The Two-Isozyme System of 3-Deoxy-d-arabino-Heptulosonate 7-Phosphate Synthase in Nicotiana silvestris and Other Higher Plants

Two isozymes of 3-deoxy-d-arabino-heptulosonate 7-phosphate (DAHP) synthase, denoted DS-Mn and DS-Co, were identified following DEAE-cellulose chromatography of crude extracts prepared from suspension-cultured cells of Nicotiana silvestris. The strikingly different properties of the isozymes allowed the development of assays for the selective detection of either isozyme in samples containing a mixture of the two. The DS-Mn isozyme required the sulfhydryl reductant, dithiothreitol, for activity and was stimulated by manganese. Activation by dithiothreitol was slow relative to catalysis, accounting for a hysteretic progress curve that was observed when reactions were started with inactive enzyme. The DS-Co isozyme was inhibited by dithiothreitol and required a divalent cation for activity. At optimal cation concentrations of 10 millimolar (magnesium), 0.5 millimolar (cobalt), and 0.5 millimolar (manganese), relative activities obtained were 100, 85, and 20, respectively. The substrate saturation curves with respect to erythrose 4-phosphate differed markedly when the two isozymes were compared. As little as 0.5 millimolar erythrose 4-phosphate saturated DS-Mn, whereas a 10-fold higher concentration was needed for saturation of DS-Co. The pH optimum of DS-Mn was 8.0, while that of the DS-Co isozyme was 8.6. Leaves of both N. silvestris and spinach also exhibited the DS-Mn/DS-Co isozyme arrangement, and the subcellular location of DS-Mn was shown to be the chloroplast compartment. By application of the differential assays for DAHP synthase isozymes, various monocotyledonous and dicotyledonous plants yielded data indicating the general presence of the DS-Mn/DS-Co isozyme pair in higher plants.     

Glyphosate Induces 3-Deoxy-d-arabino-Heptulosonate 7-Phosphate Synthase in Potato (Solanum tuberosum L.) Cells Grown in Suspension Culture

The activity of the first enzyme of the shikimate pathway, 3-deoxy-d-arabino-heptulosonate 7-phosphate synthase, varies during the growth cycle of Solanum tuberosum L. cv superior cells in suspension culture. Maximum specific enzyme activity was observed midway through the linear phase of growth. When mid-log phase cells are exposed to glyphosate, the specific activity of the enzyme increases severalfold within 24 hours. The glyphosate-induced increase in enzyme activity is due to an increase in the amount of enzyme as determined by immunoblotting. Glyphosate (up to 2 millimolar) has no effect on the enzyme activity in vitro. Dehydroquinate synthase, the second enzyme of the shikimate pathway, is not induced by glyphosate.     

Glyphosate Inhibition of 5-Enolpyruvylshikimate 3-Phosphate Synthase from Suspension-Cultured Cells of Nicotiana silvestris

Treatment of isogenic suspension-cultured cells of Nicotiana silvestris Speg. et Comes with glyphosate (N-[phosphonomethyl]glycine) led to elevated levels of intracellular shikimate (364-fold increase by 1.0 millimolar glyphosate). In the presence of glyphosate, it is likely that most molecules of shikimate originate from the action of 3-deoxy-d-arabino-heptulosonate 7-phosphate (DAHP) synthase-Mn since this isozyme, in contrast to the DAHP synthase-Co isozyme, is insensitive to inhibition by glyphosate. 5-Enolpyruvylshikimate 3-phosphate (EPSP) synthase (EC 2.5.1.19) from N. silvestris was sensitive to micromolar concentrations of glyphosate and possessed a single inhibitor binding site. Rigorous kinetic studies of EPSP synthase required resolution from the multiple phosphatase activities present in crude extracts, a result achieved by ion-exchange column chromatography. Although EPSP synthase exhibited a broad pH profile (50% of maximal activity between pH 6.2 and 8.5), sensitivity to glyphosate increased dramatically with increasing pH within this range. In accordance with these data and the pK_a values of glyphosate, it is likely that the ionic form of glyphosate inhibiting EPSP synthase is COO⁻CH₂NH₂⁺CH₂PO₃²⁻, and that a completely ionized phosphono group is essential for inhibition. At pH 7.0, inhibition was competitive with respect to phosphoenolpyruvate (K_i = 1.25 micromolar) and uncompetitive with respect to shikimate-3-P (K_i′ = 18.3 micromolar). All data were consistent with a mechanism of inhibition in which glyphosate competes with PEP for binding to an [enzyme:shikimate-3-P] complex and ultimately forms the dead-end complex of [enzyme:shikimate-3-P:glyphosate].     

3-Deoxy-d-arabino-Heptulosonate 7-Phosphate Synthase from Potato Tuber (Solanum tuberosum L.)

3-Deoxy-d-arabino-heptulosonate 7-phosphate synthase, the first enzyme of the shikimate pathway, was purified to electrophoretic homogeneity from tubers of Solanum tuberosum L. cv Superior. The enzyme is a dimer with a native molecular weight of 110,000. The enzyme appears to be hysteretic. The enzyme activity is stimulated by Mn²⁺ and l-tryptophan. Chromatofocusing resolved two forms of the enzyme with isoelectric points of 7.8 and 8.4, respectively. The enzyme closely resembles an analogous activity previously isolated from roots of Daucus carota (JA Suzich, JFD Dean, KM Herrmann 1985 Plant Physiol 79: 765-770).     

Wounding Induces One of Two Isoenzymes of 3-Deoxy-d-arabino-Heptulosonate 7-Phosphate Synthase in Solanum tuberosum L

Potato (Solanum tuberosum L.) tubers contain two isoenzymes of 3-deoxy-d-arabino-heptulosonate 7-phosphate synthase (EC 4.1.2.15), the enzyme that catalyzes the first step of aromatic amino acid biosynthesis. One of the isoenzymes is specifically activated by Mn²⁺, and the other requires Co²⁺, Mg²⁺, or another divalent cation for activity. Monospecific polyclonal antibodies against the Mn²⁺-activated isoenzyme do not cross-react with the other isoenzyme. Wounding of potato tubers induces the Mn²⁺-activated form but not the other. We conclude that two different genes encode two different isoenzymes that catalyze the first step in the shikimate pathway.     

3-Deoxy-d-arabino-Heptulosonate 7-Phosphate Synthase from Carrot Root (Daucus carota) Is a Hysteretic Enzyme

Roots of carrots (Daucus carota) contain three activities of 3-deoxy-d-arabino-heptulosonate 7-phosphate (DAHP) synthase, the enzyme that catalyzes the first step of the shikimate pathway. The three activities, enzymes I, II, and III, are separated by chromatography on phosphocellulose. Enzyme III, purified to electrophoretic homogeneity, has a native molecular weight of 103,000 and consists of two identical subunits of 53,000 daltons each. Double reciprocal plots of reaction velocity versus substrate concentration yield K_m values of 0.03 and 0.07 millimolar for P-enolpyruvate and erythrose-4-P, respectively. Both products, DAHP and orthophosphate, inhibit the enzyme. Enzyme III is a hysteretic enzyme that is activated by physiological concentrations of l-tryptophan and Mn²⁺, both of which also partially eliminate the hysteretic lag. Feedback activation of carrot DAHP synthase by tryptophan is interpreted to be an early regulatory signal for polyphenol biosynthesis. The three carrot DAHP synthase isoenzymes share antigenic determinants.     

Substrate Ambiguity of 3-Deoxy-d-manno-Octulosonate 8-Phosphate Synthase from Neisseria gonorrhoeae in the Context of Its Membership in a Protein Family Containing a Subset of 3-Deoxy-d-arabino-Heptulosonate 7-Phosphate Synthases

3-Deoxy-d-manno-octulosonate 8-phosphate (KDOP) synthase and 3-deoxy-d-arabino-heptulosonate 7-phosphate (DAHP) synthase catalyze similar phosphoenolpyruvate-utilizing reactions. The genome of Neisseria gonorrhoeae contains one gene encoding KDOP synthase and one gene encoding DAHP synthase. Of the two nonhomologous DAHP synthase families known, the N. gonorrhoeae protein belongs to the family I assemblage. KDOP synthase exhibited an ability to replace arabinose-5-P with either erythrose-4-P or ribose-5-P as alternative substrates. The results of periodate oxidation studies suggested that the product formed by KDOP synthase with erythrose-4-P as the substrate was 3-deoxy-d-ribo-heptulosonate 7-P, an isomer of DAHP. As expected, this product was not utilized as a substrate by dehydroquinate synthase. The significance of the ability of KDOP synthase to substitute erythrose-4-P for arabinose-5-P is (i) recognition of the possibility that the KDOP synthase might otherwise be mistaken for a species of DAHP synthase and (ii) the possibility that the broad-specificity type of KDOP synthase might be a relatively vulnerable target for antimicrobial agents which mimic the normal substrates. An analysis of sequences in the database indicates that the family I group of DAHP synthase has a previously unrecognized membership which includes the KDOP synthases. The KDOP synthases fall into a subfamily grouping which includes a small group of DAHP synthases. Thus, family I DAHP synthases separate into two subfamilies, one of which includes the KDOP synthases. The two subfamilies appear to have diverged prior to the acquisition of allosteric-control mechanisms for DAHP synthases. These allosteric control specificities are highly diverse and correlate with the presence of N-terminal extensions which lack homology with one another.3-Deoxy-d-arabino-heptulosonate 7-phosphate (DAHP) and 3-deoxy-d-manno-octulosonate 8-phosphate (KDOP) are analogous seven- and eight-carbon 2-keto-3-deoxy sugars that are synthesized by enzymes which belong to functionally unrelated pathways. DAHP synthase forms DAHP as the acyclic precursor of the aromatic amino acids in bacteria, lower eukaryotes, and plants (3); KDOP synthase is best known for its role in the formation of KDOP as a critical component of the lipopolysaccharide of gram-negative bacteria (37), but its distribution in nature has recently been recognized to be broader (13). Both enzymes catalyze an overall condensation of phosphoenolpyruvate (PEP) with an aldose, i.e., erythrose-4-phosphate (E4P) in the case of DAHP synthase and arabinose-5-phosphate (A5P) in the case of KDOP synthase. The reactions are irreversible and are not aldol-type condensations, which unfortunately has been implied by the Enzyme Commission naming that has been recommended for DAHP synthase.As might be expected from the close structural relationship of A5P and E4P, the reactions are strikingly similar. This similarity is reflected at the level of mechanistic detail (see reference 16 and references therein). DAHP synthase and KDOP synthase, along with enolpyruvoylshikimate 3-phosphate synthase and UDP-N-acetylglucosamine enolpyruvoyl transferase, comprise a small class of PEP-utilizing enzymes that catalyze C—O bond cleavage with respect to the release of P_i from PEP (1, 27). This contrasts with the more familiar nucleophilic attack at the phosphorous atom of PEP that results in P—O bond cleavage by the action of enzymes such as pyruvate kinase (25), PEP carboxylase (34), and PEP carboxykinase (8).In classical studies with Escherichia coli, DAHP synthase (44, 45) and KDOP synthase (41) are specific for E4P and A5P, respectively. In contrast, we found that the KDOP synthase of Neisseria gonorrhoeae possessed the ability to utilize E4P in place of A5P. We addressed the question of whether KDOP synthase of N. gonorrhoeae in the presence of E4P and PEP was able to form DAHP, in which case it would also have the potential to function as a DAHP synthase. The time-dependent cleavage of the product was investigated by the periodate-oxidation-thiobarbituric acid (TBA) assay, and these results allow some speculation on the stereospecific course of the reaction in comparison with the reaction of DAHP synthase.     

Identification of Essential Cysteine Residues in 3-Deoxy-d-Arabino-Heptulosonate-7-Phosphate Synthase from Corynebacterium glutamicum

To ascertain the functional role of cysteine residue in 3-deoxy-d-arabino-heptulosonate-7-phosphate (DAHP) synthase from Corynebacterium glutamicum, site-directed mutagenesis was performed to change each of the three residues to serine. Plasmids were constructed for high-level overproduction and one-step purification of histidine-tagged DAHP synthase. Analysis of the purified wild-type and mutant enzymes by SDS-polyacrylamide gel electrophoresis showed an apparent protein band with a molecular mass of approximately 45 kDa. Cys¹⁴⁵Ser mutant retained about 16% of the enzyme activity, while DAHP synthase activity was abolished in Cys⁶⁷Ser mutant. Kinetic analysis of Cys¹⁴⁵Ser mutant with PEP as a substrate revealed a marked increase in K _m with significant change in k _cat , resulting in a 13.6-fold decrease in k _cat /K _m ^PEP. Cys³³⁴ was found to be nonessential for catalytic activity, although it is highly conserved in DAHP synthases. From these studies, Cys⁶⁷ appears important for synthase activity, while Cys¹⁴⁵ plays a crucial role in the catalytic efficiency through affecting the mode of substrate binding. Received: 10 October 2000 / Accepted: 17 November 2000     

Expression of NADH-Dependent Glutamate Synthase in Response to the Supply of Nitrogen in Rice Cells in Suspension Culture

The effects of inorganic and organic nitrogen on the levelsof mRNA for NADH-dependent glutamate synthase (GOGAT) and theprotein were examined in rice cells in suspension culture. Asupply of NH⁺₄, NO^-₃, glutamine, or asparagine induced the accumulationof the protein and mRNA, but levels of mRNA for ferredoxin-GOGATwere hardly affected. ¹Present address: P.C. Center Wakuya-cho, Toda-gun, Miyagi,Japan.     

Response of aromatic-pathway enzymes to changing physiological states of growth in suspension cultures of Nicotiana silvestris

The activity levels of enzymes of aromatic amino acid biosynthesis respond to changing physiological states of growth, as illustrated by results obtained from suspension-cultured cells of Nicotiana silvestris Speg. et Comes line ANS 1 (2N=24). The experimental system provides a foundation for interpretations about overall regulation of enzyme levels in relationship to growth physiology. Levels of activity for shikimate dehydrogenase (EC 1.1.1.25), prephenate aminotransferase and arogenate dehydrogenase were followed throughout a growth cycle obtained by a conventional subculture protocol. Enzyme date were also obtained from cell cultures maintained in continuous exponential growth for greater than 10 generations (EE cells). Both shikimate dehydrogenase and prephenate aminotransferase exhibited elevated stationary-phase levels of enzyme, much of which was carried over into a subsequent subculture. At least 4 generations of exponential growth were required before diminution of the latter two enzymes to the levels characteristic of truly exponential-phase growth (EE cells) occurred. This is reminiscent of the overall behavior of 3-deoxy-D- arabino -heptulosonate 7-phosphate (DAHP) synthase (EC 4.1.2.15), specifically attributed to the properties of the cytosolic isozyme species (DAHP synthase-Co). Elevation of arogenate dehydrogenase also occurred in stationary-phase cells, but diminished rapidly during lag phase to reach the level characteristic of EE cells.     

Physiological and biochemical characterization of a suspensionculture system for sustained exponential growth of Nicotiana silvestris

A strong approach to understanding the regulation of enzymes in metabolic pathways, such as those responsible for amino acid biosynthesis, is to follow enzyme levels throughout the growth curve of higher plant cells in suspension culture. The rise and fall of enzyme levels can be traced as a function of physiological stage of growth Subculturing, as typically carried out by low-factor dilution of stationary phase cells, yields a system suitable for the study of changes in enzyme and metabolite levels that accompany the transition from stationary-phase physiology to exponential-phase physiology. However, the short duration of exponential growth in such subculture protocols is inadequate to avoid carryover effects from previous stationary-phase physiology. Suspension cultures of Nicotiana silvestris Speg, et Comes (2N = 24) were used to demonstrate substantial carryover levels of acid phosphatase, alkaline phosphatase and protease activities. A subculture routine is described for maintaining cell populations in exponential phase indefinitely. About 10 generations of sustained exponential growth is required to approach a true balanced state of exponential growth. Such exponential phase populations consist of cells termed EE cells. EE-cell populations were similar to cells that have been in exponential phase for only a few generations (E cells), with respect to doubling time (about 40 h) and to minimal density of diluted populations able to resume growth (about 500 cells ml^?1). EE cells possess a high content of soluble protein; they are smaller and more aggregated than are E cells. Upon dilution into fresh medium, EE cells resume exponential growth without a lag. In contrast to E cells, EE cells exhibit properties of balanced growth, since proportionate increases in cell number, dry weight, wet weight and packed-cell volume were observed. E cells, sampled at different elapsed times of growth, are likely to differ in metabolite, enzyme and cell properties, whereas EE cells exhibit near-constant properties.     

菊芋悬浮细胞对盐胁迫的生理响应

为探讨菊芋（Helianthus tuberosus）悬浮培养细胞对盐胁迫的生理响应,在0、50、100、150、200 mmol·L^-1NaCl处理下测定了细胞的生物量、相对细胞活力、抗氧化酶活性以及过氧化氢、丙二醛、脯氨酸、可溶性糖、可溶性蛋白质的含量,测定了总酚含量并鉴别、定量分析了14种酚类化合物。结果表明：盐胁迫显著减弱了菊芋悬浮细胞的活力,抑制了细胞的增长,200 mmol·L^-1NaCl处理下细胞生长基本上停止。盐胁迫诱发了细胞氧化胁迫,丙二醛含量显著增加,抗坏血酸过氧化物酶、过氧化物酶活性以及总酚含量、部分酚类化合物含量均随NaCl浓度升高而增加,酚类物质和抗氧化酶系统共同参与了应对氧化胁迫的抗氧化作用。脯氨酸在菊芋悬浮细胞应对NaCl渗透胁迫的渗透调节作用中扮演了重要的角色,而可溶性糖发挥的作用不大。     

Cytoplastic Glyceraldehyde-3-Phosphate Dehydrogenases Interact with ATG3 to Negatively Regulate Autophagy and Immunity in Nicotiana benthamiana

Autophagy as a conserved catabolic pathway can respond to reactive oxygen species (ROS) and plays an important role in degrading oxidized proteins in plants under various stress conditions. However, how ROS regulates autophagy in response to oxidative stresses is largely unknown. Here, we show that autophagy-related protein 3 (ATG3) interacts with the cytosolic glyceraldehyde-3-phosphate dehydrogenases (GAPCs) to regulate autophagy in Nicotiana benthamiana plants. We found that oxidative stress inhibits the interaction of ATG3 with GAPCs. Silencing of GAPCs significantly activates ATG3-dependent autophagy, while overexpression of GAPCs suppresses autophagy in N. benthamiana plants. Moreover, silencing of GAPCs enhances N gene-mediated cell death and plant resistance against both incompatible pathogens Tobacco mosaic virus and Pseudomonas syringae pv tomato DC3000, as well as compatible pathogen P. syringae pv tabaci. These results indicate that GAPCs have multiple functions in the regulation of autophagy, hypersensitive response, and plant innate immunity.     

Differentially Regulated Isozymes of 3-Deoxy-d-arabino-Heptulosonate-7-Phosphate Synthase from Seedlings of Vigna radiata [L.] Wilczek

Two isozymes of 3-deoxy-d-arabino-heptulosonate-7-phosphate synthase (EC 4.1.2.15) designated DS-Mn and DS-Co were separated from seedlings of Vigna radiata [L.] Wilczek by DEAE-cellulose column chromatography. DS-Mn was activated 2.6-fold by 0.4 millimolar manganese, had an activity optimum of 7.0, and was substrate inhibited by erythrose-4-phosphate (E4P) concentrations in excess of 0.5 millimolar. In contrast, DS-Co had an activity optimum at pH 8.8, required E4P concentrations of at least 4 millimolar to approach saturation, and exhibited an absolute requirement for divalent cation (cobalt being the best). Regulatory properties of the two isozymes differed dramatically. The activity of DS-Mn was activated by chorismate (noncompetitively against E4P and competitively against phosphoenolpyruvate), and was inhibited by prephenate and l-arogenate (competitively against E4P and noncompetitively against phosphoenolpyruvate in both cases). Under standard assay conditions, l-arogenate inhibited the activity of DS-Mn 50% at a concentration of 155 micromolar and was at least 3 times more potent than prephenate on a molar basis. Weak inhibition of DS-Mn by l-tryptophan was also observed, the magnitude of inhibition increasing with decreasing pH. The pattern of allosteric control found for DS-Mn is consistent with the operation of a control mechanism of sequential feedback inhibition governing overall pathway flux. DS-Co was not subject to allosteric control by any of the molecules affecting DS-Mn. However, DS-Co was inhibited by caffeate (3,4-dihydroxycinnamate), noncompetitively with respect to either substrate. The striking parallels between the isozyme pairs of 3-deoxy-d-arabino-heptulosonate-7-phosphate synthase and chorismate mutase are noted—one isozyme in each case being tightly regulated, the other being essentially unregulated.     

NAD- and NADP-Linked Glyceraldehyde-3-Phosphate Dehydrogenases in Light- and Dark-Germinated Seeds of Scots Pine (Pinus silvestris)

By means of arsenate in optical tests the occurrence of both NAD- and NADP-linked glyceraldehyde-3-phosphate dehydrogenase activity has been found in embryo and endosperm tissues from Scots pine seeds. By exclusion of arsenate also evidences for the activity of a non-phosphorylating NADP-linked form have been demonstrated. The results are sustained by inhibition experiments. The enzyme activities have been followed principally during the first 24 hours of the germination process and in relation to the germination-controlling light factor. Homogenates from complete seeds and partly also from separated embryo and endosperm tissues have been studied. The predominant activity in both of the tissues was linked to the NAD-form, expressed either on seed (part) or protein basis. No indications for a light-regulation of anyone of these enzymes have been found contrary to findings from other plant materials.     

The essential role of cobalt in the inhibition of the cytosolic isozyme of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase from Nicotiana silvestris by glyphosate

The prime molecular target of glyphosate (N-[phosphonomethyl]glycine), a potent herbicidal and antimicrobial agent, is known to be the shikimate-pathway enzyme, 5-enol-pyruvylshikimate-3-phosphate synthase. Inhibition by glyphosate of an earlier pathway enzyme that is located in the cytosol of higher plants, 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (DS-Co), has raised the possibility of dual enzyme targets in vivo. With the recent appreciation that magnesium (and manganese) can replace cobalt as the divalent-metal activator of DS-Co, it has now been possible to show that sensitivity of DS-Co to inhibition by glyphosate is obligately dependent upon the presence of cobalt. Evidence for a cobalt(II):glyphosate complex with octahedral coordination was obtained through examination of the effect of glyphosate upon the visible electronic spectrum of aqueous solutions of cobalt(II) chloride. The presence of glyphosate increased the concentration of cobalt(II) chloride required for enzyme activity, and the concentration of cobalt(II) chloride markedly affected the concentration of glyphosate required for inhibition of DS-Co activity. The extent to which DS-Co is vulnerable to inhibition by glyphosate in vivo depends, therefore, upon the unknown extent to which DS-Co molecules in the cytosol might be associated with cobalt.     

适合于悬浮培养的枇杷愈伤组织诱导及状态调控

以枇杷不同品种和不同器官为外植体,通过对愈伤组织诱导率、诱导状态、熊果酸（UA）和齐墩果酸（OA）含量的综合考虑,筛选适合于悬浮培养的枇杷愈伤组织,并通过调整2,4-D、6-BA浓度,同时加入酸水解酪蛋白（CH）、谷氨酰胺（Gln）、KCl等物质对花丝愈伤组织状态进行调控。结果表明：枇杷‘早钟6号’幼胚诱导的愈伤组织最适用于枇杷悬浮培养,愈伤诱导率为83.70%,愈伤组织嫩黄、疏松、颗粒明显,UA、OA含量分别为6.90、2.34 mg/g dw。在MS中附加2,4-D 6.0 mg/L、6-BA 0.8 mg/L、CH 200 mg/L、KCl 500 mg/L,花丝诱导的坚硬愈伤出现细软、嫩黄的愈伤组织,但多次继代后水渍化,状态不佳。