroxy acid synthase activity in the halophilic archaea Haloferax volcanii. In addressing ion requirements, enzyme inhibition and antibody labeling, the results reveal that, except for its elevated salt requirements, the haloarchaeal enzyme is remarkably similar to its bacterial counterpart.

Multiplex PCR screening to detect cry9 genes in Bacillus thuringiensis strains

An extended PCR method was established to rapidly identify and classify Bacillus thuringiensis strains containing cry (crystal protein) genes toxic to lepidopteran, coleopteran, and dipteran pests (Ben-Dov et al., Appl. Environ. Microbiol. 63:4883-4890, 1997). To optimize identification of all reported cry genes, this methodology needs a complete PCR set of primers. In the study reported here, a set of universal (Un9) and specific primers for multiplex rapid screening for all four known genes from the cry9 group was designed. PCR analyses were performed for cry9 genes on 16 standard strains and 215 field isolates of B. thuringiensis. Among the standard strains, only B. thuringiensis subsp. aizawai HD-133, which harbors cry1 and cry2 genes, was positive with Un9 but negative to all four specific primers for cry9 genes. DNA of 22 field-collected isolates was also found to be positive with Un9. These isolates were classified into three cry9 profiles using specific primers; all of them harbor cry1 and cry2. This newly designed set of primers complements the existing PCR methodology for most currently known cry genes.

Branched-chain amino acid biosynthesis in Salmonella typhimurium: a quantitative analysis

We report here the first quantitative study of the branched-chain amino acid biosynthetic pathway in Salmonella typhimurium LT2. The intracellular levels of the enzymes of the pathway and of the 2-keto acid intermediates were determined under various physiological conditions and used for estimation of several of the fluxes in the cells. The results led to a revision of previous ideas concerning the way in which multiple acetohydroxy acid synthase (AHAS) isozymes contribute to the fitness of enterobacteria. In wild-type LT2, AHAS isozyme I provides most of the flux to valine, leucine, and pantothenate, while isozyme II provides most of the flux to isoleucine. With acetate as a carbon source, a strain expressing AHAS II only is limited in growth because of the low enzyme activity in the presence of elevated levels of the inhibitor glyoxylate. A strain with AHAS I only is limited during growth on glucose by the low tendency of this enzyme to utilize 2-ketobutyrate as a substrate; isoleucine limitation then leads to elevated threonine deaminase activity and an increased 2-ketobutyrate/2-ketoisovalerate ratio, which in turn interferes with the synthesis of coenzyme A and methionine. The regulation of threonine deaminase is also crucial in this regard. It is conceivable that, because of fundamental limitations on the specificity of enzymes, no single AHAS could possibly be adequate for the varied conditions that enterobacteria successfully encounter.

Biosynthesis of 2-aceto-2-hydroxy acids: acetolactate synthases and acetohydroxyacid synthases

Two groups of enzymes are classified as acetolactate synthase (EC 4. 1.3.18). This review deals chiefly with the FAD-dependent, biosynthetic enzymes which readily catalyze the formation of acetohydroxybutyrate from pyruvate and 2-oxobutyrate, as well as of acetolactate from two molecules of pyruvate (the ALS/AHAS group). These enzymes are generally susceptible to inhibition by one or more of the branched-chain amino acids which are ultimate products of the acetohydroxyacids, as well as by several classes of herbicides (sulfonylureas, imidazolinones and others). Some ALS/AHASs also catalyze the (non-physiological) oxidative decarboxylation of pyruvate, leading to peracetic acid; the possible relationship of this process to oxygen toxicity is considered. The bacterial ALS/AHAS which have been well characterized consist of catalytic subunits (around 60 kDa) and smaller regulatory subunits in an alpha2beta2 structure. In the case of Escherichia coli isozyme III, assembly and dissociation of the holoenzyme has been studied. The quaternary structure of the eukaryotic enzymes is less clear and in plants and yeast only catalytic polypeptides (homologous to those of bacteria) have been clearly identified. The presence of regulatory polypeptides in these organisms cannot be ruled out, however, and genes which encode putative ALS/AHAS regulatory subunits have been identified in some cases. A consensus sequence can be constructed from the 21 sequences which have been shown experimentally to represent ALS/AHAS catalytic polypeptides. Many other sequences fit this consensus, but some genes identified as putative 'acetolactate synthase genes' are almost certainly not ALS/AHAS. The solution of the crystal structures of several thiamin diphosphate (ThDP)-dependent enzymes which are homologous to ALS/AHAS, together with the availability of many amino acid sequences for the latter enzymes, has made it possible for two laboratories to propose similar, reasonable models for a dimer of catalytic subunits of an ALS/AHAS. A number of characteristics of these enzymes can now be better understood on the basis of such models: the nature of the herbicide binding site, the structural role of FAD and the binding of ThDP-Mg2+. The models are also guides for experimental testing of ideas concerning structure-function relationships in these enzymes, e.g. the nature of the substrate recognition site. Among the important remaining questions is how the enzyme suppresses alternative reactions of the intrinsically reactive hydroxyethylThDP enamine formed by the decarboxylation of the first substrate molecule and specifically promotes its condensation with 2-oxobutyrate or pyruvate.

Germination, growth, and sporulation of Bacillus thuringiensis subsp. israelensis in excreted food vacuoles of the protozoan Tetrahymena pyriformis

Spores of Bacillus thuringiensis subsp. israelensis and their toxic crystals are bioencapsulated in the protozoan Tetrahymena pyriformis, in which the toxin remains stable. Each T. pyriformis cell concentrates the spores and crystals in its food vacuoles, thus delivering them to mosquito larvae, which rapidly die. Vacuoles containing undigested material are later excreted from the cells. The fate of spores and toxin inside the food vacuoles was determined at various times after excretion by phase-contrast and electron microscopy as well as by viable-cell counting. Excreted food vacuoles gradually aggregated, and vegetative growth of B. thuringiensis subsp. israelensis was observed after 7 h as filaments that stemmed from the aggregates. The outgrown cells sporulated between 27 and 42 h. The spore multiplication values in this system are low compared to those obtained in carcasses of B. thuringiensis subsp. israelensis-killed larvae and pupae, but this bioencapsulation represents a new possible mode of B. thuringiensis subsp. israelensis recycling in nontarget organisms.

Extended screening by PCR for seven cry-group genes from field-collected strains of Bacillus thuringiensis

An extended multiplex PCR method was established to rapidly identify and classify Bacillus thuringiensis strains containing cry (crystal protein) genes toxic to species of Lepidoptera, Coleoptera, and Diptera. The technique enriches current strategies and simplifies the initial stages of large-scale screening of cry genes by pinpointing isolates that contain specific genes or unique combinations of interest with potential insecticidal activities, thus facilitating subsequent toxicity assays. Five pairs of universal primers were designed to probe the highly conserved sequences and classify most (34 of about 60) genes known in the following groups: 20 cry1, 3 cry2, 4 cry3, 2 cry4, 2 cry7, and 3 cry8 genes. The DNA of each positive strain was probed with a set of specific primers designed for 20 of these genes and for cry11A. Twenty-two distinct cry-type profiles were identified from 126 field-collected B. thuringiensis strains. Several of them were found to be different from all published profiles. Some of the field-collected strains, but none of the 16 standard strains, were positive for cry2Ac. Three standard and 38 field-collected strains were positive by universal primers but negative by specific primers for all five known genes of cry7 and cry8. These field-collected strains seem to contain a new gene or genes that seem promising for biological control of insects and management of resistance.

Homology modeling of the structure of bacterial acetohydroxy acid synthase and examination of the active site by site-directed mutagenesis

Acetohydroxy acid synthase (AHAS, EC 4.1.3.18) catalyzes the thiamin pyrophosphate (TPP)-dependent decarboxylation of pyruvate and condensation of the resulting two-carbon moiety with a second alpha-keto acid. It belongs to a family of homologous, TPP-dependent enzymes which catalyze different reactions which start from decarboxylation of alpha-keto acids. A model for the structure of Escherichia coli AHAS isozyme II, based on its homology with pyruvate oxidase and experimental testing of the model by site-directed mutagenesis, has been used here to study how AHAS controls the chemical fate of a decarboxylated keto acid. Because of the potential conformational freedom of the reacting substrates, residues interacting with the substrate could not be identified directly from the model of AHAS. Three residues were considered as candidates for involvement in the recognition of alpha-ketobutyrate, as the amino acids at these sites in a unique low-specificity AHAS are different from those in typical AHASs, which are highly specific for reaction with alpha-ketobutyrate as second substrate, in preference to pyruvate. These residues were altered in AHAS II by site-directed mutagenesis. Replacement of Trp464 lowers the specificity by at least 1 order of magnitude, with minor effects on the activity or stability of the enzyme, suggesting that Trp464 contributes > or = 1.3 kcal mol-1 to interaction with the "extra" methyl of alpha-ketobutyrate. Mutations of Met460 or Thr70 have small effects on specificity and do affect other properties of the protein. A model for enzyme-substrate interactions can be proposed on the basis of these results. The model of AHAS also explains previously reported spontaneous mutants of AHAS resistant to sulfonylurea herbicides, which probably bind in the narrow depression which provides access to the bound TPP. A role for the C terminus of the enzyme polypeptide in determination on the reaction pathway is also possible.

Raising activity of Bacillus thuringiensis var. israelensis against Anopheles stephensi larvae by encapsulation in Tetrahymena pyriformis (Hymenostomatida:Tetrahymenidae)

Toxicity of Bacillus thuringiensis var israelensis (B.t.i.) against surface-feeding mosquito larvae of Anopheles stephensi was enhanced by encapsulation in the protozoan Tetrahymena pyriformis. In the laboratory, larvae died about 8 times faster when exposed to protozoan cells filled with B.t.i. than when exposed to the same concentrations of B.t.i. alone. Best larvicidal activities were achieved with ratios of 1:200-1:500 T. pyriformis cells to B.t.i. spores. The concentration of B.t.i. needed to kill 50% of exposed populations was 4-fold lower with T. pyriformis than with B.t.i. alone in 100 ml-test cups. Toxicity enhancement is very likely a consequence of concentrating B.t.i. insecticidal crystal proteins in T. pyriformis cells and floating them to the water surface in the larval feeding zone. Reduction in the exposure time of B.t.i. to unfavorable field conditions, as a result of the decrease in larval mortality time, might improve the persistence of this biological control agent in nature.

Enhanced ribosome frameshifting in stationary phase cells

We have examined the effect of growth phase in Escherichia coli on the translation of a plasmid-borne lacZ gene in which active enzyme synthesis requires a leftward frameshift. During the log phase of growth, the differential rate of enzyme synthesis is very low. It increases by about two orders of magnitude during the small amount of protein synthesis which occurs at the end of log phase and the early part of stationary phase. The increase is sufficient to increase the enzyme's specific activity in crude extracts to 30 times more than it would be if the log-phase differential rate continued unchanged. No such large increase is observed with a zero-frame lacZ+ control gene on the same plasmid under the control of the same promoter; a significant but much smaller increase is observed with a zero-frame control containing an in-frame terminator triplet in the region of the required frameshift. Protein sequence analysis of the enzyme made from the frameshift reporter in stationary cells shows that the increased enzyme synthesis is due to frameshifting, and not due to termination and reinitiation. The frameshift occurs at or right after the sequence U UUC AAG, an intrinsically shifty site.

Isolation and characterization of subunits of acetohydroxy acid synthase isozyme III and reconstitution of the holoenzyme

The separately cloned large and small subunits of AHAS isozyme III from Escherichia coli have been isolated and purified. The essentially pure small subunit (17 kDa ilvH product) was obtained by a procedure exploiting its low solubility. The large, catalytic subunit (62 kDa ilvI product) was isolated by standard techniques, to > or = 95% purity. The large subunit has low catalytic activity relative to holoenzyme (approximately 5%) but shows similar substrate specificity and qualitatively similar cofactor dependence and inhibition by a sulfonylurea herbicide. Its activity is insensitive to valine, and the protein does not bind valine. The small subunit binds valine with Kd = 0.2 mM. Reconstitution of the holoenzyme from its subunits leads to a complex with the properties of the native protein, including valine inhibition of activity with Ki = 12 microM. Reconstitution titrations confirm the 1:1 stoichiometry of subunit assembly and a tendency to dissociation (about 50% dissociation near 0.1 microM subunit). Size exclusion HPLC indicates that either subunit alone is largely monomeric, and that assembly of the holoenzyme (two large + two small subunits, 150-160 kDa) requires FAD. On the basis of its homology with pyruvate oxidase and pyruvate decarboxylase, we suggest that the active sites of AHAS III are located at the interface of a dimer of catalytic subunits. Our experiments suggest that such a dimer is not stable except in the presence of the small subunits. The association of valine with sites on the regulatory subunits presumably influences the active sites by an allosteric conformational effect.

On the mechanism of leftward frameshifting at several hungry codons

We have used lacZ reporter genes to assess leftward ribosome frameshifting on sequences containing the quadruplet U UUC followed by several different triplets coding for lysine, isoleucine, or leucine. Limitation for lysine-tRNA provokes leftward frameshifting when the slippery quadruplet is followed by either lysine codon aag or aaa, but not when followed by an isoleucine or leucine codon. Limitation for isoleucine provokes frameshifting when the quadruplet is followed by either isoleucine codon aua or auc, but not when it is followed by a lysine codon. We conclude that the quadruplet promotes shifting when the ribosome is stalled at any "hungry" codon immediately after it. Changing the quadruplet to U AGC, at which peptidyl-tRNA cognate to the AGC triplet will be mismatched at all three anticodon positions if it slips left, abolishes frameshifting when the ribosome is stalled at the next position. We conclude that the U UUC quadruplet promotes frameshifting by virtue of its ability to pair with a left-slipped peptidyl-tRNA. The frameshift promoted by isoleucine-tRNA limitation of the U UUC aua sequence was analyzed by amino acid sequencing of the protein product. It occurs through reading of the Cau histidine codon overlapping the hungry codon from the left. This result rules out a "simultaneous slippage" type of mechanism. It strongly suggests instead that starvation-promoted frameshifting occurs primarily by slippage of peptidyl-tRNA just upstream of the stall site, followed by decoding of the triplet overlapping the stall site from the left or 5' side. A secondary finding is that the last base of the "hungry" codon has a moderate effect on its shiftiness, aag being shiftier than aaa, and aua being shiftier than auc.

Metabolic effects of inhibitors of two enzymes of the branched-chain amino acid pathway in Salmonella typhimurium

The metabolic effects of inhibitors of two enzymes in the pathway for biosynthesis of branched-chain amino acids were examined in Salmonella typhimurium mutant strain TV105, expressing a single isozyme of acetohydroxy acid synthase (AHAS), AHAS isozyme II. One inhibitor was the sulfonylurea herbicide sulfometuron methyl (SMM), which inhibits this isozyme and AHAS of other organisms, and the other was N-isopropyl oxalylhydroxamate (IpOHA), which inhibits ketol-acid reductoisomerase (KARI). The effects of the inhibitors on growth, levels of several enzymes of the pathway, and levels of intermediates of the pathway were measured. The intracellular concentration of the AHAS substrate 2-ketobutyrate increased on addition of SMM, but a lack of correlation between increased ketobutyrate and growth inhibition suggests that the former is not the immediate cause of the latter. The levels of the keto acid precursor of valine, but not of the precursor of isoleucine, were drastically decreased by SMM, and valine, but not isoleucine, partially overcame SMM inhibition. This apparent stronger effect of SMM on the flux into the valine arm, as opposed to the isoleucine arm, of the branched-chain amino acid pathway is explained by the kinetics of the AHAS reaction, as well as by the different roles of pyruvate, ketobutyrate, and the valine precursor in metabolism. The organization of the pathway thus potentiates the inhibitory effect of SMM. IpOHA has strong initial effects at lower concentrations than does SMM and leads to increases both in the acetohydroxy acid substrates of KARI and, surprisingly, in ketobutyrate. Valine completely protected strain TV105 from IpOHA at the MIC. A number of explanations for this effect can be ruled out, so that some unknown arrangement of the enzymes involved must be suggested. IpOHA led to initial cessation of growth, with partial recovery after a time whose duration increased with the inhibitor concentration. The recovery is apparently due to induction of new KARI synthesis, as well as disappearance of IpOHA from the medium.

Protozoan-enhanced toxicity of Bacillus thuringiensis var. israelensis delta-endotoxin against Aedes aegypti larvae

The toxicity of Bacillus thuringiensis var. israelensis (Bti) in mosquito larvae was enhanced by encapsulation in the protozoan Tetrahymena pyriformis. Aedes aegypti larvae which fed on T. pyriformis loaded with Bti died about three times faster than when fed on the same concentrations of Bti alone due to ingestion of higher toxin concentrations, reflected by shorter death times of exposed populations. The best larvicidal activities were achieved at ratios of cell/spore numbers in the range of 1:200 to 1:500. This enhancement of mortality by preincubation with T. pyriformis was higher at low Bti concentrations or in late third-instar larvae. Ninety minutes of preincubation yielded the best enhancement effect. Toxicity enhancement is very likely a consequence of concentrating large quantities of Bti spores and crystals (containing delta-endotoxin) by T. pyriformis cells and delivering them to the larvae. Shortening larval mortality time by encapsulation in T. pyriformis should reduce the exposure time of Bti to unfavorable field conditions that inactivate its larvicidal activity. Whether this method will indeed improve Bti efficacy is still to be determined.

Subunit association in acetohydroxy acid synthase isozyme III

Acetohydroxy acid synthase isozyme III (AHAS III) from Escherichia coli is composed of large and small subunits (encoded by the genes ilvI and ilvH) in an alpha 2 beta 2 structure. The large (61-kDa) subunit apparently contains the catalytic machinery of the enzyme, while the small (17-kDa) subunit is required for specific stabilization of the active conformation of the large subunit as well as for valine sensitivity. The interaction between subunits has been studied by using purified enzyme and extracts containing subcloned subunits. The association between large and small subunits is reversible, with a dissociation constant sufficiently high to have important experimental consequences: the activity of the enzyme shows a concentration dependence curve which is concave upward, and this dependence becomes linear upon the addition of excess large or small subunits. We estimate that at a concentration of 10(-7) M for each subunit (7 micrograms of enzyme ml-1), the large subunits are only half associated as the I2H2 active holoenzyme. This dissociation constant is high enough to cause underestimation of the activity of AHAS III in bacterial extracts. The true activity of this isozyme in extracts is observed in the presence of excess small subunits, which maintain the enzyme in its associated form. Reexamination of an E. coli K-12 ilvBN+ ilvIH+ strain grown in glucose indicates that AHAS III is the major isozyme expressed. As an excess of small subunits does not influence the apparent Ki for valine inhibition of the purified enzyme, it is likely that valine binds to and inhibits I2H2 rather than inducing dissociation. AHAS I and II seem to show a much lower tendency to dissociate than does AHAS III.

Properties of subcloned subunits of bacterial acetohydroxy acid synthases

The acetohydroxy acid synthase (AHAS) isozymes from enterobacteria are each composed of a large and small subunit in an alpha 2 beta 2 structure. It has been generally accepted that the large (ca. 60-kDa) subunits are catalytic, while the small ones are regulatory. In order to further characterize the roles of the subunits as well as the nature and the specificities of their interactions, we have constructed plasmids encoding the large or small subunits of isozymes AHAS I and AHAS III, each with limited remnants of the other peptide. The catalytic properties of the large subunits have been characterized and compared with those of extracts containing the intact enzyme or of purified enzymes. Antisera to the isolated subunits have been used in Western blot (immunoblot) analyses for qualitative and semiquantitative determinations of the presence of the polypeptides in extracts. The large subunits of AHAS isozymes I and III have lower activities than the intact enzymes: Vmax/Km is 20 to 50 times lower in both cases. However, for AHAS I, most of this difference is due to the raised Km of the large subunit alone, while for AHAS III, it is due to a lowered Vmax. The substrate specificities, R, of large subunits are close to those of the intact enzymes. The catalytic activity of the large subunits of AHAS I is dependent on flavin adenine dinucleotide (FAD), as is that of the intact enzyme, although the apparent affinities of the large subunits alone for FAD are 10-fold lower. Isolated subunits are insensitive to valine inhibition. Nearly all of the properties of the intact AHAS isozyme I or III can be reconstituted by mixing extracts containing the respective large and small subunits. The mixing of subunits from different enzymes does not lead to activation of the large subunits. It is concluded that the catalytic machinery of these AHAS isozymes is entirely contained within the large subunits. The small subunits are required, however, for specific stabilization of an active conformation of the large subunits as well as for value sensitivity.

Determination of products of acetohydroxy acid synthase by the colorimetric method, revisited

The enzyme acetohydroxy acid synthase (AHAS, EC 4.1.3.18) catalyzes two competing reactions of physiological importance: condensation of two molecules of pyruvate to form acetolactate (AL) or condensation of pyruvate and 2-ketobutyrate to form acetohydroxybutyrate (AHB). The activity of AHAS is most frequently analyzed using the Westerfeld method, in which the acetoin formed upon decarboxylation of AL is determined by colorimetric reaction with creatine and alpha-naphthol. However, there has been confusion as to the interpretation of the results of this assay in the presence of both substrates, conditions which lead to formation of both AL and AHB. By applying this assay to enzymatically prepared samples of AL and AHB which have also been analyzed by two other independent methods, we show here that the color yield for AHB in the commonly used assay is 35-40% that for equivalent amounts of acetoin or AL. The relative color yield is not significantly affected by varying the time or temperature of various steps in the color-forming reaction. This information could in principle be used, together with an independent specific assay for AHB, to determine the composition of an AHAS product mixture; it would, however, be less accurate than a simultaneous chromatographic method.

The fate of Bacillus thuringiensis var. israelensis in B. thuringiensis var. israelensis-killed pupae of Aedes aegypti

Carcasses of mosquito larvae killed by Bacillus thuringiensis var. israelensis allow its complete growth cycle (germination, vegetative growth, and sporulation), thus becoming toxic themselves to scavenging larvae. In this study, we demonstrate that the bacterium is capable of inducing death of Aedes aegypti pupae and of recycling in the resulting carcasses. B. thuringiensis var. israelensis-killed pupae were obtained by treating 40-hr-old synchronized fourth instar larvae with a low dose of spores (8000/ml). The fraction of dead pupae was reduced by higher or lower spore concentrations as well as by treating younger or older larval populations (both fourth instar): Increased proportions of dead larvae were obtained at higher concentration or by earlier treatment, whereas lower concentrations or later treatment resulted in more living pupae. Multiplication of B. thuringiensis var. israelensis is shown to occur in the carcasses of dead pupae. The number of spores in each pupal carcass followed a similar kinetic as in larval carcasses, but the final yield was about 10-fold higher, apparently reflecting the difference in dry weight between the two mosquito developmental stages (426 micrograms vs 83 micrograms, respectively). The specific larvicidal activity in a homogenized dead pupa was similar to that of B. thuringiensis var. israelensis powder, LC50 of about 600 spores/ml.

Acetohydroxy Acid Synthase Activity in Chlorella emersonii under Auto- and Heterotrophic Growth Conditions

Acetohydroxyacid synthase (AHAS) activity was studied in the green unicellular alga Chlorella emersonii. This activity and its regulation was compared in the algae grown autotrophically and heterotrophically on glucose in the dark. No evidence for the existence of more than one enzyme was found. The activity in crude extracts from either heterotrophically or autotrophically grown cells showed a K(m) for pyruvate of 9 millimolar, a 22-fold preference for 2-ketobutyrate over pyruvate as the second substrate, 50% inhibition by 0.5 millimolar valine, and 50% inhibition by 0.3 micromolar sulfometuron methyl (SMM). Spontaneous mutants of the alga resistant to SMM were isolated, which appeared to be single gene mutants containing SMM-resistant AHAS activity. Hence, AHAS appears to be the sole direct target site of SMM in C. emersonii. The fact that the mutants had equivalent SMM resistance under auto- and heterotrophic conditions further supports the conclusion that the same enzyme functions under both physiological regimes. The addition of valine and isoleucine leads to partial relief of SMM inhibition of biomass increase, but not of SMM inhibition of cell division.

Physiological implications of the substrate specificities of acetohydroxy acid synthases from varied organisms

Acetohydroxy acid synthase (AHAS; EC 4.1.3.18) catalyzes the following two parallel, physiologically important reactions: condensation of two molecules of pyruvate to form acetolactate (AL), in the pathway to valine and leucine, and condensation of pyruvate plus 2-ketobutyrate to form acetohydroxybutyrate (AHB), in the pathway to isoleucine. We have determined the specificity ratio R with regard to these two reactions (where VAHB and VAL are rates of formation of the respective products) as follows: VAHB/VAL = R [2-ketobutyrate]/[pyruvate] for 14 enzymes from 10 procaryotic and eucaryotic organisms. Each organism considered has at least one AHAS of R greater than 20, and some appear to contain but a single biosynthetic AHAS. The implications of this for the design of the pathway are discussed. The selective pressure for high specificity for 2-ketobutyrate versus pyruvate implies that the 2-ketobutyrate concentration is much lower than the pyruvate concentration in all these organisms. It seems important for 2-ketobutyrate levels to be relatively low to avoid a variety of metabolic interferences. These results also reinforce the conclusion that biosynthetic AHAS isozymes of low R (1 to 2) are a special adaptation for heterotrophic growth on certain poor carbon sources. Two catabolic "pH 6 AL-synthesizing enzymes" are shown to be highly specific for AL formation only (R less than 0.1).

Induction of an ATP-polymerizing enzyme in TMV-infected tobacco and its homology to the human 2'-5' A synthetase

Several reports have indicated that tobacco carries an enzyme (APE) that, in the presence of poly (rI):(rC), polymerizes ATP to oligoadenylates. This paper demonstrates that the tobacco APE system comprises several proteins (estimated sizes: 32, 42, 67, and 84 +/- 10% kD). Only one of these proteins (the "67-kD" form) binds to poly (rI):(rC). This APE form has been purified by affinity chromatography on a synthetic ds-RNA column. Four tobacco proteins, including the purified one, crossreact with antibodies against the human enzyme, 2'-5' A synthetase. The ATP-binding capacity of some of these proteins has also been demonstrated. The amount of plant oligoadenylates obtained by polymerizing ATP with the purified APE form allows, for the first time, their direct analysis by TLC. The TLC analysis indicated that the oligomer produced by APE is not identical to the 2'-5' oligoadenylate. The appearance of the 2'-5' A-related proteins correlates with the build up of TMV infection, and the pattern of their stimulation and turnover was established. Nucleic acid hybridization indicates homology of tobacco DNA and RNA sequences with cloned cDNA of the human 2'-5' A synthetase gene. The stimulation in tobacco, upon TMV infection, of mRNA species homologous to the above human cDNA has been demonstrated. The analogy between the plant and the human system is discussed.

Structural correlates of a regulatory idiotope

The A48RI expressed on the ABPC48 and UPC10 beta 2----6 fructosan-binding myeloma proteins is a conformational antigenic determinant encoded by V genes deriving from the VHX24 and VK10 families. In the preimmune repertoire the clones using VHX24 genes rarely express A48 idiotopes, clearly demonstrating that this regulatory idiotope is a minor or silent idiotope. Furthermore, these same VHX24-utilizing preimmune clones are frequently associated with the VK1 gene family which is highly represented in the neonatal and adult repertoires. The clonal expansion occurring subsequent to neonatal injection of minute amounts of anti-Id antibodies leads to selective expansion of A48Id+ clones associated with class switching. Few somatic mutations are observed in preimmune clones, or in those expanded by anti-Id antibodies. The fact that few mutations were observed in the IgG1 clones obtained from animals injected with anti-A48Id antibodies after birth indicates that, in contrast to antigen-induced class-switching, the anti-Id-induced switching is not associated with a highly active mutational process. In contrast to the preimmune clones, or those expanded by anti-Id (in the absence of antigenic stimulation) in which VHX24 is associated with VK regions deriving from various gene families, the clones expanded by anti-Id and fructan resemble A48 by using VHX24 and VK10 genes. Few apparent mutations were also observed in these IgM or IgG3 clones expressing A48 idiotopes. The A48 RI can be expressed on clones producing antibodies specific for various self and foreign antigens, and encoded by V genes deriving from various VH and VK families. These results indicate that key contacting residues bearing A48 conformational idiotypic determinants can be made up by various VH-VK combinations. A comparison of the VH and VL sequences of A48 RI+ mAbs showed that many of the observed somatic mutations could be correlated to decreased IDA10 binding. This comparison allowed identification of specific idiotope-determining regions of VH and VK which could represent contacting residues with anti-idiotypic antibodies. The contributions of these regions to the expression of the A48Id was tested by generating a transfectoma antibody expressing the rearranged VHJ558 gene of the ricin 45 hybridoma and the VK10-Ars-a gene of the 36-65 hybridoma. This transfectoma antibody expresses the idiotope recognized by IDA10 and confirms the conformational nature of this idiotope. There are three amino acid residues shared by VHX24 and VHJ558 antibodies expressing the A48 RI which are important for its expression.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetics and mechanism of acetohydroxy acid synthase isozyme III from Escherichia coli

Acetohydroxy acid synthase (AHAS, EC 4.1.3.18) isozyme III from Escherichia coli has been studied in steady-state kinetic experiments in which the rates of formation of acetolactate (AL) and acetohydroxybutyrate (AHB) have been determined simultaneously. The ratio between the rates of production of the two alternative products and the concentrations of the substrates pyruvate and 2-ketobutyrate (2KB) leading to them, R, VAHB/VAL = R[( 2KB]/[pyruvate]), was found to be 40 +/- 3 under a wide variety of conditions. Because pyruvate is a common substrate in the reactions leading to both products and competes with 2-ketobutyrate to determine whether AL or AHB is formed, steady-state kinetic studies are unusually informative for this enzyme. At a given pyruvate concentration, the sum of the rates of formation of AL and AHB was nearly independent of the 2-ketobutyrate concentration. On the basis of these results, a mechanism is proposed for the enzyme that involves irreversible and rate-determining reaction of pyruvate, at a site which accepts 2-ketobutyrate poorly, if at all, to form an intermediate common to all the reactions. In the second phase of the reaction, various 2-keto acids can compete for this intermediate to form the respective acetohydroxy acids. 2-Keto acids other than the natural substrates pyruvate and 2-ketobutyrate may also compete, to a greater or lesser extent, in the second phase of the reaction to yield alternative products, e.g., 2-ketovalerate is preferred by about 2.5-fold over pyruvate. However, the presence of an additional keto acid does not affect the relative specificity of the enzyme for pyruvate and 2-ketobutyrate; this further supports the proposed mechanism. The substrate specificity in the second phase is an intrinsic property of the enzyme, unaffected by pH or feedback inhibitors.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological implications of the specificity of acetohydroxy acid synthase isozymes of enteric bacteria

The rates of formation of the two alternative products of acetohydroxy acid synthase (AHAS) have been determined by a new analytical method (N. Gollop, Z. Barak, and D. M. Chipman, Anal. Biochem., 160:323-331, 1987). For each of the three distinct isozymes of AHAS in Escherichia coli and Salmonella typhimurium, a specificity ratio, R, was defined: Formula: see text, which is constant over a wide range of substrate concentrations. This is consistent with competition between pyruvate and 2-ketobutyrate for an active acetaldehyde intermediate formed irreversibly after addition of the first pyruvate moiety to the enzyme. Isozyme I showed no product preference (R = 1), whereas isozymes II and III form acetohydroxybutyrate (AHB) at approximately 180- and 60-fold faster rates, respectively, than acetolactate (AL) at equal pyruvate and 2-ketobutyrate concentrations. R values higher than 60 represent remarkably high specificity in favor of the substrate with one extra methylene group. In exponentially growing E. coli cells (under aerobic growth on glucose), which contain about 300 microM pyruvate and only 3 microM 2-ketobutyrate, AHAS I would produce almost entirely AL and only 1 to 2% AHB. However, isozymes II and III would synthesize AHB (on the pathway to Ile) and AL (on the pathway to valine-leucine) in essentially the ratio required for protein synthesis. The specificity ratio R of any AHAS isozyme was affected neither by the natural feedback inhibitors (Val, Ile) nor by the pH. On the basis of the specificities of the isozymes, the known regulation of AHAS I expression by the catabolite repression system, and the reported behavior of bacterial mutants containing single AHAS isozymes, we suggest that AHAS I enables a bacterium to cope with poor carbon sources, which lead to low endogenous pyruvate concentrations. Although AHAS II and III are well suited to producing the branched-chain amino acid precursors during growth on glucose, they would fail to provide appropriate quantities of AL when the concentration of pyruvate is relatively low.

Fate of Bacillus thuringiensis subsp. israelensis under Simulated Field Conditions

The fate of Bacillus thuringiensis subsp. israelensis in a natural aquatic habitat was studied in a model system by using laboratory-simulated field waters and a mutant of the bacterium resistant to three antibiotics. Contact with mud of a sporal culture of the mutant resulted in an immediate disappearance of the larvicidal activity but had no influence on viability. The cessation of toxicity was caused by bacterial adsorption on soil particles, since 99.8% of the bacteria was found in the mud fraction within 45 min, with concurrent disappearance from the supernatant. When the mud was stirred, the bacteria could be redetected. The viability count of the mud suspension remained practically constant for at least 22 days, indicating that the spores were still fully viable but were incapable of germinating and multiplying in the mud under our experimental conditions. Approximately 8% of the colony forming ability of the bacteria could be separated from the mud by vigorous mixing followed by immediate filtration. The filtrated spores retained their toxicity, killing 90% of the larval populations even after 22 days incubation in the soil. The inactivation of the toxic activity of B. thuringiensis subsp. israelensis in the mud was therefore a reversible process and was probably due to masking of the bacteria, thus making the bacteria and their toxin inaccessible to the larvae. In the simulated field waters without mud, we observed only a very slow inhibition of the larvicidal activity. In contrast to the activity in the mud suspension, this activity could not be restored.

A method for simultaneous determination of the two possible products of acetohydroxy acid synthase

A method for the simultaneous assay of 2-acetolactate and 2-aceto-2-hydroxybutyrate formation catalyzed by acetohydroxy acid synthase in the presence of its substrates pyruvate and 2-ketobutyrate is described. The method, appropriate for the study of the physiologically and mechanistically significant competition between the two reactions, involves oxidative decarboxylation of the acetohydroxy acids to the corresponding 2,3-diketones, transfer of the volatile diketones to methanol, and gas chromatographic analysis with electron-capture detection. Oxidative decarboxylation by air requires catalytic activation, and addition of iron salts is crucial to the success of the method with purified enzymes.

Inhibition of acetohydroxy acid synthase by leucine

The enzymatic reaction of acetohydroxy acid synthase in crude extracts of Escherichia coli K-12 is inhibited by leucine. Inhibition is most pronounced at low pH values and is low at pH values higher than 8.0. Both isoenzymes of acetohydroxy acid synthase present in E. coli K-12 (isoenzyme I and isoenzyme III) are inhibited by leucine. Isoenzyme I, which is responsible for the majority of acetohydroxy acid synthase activity in E. coli K-12 at physiological pH, is inhibited almost completely by 30 mM leucine at pH 6.25-7.0 and is not affected at all at pH values higher than 8.4. Inhibition of isoenzyme I by leucine is a mixed noncompetitive process. Leucine inhibition of isoenzyme III is pH-independent and reaches only 40% at 30 mM leucine. The inhibition of acetohydroxy acid synthase by leucine at physiological pH, observed in vitro in this study, correlates with the idea that acetohydroxy acid synthase is a target for the toxicity of the abnormally high concentrations of leucine in E. coli K-12.

Acetohydroxy acid synthase is a target for leucine containing peptide toxicity in Escherichia coli

Acetohydroxy acid synthase from a mutant resistant to leucine-containing peptides was insensitive to leucine inhibition. It is concluded that acetohydroxy acid synthase is a target for the toxicity of the high concentrations of leucine brought into Escherichia coli K-12 by leucine-containing peptides.

Toxicity of leucine-containing peptides in Escherichia coli caused by circumvention of leucine transport regulation

A variety of leucine-containing peptides (LCP), Phe-Leu, Gly-Leu, Pro-Leu, Ala-Leu, Ala-Leu-Lys, Leu-Phe-Ala, Leu-Leu-Leu, and Leu-Gly-Gly, inhibited the growth of a prototrophic strain of Escherichia coli K-12 at concentrations between 0.05 and 0.28 mM. Toxicity requires normal uptake of peptides. When peptide transport was impaired by mutations, strains became resistant to the respective LCP. Inhibition of growth occurred immediately after the addition of LCP, and was relieved when 0.4 mM isoleucine was added. The presence of Gly-Leu in the medium correlated with the inhibition of growth, and the bacteria began to grow at the normal rate 70 min after Gly-Leu became undetectable. Disappearance of the peptide corresponded with the appearance of free leucine and glycine in the medium. The concentration of leucine inside the LCP-treated bacteria was higher than that in the leucine-treated and the control cultures. We suggest that entry of LCP into the cells via peptide transport systems circumvents the regulation of leucine transport, thereby causing abnormality high concentrations of leucine inside the cells. This accumulation of leucine interferes with the biosynthesis of isoleucine and inhibits the growth of the bacteria.