1,721,009 research outputs found
Dark complexes of the Calvin-Benson cycle in a physiological perspective
Full text link: Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and phosphoribulokinase (PRK) are two enzymes of the Calvin Benson cycle that stand out for some peculiar properties they have in common: (i) they both use the products of light reactions for catalysis (NADPH for GAPDH, ATP for PRK), (ii) they are both light-regulated through thioredoxins and (iii) they are both involved in the formation of regulatory supramolecular complexes in the dark or low photosynthetic conditions, with or without the regulatory protein CP12. In the complexes, enzymes are transiently inactivated but ready to recover full activity after complex dissociation. Fully active GAPDH and PRK are in large excess for the functioning of the Calvin-Benson cycle, but they can limit the cycle upon complex formation. Complex dissociation contributes to photosynthetic induction. CP12 also controls PRK concentration in model photosynthetic organisms like Arabidopsis thaliana and Chlamydomonas reinhardtii. The review combines in vivo and in vitro data into an integrated physiological view of the role of GAPDH and PRK dark complexes in the regulation of photosynthesis
NADH: Fe(III)-chelate reductase of maize roots is an active cytochrome b5 reductase
No full textMicrosomal NADH:Fe(III)-chelate reductase (NFR) of maize roots has been purified as a monomeric flavoprotein of 32 kDa with non-covalently bound FAD. In the presence of NADH, NFR efficiently reduced the physiological iron-chelate Fe(III)-citrate (K(cat)/K(m(Fe(III)-(citrate)) = 6.0 X 106 M-1 s-1) with a sequential reaction mechanism. Purified NFR was totally inhibited by the sulfhdryl reagent PHMB at 10-9 M, and it could use cyt b5 as alternative electron acceptor with a maximal reduction rate as high as with Fe(III)-citrate. We conclude that in maize roots the reduction of Fe(III)-citrate is chiefly performed by a cytochrome b5 reductase, mostly associated with intracellular membranes and in part with the plasma membrane
Calvin–Benson cycle regulation is getting complex
Full text linkOxygenic phototrophs use the Calvin–Benson cycle to fix CO2 during photosynthesis. In the dark, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and phosphoribulokinase (PRK), two enzymes of the Calvin–Benson cycle, form an inactive complex with the regulatory protein CP12, mainly under the control of thioredoxins and pyridine nucleotides. In the light, complex dissociation allows GAPDH and PRK reactivation. The GAPDH/CP12/PRK complex is conserved from cyanobacteria to angiosperms and coexists in land plants with an autoassembling GAPDH that is analogously regulated. With the recently described 3D structures of PRK and GAPDH/CP12/PRK, the structural proteome of this ubiquitous regulatory system has been completed. This outcome opens a new avenue for understanding the regulatory potential of photosynthetic carbon fixation by laying the foundation for its knowledge-based manipulation
Impact of drought on soluble sugars and free proline content in selected arabidopsis mutants
No full textWater shortage is an increasing problem affecting crop yield. Accumulation of compatible osmolytes is a typical plant response to overcome water stress. Sucrose synthase 1 (SUS1), and glucan, water dikinase 2 (GWD2) and δ1-pyrroline-5-carboxylate synthetase 1 (P5CS1) are members of small protein families whose role in the response of Arabidopsis thaliana plants to mild osmotic stress has been studied in this work. Comparative analysis between wild-type and single loss-of-function T-DNA plants at increasing times following exposure to drought showed no differences in the content of water-insoluble carbohydrate (i.e., transitory starch and cell wall carbohydrates) and in the total amount of amino acids. On the contrary, water-soluble sugars and proline contents were significantly reduced compared to wild-type plants regardless of the metabolic pathway affected by the mutation. The present results contribute to assigning a physiological role to GWD2, the least studied member of the GWD family; strengthening the involvement of SUS1 in the response to osmotic stress; showing a greater contribution of soluble sugars than proline in osmotic adjustment of Arabidopsis in response to drought. Finally, an interaction between proline and soluble sugars emerged, albeit its nature remains speculative and further investigations will be required for complete comprehension
Arabidopsis thaliana Sucrose Phosphate Synthase A2 Affects Carbon Partitioning and Drought Response
Full text linkSucrose is essential for plants for several reasons: It is a source of energy, a signaling molecule, and a source of carbon skeletons. Sucrose phosphate synthase (SPS) catalyzes the conversion of uridine diphosphate glucose and fructose-6-phosphate to sucrose-6-phosphate, which is rapidly dephosphorylated by sucrose phosphatase. SPS is critical in the accumulation of sucrose because it catalyzes an irreversible reaction. In Arabidopsis thaliana, SPSs form a gene family of four members, whose specific functions are not clear yet. In the present work, the role of SPSA2 was investigated in Arabidopsis under both control and drought stress conditions. In seeds and seedlings, major phenotypic traits were not different in wild-type compared with spsa2 knockout plants. By contrast, 35-day-old plants showed some differences in metabolites and enzyme activities even under control conditions. In response to drought, SPSA2 was transcriptionally activated, and the divergences between the two genotypes were higher, with spsa2 showing reduced proline accumulation and increased lipid peroxidation. Total soluble sugars and fructose concentrations were about halved compared with wild-type plants, and the plastid component of the oxidative pentose phosphate pathway was activated. Unlike previous reports, our results support the involvement of SPSA2 in both carbon partitioning and drought response
FeON-FeOFF: the Helicobacter pylori Fur regulator commutates iron-responsive transcription by discriminative readout of opposed DNA grooves
No full textMost transcriptional regulators bind nucleotide motifs in the major groove, although some are able to recognize molecular determinants conferred by the minor groove of DNA. Here we report a transcriptional commutator switch that exploits the alternative readout of grooves to mediate opposite output regulation for the same input signal. This mechanism accounts for the ability of the Helicobacter pylori Fur regulator to repress the expression of both iron-inducible and iron-repressible genes. When iron is scarce, Fur binds to DNA as a dimer, through the readout of thymine pairs in the major groove, repressing iron-inducible transcription (FeON). Conversely, on iron-repressible elements the metal ion acts as corepressor, inducing Fur multimerization with consequent minor groove readout of AT-rich inverted repeats (FeOFF). Our results provide first evidence for a novel regulatory paradigm, in which the discriminative readout of DNA grooves enables to toggle between the repression of genes in a mutually exclusive manner. © The Author(s) 2013
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Dynamic regulation of Arabidopsis β-AMYLASE1 by glutathione and thioredoxins affects starch in guard cells
No full textGuard cells control the opening and closure of stomatal pores in response to internal and external stimuli, ensuring gas exchange in plants. In Arabidopsis (Arabidopsis thaliana), beta-AMYLASE1 (BAM1), assisted by alpha-AMYLASE3, begins degrading starch at dawn in guard cells to promote stomatal opening. Both enzymes are controlled by reversible disulfide bond formation, which decreases their activity. In the present study, we investigated the sensitivity of BAM1 to other redox-dependent post-translational modifications (PTM) both in vitro and in vivo. In vitro, H2O2 reversibly inactivates BAM1 and, in the presence of glutathione (GSH), induces S-glutathionylation of BAM1. Glutathionylated BAM1 is active and transiently protected from H2O2 inhibition. However, the glutathionylated state of BAM1 has limited stability and can be slowly resolved by a second cysteine with the formation of the intramolecular disulfide bond that inhibits BAM1 activity. Thioredoxin f can fully revert the inhibition by reducing the disulfide to a dithiol. In vivo, Arabidopsis mutants with lower plastidial GSH reductase activity, and consequently modified GSH homeostasis, showed higher BAM1 activity, lower starch levels in guard cells, and altered stomata aperture, indicating that GSH redox potential impacts stomatal physiology, possibly through BAM1. Moreover, plastidial BAM1 presents a prime example for the role of glutathionylation functioning as a transiently protective PTM, interfering with the formation of inhibitory disulfide bonds. This example illustrates how transitions between protein cysteinyl thiol PTMs can orchestrate dynamic responses involving several redox systems.The redox homeostasis of GSH regulates starch metabolism in guard cells by influencing the activation state of the beta-AMYLASE1, a key enzyme in guard cell starch degradation
Insight into the assembly of the Calvin cycle regulatory GAPDH/CP12/PRK complex by SAXS
No full textPhotosynthetic organisms produce sugars through the Calvin-Benson cycle, consuming carbon dioxide and energy provided by the conversion of light to chemical energy. The smooth proceeding of photosynthesis is controlled by different regulatory systems including the transient formation of protein complexes. Through the scaffold protein CP12, which is predicted to be intrinsically disordered, two enzymes of the cycle, glyceraldehyde-3-phosphate dehydrogenase (tetrameric GAPDH) and phosphoribulokinase (dimeric PRK), are regulated by formation of a supramolecular ternary complex of 498 kDa with stoichiometry [GAPDH-(CP12)2-(PRK)]2. The activities of GAPDH and PRK enzymes are inhibited by complex formation and fully recovered upon dissociation of the complex at the onset of light, providing an effective means for regulation of the Calvin cycle in vivo.Small angle X-ray scattering analysis was performed on the pre-formed complex and its free components all from Arabidopsis thaliana, and the ATSAS package was used for data analysis and modelling. A concave bent and screwed ab-initio shape of the PRK dimer was recovered, while a combined rigid-body/dummy-residue model was obtained for the GAPDH-(CP12)2 binary complex in order to take into account a small rearrangement of the known crystallographic subunits positions and the missing CP12 amino acids. These models were then used in the rigid-body modelling of the ternary complex against the experimental scattering curve, allowing for partial dissociation. The known stoichiometry of the complex was confirmed and from the sorting of a big number of models obtained with multiple runs of the minimization procedure, an overall reproducible assembly emerged. The structure of the ternary complex appears more compact with respect to the previous pictorial models and the two GAPDHs proximity suggests an unsuspected involvement of an interaction between them in the overall complex stabilization
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