Aspartokinase in Lemna minor L: Studies on the in Vivo and in Vitro Regulation of the Enzyme

Changes of Chemical Composition and Hemicelluloses Structure in Differently Aged Bamboo ( Neosinocalamus affinis) Culms

To study the differences in chemical composition analysis and spatial distribution of young Neosinocalamus affinis bamboo, we used the methods of standard of National Renewable Energy Laboratory and confocal Raman microscopy, respectively. It was found that the acid-soluble lignin and acid-insoluble lignin content showed an inverse relationship with the increasing bamboo age. Raman analysis revealed that Raman signal intensity of lignin in both the secondary cell wall and the compound middle lamella regions showed a similar increase trend with growth of bamboo. In addition, eight hemicellulosic fractions were obtained by successively treating holocellulose of the 2-, 4-, 8-, and 12-month-old Neosinocalamus affinis bamboo culms with DMSO and alkaline solution. The ratio of arabinose to xylose of hemicelluloses was increased with the growth of bamboo. FT-IR and NMR analyses revealed that DMSO-soluble hemicelluloses of young bamboo culms are mainly composed of highly substituted xylans and β-d-glucans.

Aspartokinase of Lemna paucicostata Hegelm. 6746

A sensitive and specific method was developed for assay of aspartokinase (EC 2.7.2.4) in crude extracts of Lemna paucicostata. Lysine inhibited approximately 93%, and threonine approximately 6%; together, these amino acids inhibited 99%. Inhibition by lysine was synergistically increased by S-adenosylmethionine, which by itself had no effect on activity. Essentially complete inhibition of threonine-resistant activity was obtained with lysine, and of lysine-resistant activity with threonine. Inhibition by lysine and threonine was additive, with no indication of concerted inhibition. Aspartate concentration had no effect on the relative proportions of lysine- and threonine-sensitive activities. Aspartokinase activity was in large excess of that reported by other workers, the maximum capacity (V(max)) far exceeding the in vivo requirements. Estimations of rates of aspartokinase in vivo suggest that the step catalyzed by this enzyme may not be the overall ;rate-limiting' one for entry of 4-carbon units into the aspartate family of amino acids, and that feedback inhibition of this enzyme by lysine and threonine may not be a major factor in regulating flux through this step.

Effects of exogenous methionine on storage protein composition of soybean cotyledons cultured in vitro

Supplemental methionine in a complete culture medium increased the methionine content of the protein fraction of cultured soybean (Glycine max L. Merrill) cotyledons (Thompson, Madison, Muenster 1981 Phytochemistry 20: 941-945). To explain the observed increase in protein methionine, we have measured the amounts and subunit compositions of 7S and 11S storage proteins and determined the amino acid compositions of the three major protein fractions (2-5S, 7S, 11S) of seeds developed on plants and of cultured cotyledons grown in the presence or absence of supplemental l-methionine. Development of cultured cotyledons was representative of development of seeds on plants. The ratios of 11S to 7S proteins, the subunit contents, and amino acid compositions of their storage protein fractions were similar, but not identical. Supplemental methionine increased the mole percent methionine in each of the three protein fractions of cultured cotyledons and changed the amounts of several other amino acids. Supplemental methionine inhibited expression of the 7S beta-subunit gene. Concomitant with the absence of the beta-subunit, which contains no methionine, was an increase in the ratio of 11S to 7S proteins, and an increase in the methionine content of the subunits composing these fractions. Inhibition of beta-subunit gene expression by methionine in cultured cotyledons provides a reproducible, easily controlled system for the study of eucaryotic gene expression.

Lemna paucicostata Hegelm. 6746: DEVELOPMENT OF STANDARDIZED GROWTH CONDITIONS SUITABLE FOR BIOCHEMICAL EXPERIMENTATION

Photoautotrophic and mixotrophic growth of Lemna paucicostata Hegelm. 6746 (formerly Lemna perpusilla Torr. 6746) was investigated to establish standardized conditions for biochemical studies. Optimal temperature for growth was 29 to 30 C. The medium used previously (Datko AH, Mudd SH, Giovanelli J 1977 J Biol Chem 252: 3436-3445) was modified by inclusion of NH(4)Cl, decreasing macronutrient and ethylenediamine tetraacetate concentration, increasing micronutrient concentration, and inclusion of bicarbonate (for photoautotrophic growth) or 2-(N-morpholino)ethanesulfonic acid (for mixotrophic growth) buffers. Varying the sulfate concentration between 14 and 1 millimolar had no effect on growth. For photoautotrophic growth in the new medium (medium 4), the effects of CO(2) concentration, light intensity, and pH were measured. Under the optimal conditions, a multiplication rate (MR) of 300 to 315, equivalent to a doubling time of 23 to 24 hours was obtained. Addition of glutamine or asparagine did not increase this MR. For mixotrophic growth in low light, the effects of sucrose concentration and pH were determined. Under optimal conditions, MR was 210. A concentration of sucrose less than maximal for growth was chosen for the medium for experiments which will include (14)C-labeling of intermediates. MR under these conditions was 184. Growth was equally good in medium 4 and in half-strength Hutner's medium when sulfate was high (0.4 to 1 millimolar), but better in medium 4 when sulfate was low (20 micromolar). Growth rates could be restored to normal in half-strength Hutner's with low sulfate by decreasing the molybdate concentration.By modifying medium 4 to contain very low amounts of sulfate, and by preconditioning medium and plants, it was shown that there was an increment in plant protein of approximately 2.5 micrograms per nanomole of added MgSO(4). Colonies undergoing sulfur limitation exhibited a slow growth rate and a high frond to colony ratio. Molybdate and selenate produced growth inhibition reversible by sulfate. Conditions were developed in which the plants could be maintained indefinitely in the presence of either molybdate or selenate in altered metabolic steady-states with lowered growth rates and protein per frond.

Amino Acid Uptake into Cultivated Mesophyll Cells from Asparagus officinalis L

The uptake of threonine, aspartic acid, and isoleucine into cultivated asparagus cells was examined under culture conditions.Transport was linear with time and with the number of cells present in the assay. The upake of the three amino acids obeyed Michaelis-Menten kinetics with apparent affinities in the range of 0.1 to 0.02 millimolar. Azide and 2,4-dinitrophenol caused inhibition of threonine uptake. Reciprocal competitive inhibitions were found between threonine, aspartic acid, and isoleucine. Methionine and lysine competitively inhibited the uptake of these three amino acids. These results and the comparison of apparent affinities and inhibition constants are in favor of only one uptake system for threonine, aspartic acid, isoleucine, methionine, and lysine with different affinities for the different amino acids. All other amino acids with the exception of histidine, arginine, asparagine, and hydroxyproline inhibited threonine uptake. It is most likely that at least one component of the uptake system is shared by all amino acids.

Studies on lysine analogs, aspartate-derived amino acids, and attempted mutant selection on oat seedlings

The lysine analogs S-2-aminoethyl-L-cysteine (AEC) and DL-Δ-hydroxylysine (DHL) caused severe growth inhibition in dark-grown oat seedlings (Avena sativa L. and A. nuda L.) at similar concentrations while L-lysine methyl ester (LME) had little effect. Lysine, arginine, and ornithine reversed the inhibition caused by AEC and DHL, the order of effectiveness being lysine>arginine>ornithine. Of aspartate-pathway amino acids, tested individually and in combinations for inhibitory effects on seedling growth, lysine and combinations containing lysine were the most inhibitory, but the inhibition was much less than that produced by AEC. Only slight synergistic effects occurred when oat seedlings were grown in the presence of paired combinations of aspartatepathway endproduct amino acids.Ca. 54,000 seeds obtained from 3,463 plants grown from ethyl-methanesulfonate (EMS) treated seed were screened for resistance to AEC. Three resistant variants were identified but the resistance was not recovered among their self-pollinated progeny.

Change in the proportion of two aspartokinases in carrot root tissue in response to in vitro culture

Two isofunctional aspartokinases (EC 2.7.2.4) exist in fresh root tissue of carrot (Daucus carota, cv. Oogata sanzun). The threoninesensitive portion constitutes about 70% of the activity; the lysinesensitive, less than 20%. Culture of slices of carrot tissue for 3 days reversed the ratio as the lysine-sensitive activity preferentially increased. Inhibition by threonine and lysine was additive in both enzyme preparations from fresh and cultured tissues. The activities were resolved into two distinct fractions of different sensitivity to threonine and lysine by DEAE-Sephadex A-50 column chromatography.

The effect of aspartate-derived amino acids (lysine, threonine, methionine) on the growth of excised embryos of wheat and barley

Excised wheat (Triticum aestivum L. var. Maris Freeman) and barley (Hordeum vulgare L. var. Maris Mink) embryos were grown on medium containing both nitrate and ammonium ions. Addition of lysine (1 mM) plus threonine (1 mM) caused a synergistic inhibition of growth measured by length of first leaf or dry weight. The inhibition was specifically relieved by methionine, homocysteine and homoserine. Threonine at 0.2-0.3 mM caused half-maximal inhibition of growth at all lysine concentrations whereas lysine increased the synergistic inhibition up to 3 mM. The inhibition is explained by a model in which lysine acts as a feedback inhibitor of aspartate kinase and threonine of homoserine dehydrogenase. This is compatible with published studies of the enzymes involved. The implications of these findings for using lysine plus threonine as a selection system for lysine-overproducing cereals are discussed.

Aspartate kinase and the synthesis of aspartate-derived amino acids in wheat

Aspartate kinase (EC 2.7.2.4.) has been purified from 7 day etiolated wheat (Triticum aestivum L. var. Maris Freeman) seedlings and from embryos imbibed for 8 h. The enzyme was 50% inhibited by 0.25 mM lysine. In this study wheat aspartate kinase was not inhibited by threonine alone or cooperatively with lysine; these results contrast with those published previously. In vivo regulation of the synthesis of aspartate-derived amino acids was examined by feeding [(14)C]acetate and [(35)S]sulphate to 2-3 day germinating wheat embryos in culture in the presence of exogenous amino acids. Lysine (1 mM) inhibited lysine synthesis by 86%. Threonine (1 mM) inhibited threonine synthesis by 79%. Lysine (1 mM) plus threonine (1 mM) inhibited threonine synthesis by 97%. Methionine synthesis was relatively unaffected by these amino acids, suggesting that there are important regulatory sites other than aspartate kinase and homoserine dehydrogenase. [(35)S]sulphate incorporation into methionine was inhibited 50% by lysine (2 mM) plus threonine (2 mM) correlating with the reported 50% inhibition of growth by these amino acids in this system. The synergistic inhibition of growth, methionine synthesis and threonine synthesis by lysine plus threonine is discussed in terms of lysine inhibition of aspartate kinase and threonine inhibition of homoserine dehydrogenase.

Properties and Regulation of Aspartate Kinase from Barley Seedlings (Hordeum vulgare L.)

Aspartate kinase (EC 2.7.2.4) has been purified 8-fold and characterized from germinating barley (Hordeum vulgare) seedlings. The enzyme is inhibited 50% by 0.7 mm l-lysine and almost completely at 5 mm. l-Methionine does not affect the enzyme on its own, but at low concentrations (0.1-1 mm) increases the inhibition in the presence of lysine, indicating that the two amino acids act as cooperative feedback regulators.

Lysine-methionine-threonine interactions in growth and development of Mimulus cardinalis seedlings

Plants of Mimulus cardinalis Douglas (Scrophulariaceae) were grown in axenic culture from seed for 28 days on a minimal salts medium supple-mented with L-lysine, L-methionine, L-threonine, L-isoleucine, DL-or L-homoserine or DL-homocysteine, alone or in combinations ranging from 5 to 1000 μM. Abnormal growth was observed at the higher concentrations of all these aminoacidsexcept homocysteine. The lysine inhibition was significantly reduced by methionine, homocysteine or isoleucine. The threonine inhibition was significantly reduced by methionine or homocysteine. A combination of lysine and threonine at 1 MM was lethal. This synergistic effect was prevented when methionine, homoserine or homocysteine were added to the lysine-threonine combination. These results can be explained in terms of end-product control of aspartokinase and homoserine dehydrogenase by lysine and threonine, respectively, in the biosynthetic pathway to these aspartic-acid-derived amino acids.

Control of Free Methionine Production in Wild Type and Ethionine-resistant Mutants of Chlorella sorokiniana

Mutants of Chlorella sorokiniana selected for resistance to the methionine analogue ethionine took up ethionine at the same rate as did the wild type strain. Cells of two ethionine-resistant mutants produced severalfold higher levels of free methionine and cysteine than did wild type cells.Exogenous methionine had no apparent effect on free methionine production in a mutant that produces excessive levels of free methionine. Under the same conditions, production of free methionine was relatively inhibited in wild type cells and in a mutant that produces wild type levels of free methionine.The results suggest that free methionine production in the wild type strain is subject to endproduct control, and that this control is lacking in one class of ethionine-resistant mutants.

In-vivo evidence for metabolic control of amino-acid and protein synthesis by exogenous lysine and threonine in Mimulus cardinalis

Young developing seedlings of Mimulus cardinalis Douglas (Scrophulariaceae) were grown in Hoagland's medium supplemented with lysine (LYS; 1.0 mM), threonine (THR; 1.0 mM), LYS+THR (1.0 mM ea.) or LYS+THR+methionine (MET; 1.0-1.0-0.1 mM, respectively). LYS or THR reduced the total amount of protein in the developing seedlings and when added in combination completely inhibited the accumulation of protein after 7 days. The combination of LYS+THR+MET enhanced the accumulation of protein. When MET was added to seedlings grown on LYS+THR for 9 days, the rate of protein accumulation reverted to normal. Studies using [(14)C]leucine demonstrated similar inhibitory patterns on protein synthesis. The effects of LYS and THR on the distribution of [(14)C]aspartate into the aspartate amino acids including MET were studied. LYS and THR, singly and synergistically when in combination, inhibited the synthesis of MET. Thus the simultaneous application of LYS and THR can be interpreted as producing an artificially induced MET auxotroph. These results indicate that LYS and THR act singly and together in vivo to inhibit the accumulation of soluble LYS, MET and THR in a manner indicative of end-product inhibition. Indirect evidence for end-product control of alanine, arginine, glycine, phenylalanine, proline, tryptophan, tyrosine and valine is also given.

Aspartokinase from wheat germ: isolation, characterization, and regulation

Aspartokinase has been isolated from wheat germ and a preliminary survey made of its properties in a partially purified extract. The enzyme has an absolute requirement for ATP and a divalent metal ion. The phosphate donor can be either ATP or GTP, but other nucleotides are ineffective. Both magnesium and manganese will activate the enzyme, whereas calcium shows a trace amount of activity. The enzyme has a Km of 16.7 mm for aspartate, 1.2 mm for ATP, and 3.3 mm for MgCl(2). Lysine inhibits the reaction at fairly low concentrations, and threonine inhibits at high concentrations. Other amino acids which are derived from aspartate (methionine, homoserine, threonine, and isoleucine) have little effect. When lysine and threonine are added together, they show a concerted inhibition of the reaction. The enzyme is also stabilized against heat inactivation by lysine and threonine together but not by either when added separately. It is suggested that aspartokinase from plants is a regulatory enzyme and exhibits a concerted feedback mechanism.