Characterization of an Arabidopsis thaliana cDNA encoding an S-adenosylmethionine-sensitive threonine synthase Threonine synthase from higher plants

AbstractAn Arabidopsis thaliana cDNA encoding an Sadenosylmethionine-sensitive threonine synthase (EC 4.2.99.2) has been isolated by functional complementation of an Escherichia coli mutant devoid of threonine synthase activity. Threonine synthase from A. thaliana was shown to be synthesized with a transit peptide. The recombinant protein is activated by Sadenosylmethionine in the same range as the plant threonine synthase and evidence is presented for an involvement of the N-terminal part of the mature enzyme in the sensitivity to Sadenosylmethionine

Analytical ultracentrifugation and preliminary X-ray studies of the chloroplast envelope quinone oxidoreductase homologue from Arabidopsis thaliana.

International audienceQuinone oxidoreductases reduce a broad range of quinones and are widely distributed among living organisms. The chloroplast envelope quinone oxidoreductase homologue (ceQORH) from Arabidopsis thaliana binds NADPH, lacks a classical N-terminal and cleavable chloroplast transit peptide, and is transported through the chloroplast envelope membrane by an unknown alternative pathway without cleavage of its internal chloroplast targeting sequence. To unravel the fold of this targeting sequence and its substrate specificity, ceQORH from A. thaliana was overexpressed in Escherichia coli, purified and crystallized. Crystals of apo ceQORH were obtained and a complete data set was collected at 2.34 Å resolution. The crystals belonged to space group C2221, with two molecules in the asymmetric unit

Tyrosine metabolism: identification of a key residue in the acquisition of prephenate aminotransferase activity by 1β aspartate aminotransferase

International audienceAlternative routes for the post-chorismate branch of the biosynthetic pathway leading to tyrosine exist, the 4-hydroxyphenylpyruvate or the arogenate route. The arogenate route involves the transamination of prephenate into arogenate. In a previous study, we found that, depending on the microorganisms possessing the arogenate route, three different aminotransferases evolved to perform prephenate transamination, that is, 1β aspartate aminotransferase (1β AAT), N-succinyl-l,l-diaminopimelate aminotransferase, and branched-chain aminotransferase. The present work aimed at identifying molecular determinant(s) of 1β AAT prephenate aminotransferase (PAT) activity. To that purpose, we conducted X-ray crystal structure analysis of two PAT competent 1β AAT from Arabidopsis thaliana and Rhizobium meliloti and one PAT incompetent 1β AAT from R. meliloti. This structural analysis supported by site-directed mutagenesis, modeling, and molecular dynamics simulations allowed us to identify a molecular determinant of PAT activity in the flexible N-terminal loop of 1β AAT. Our data reveal that a Lys/Arg/Gln residue in position 12 in the sequence (numbering according to Thermus thermophilus 1β AAT), present only in PAT competent enzymes, could interact with the 4-hydroxyl group of the prephenate substrate, and thus may have a central role in the acquisition of PAT activity by 1β AAT

Identification of the Arabidopsis calmodulin-dependent NAD+ kinase that sustains the elicitor-induced oxidative burst

International audience17 NADP(H) is an essential cofactor of multiple metabolic processes in all living organisms. In plants, 18 NADP(H) is required as the substrate of Ca 2+-dependent NADPH oxidases which catalyze a reactive 19 oxygen species burst in response to various stimuli. While NADP + production in plants has long been 20 known to involve a Calmodulin and Calcium (CaM)/Ca 2+-dependent NAD + kinase, the nature of the 21 enzyme catalyzing this activity has remained enigmatic, as well as its role in plant physiology. Here, 22 thanks to a combination of proteomics, biochemistry, molecular biology and in vivo studies, we have 23 identified an Arabidopsis protein that catalyzes NADP + production exclusively in the presence of 24 CaM/Ca 2+. This new enzyme (NADKc) has a CaM-binding peptide located in its N-terminal region and 25 displays peculiar biochemical properties as well as different domain organization compared to known 26 plant NAD + kinases. In response to a pathogen elicitor, activity of NADKc, which is associated with the 27 mitochondrial periphery, contributes to an increase in the cellular NADP + concentration and to the 28 amplification of the elicitor-induced oxidative burst. Based on a phylogenetic analysis and enzymatic 29 assays, we propose that the CaM/Ca 2+-dependent NAD + kinase activity found in photosynthetic 3

Understanding the regulation of aspartate metabolism using a model based on measured kinetic parameters

The aspartate-derived amino-acid pathway from plants is well suited for analysing the function of the allosteric network of interactions in branched pathways. For this purpose, a detailed kinetic model of the system in the plant model Arabidopsis was constructed on the basis of in vitro kinetic measurements. The data, assembled into a mathematical model, reproduce in vivo measurements and also provide non-intuitive predictions. A crucial result is the identification of allosteric interactions whose function is not to couple demand and supply but to maintain a high independence between fluxes in competing pathways. In addition, the model shows that enzyme isoforms are not functionally redundant, because they contribute unequally to the flux and its regulation. Another result is the identification of the threonine concentration as the most sensitive variable in the system, suggesting a regulatory role for threonine at a higher level of integration

Sur les cartels internationaux

Curien Gilles. Sur les cartels internationaux. In: Annales. Économies, Sociétés, Civilisations. 2ᵉ année, N. 2, 1947. pp. 191-193

Rythmes du monde. Les accords économiques internationaux depuis 1860

Curien Gilles. Rythmes du monde. Les accords économiques internationaux depuis 1860. In: Annales. Économies, Sociétés, Civilisations. 1ᵉ année, N. 3, 1946. pp. 219-234