Fructose Bisphosphatase Research Articles

Unexpected similarity in regulation between an archaeal inositol monophosphatase/fructose bisphosphatase and chloroplast fructose bisphosphatase.

Hyperthermophilic archaea have an unusual phosphatase that exhibits activity toward both inositol-1-phosphate and fructose-1,6-bisphosphate, activities carried out by separate gene products in eukaryotes and bacteria. The structures of phosphatases from Archaeoglobus fulgidus (AF2372) and Methanococcus jannaschii (MJ0109), both anaerobic organisms, resemble the dimeric unit of the tetrameric pig kidney fructose bisphosphatase (FBPase). A striking feature of AF2372, but not of MJ0109, is that the sulfhydryl groups of two cysteines, Cys150 and Cys186, are in close proximity (4 A). A similar arrangement of cysteines has been observed in chloroplast FBPases that are regulated by disulfide formation controlled by redox signaling pathways (ferredoxin/thioredoxin). This mode of regulation has not been detected in any other FBPase enzymes. Biochemical assays show that the AF2372 phosphatase activity can be abolished by incubation with O(2). Full activity is restored by incubation with thiol-containing compounds. Neither the C150S variant of AF2372 nor the equivalent phosphatase from M. jannaschii loses activity with oxidation. Oxidation experiments using Escherichia coli thioredoxin, in analogy with the chloroplast FBPase system, indicate an unexpected mode of regulation for AF2372, a key phosphatase in this anaerobic sulfate reducer.

A possible role of the key enzymes of the glyoxylate and gluconeogenesis pathways for fruit-body formation of the wood-rotting basidiomycete Flammulina velutipes

Biochemical roles of the representative enzymes involved in carbon metabolism of glucose were investigated in relation to the fruit-body formation of the basidiomycete Flammulina velutipes. Changes in specific activities of the enzymes of the tricarboxylic acid (TCA) cycle and glyoxylate (GLOX) and gluconeogenesis pathways were measured at different stages of development of the fungus. The enzyme activities of malate synthase (MS) and fructose bisphosphatase (FBP) as the key enzymes for the GLOX-gluconeogenesis pathways increased in mycelia during the fruit-body formation. The activities of isocitrate dehydrogenase (IDH) for the TCA cycle and NADP-linked glutamate dehydrogenase (GLTDH (NADP)) for glutamate synthesis increased more markedly. Moreover, the mycelial mat of the cultures producing fruit bodies yielded greater enzyme activities of isocitrate lyase (ICL), MS, FBP, and IDH than that of the cultures that did not produce fruit bodies. These results suggest that the GLOX-gluconeogenesis pathways as well as the glutamate synthesis have a strong correlation with the fruit-body formation of F. velutipes.

Estimating the genetic divergence and identification of three Trichinella species by isoenzyme analysis.

Isoenzyme-based approach was applied to compare Trichinella spiralis, T. britovi and T. pseudospiralis species. Among 13 enzyme systems examined, esterase (EST), malic enzyme (ME) and phosphoglucomutase (PGM) have been found as fully diagnostic, with no common allele in species studied. Adenosine deaminase (ADA), adenylate kinase (AK), hexokinase (HK), peptidase leucyl-alanine (PEP-C) and fructose-bis-phosphatase (FBP) have been capable of distinguishing the two species from resulting profiles. In addition, ADA, AK and PGM displayed the enzyme expression in the lowest amounts of muscle larvae in systems tested (100 larvae/100 microliters of extracts). Based on allozyme data, T. pseudospiralis has been found as the most distinct species within the group of taxa. Only a subtle genetic variability was recorded for T. pseudospiralis in which solely phosphoglucomutase exhibited variant patterns. In addition to the study of reference isolates, T. spiralis from lowland fox in Eastern Slovakia has been evidenced by use of genetic markers. This finding has proved that T. britovi is not the exclusive species parasitizing in the sylvatic ecosystem of the Slovak region.

Crystal structure and catalytic mechanism of the MJ0109 gene product: a bifunctional enzyme with inositol monophosphatase and fructose 1,6-bisphosphatase activities.

Inositol monophosphatase (EC 3.1.3.25) in hyperthermophilic archaea is thought to play a role in the biosynthesis of di-myo-inositol-1,1'-phosphate (DIP), an osmolyte unique to hyperthermophiles. The Methanococcus jannaschii MJ109 gene product, the sequence of which is substantially homologous to that of human inositol monophosphatase, exhibits inositol monophosphatase activity but with substrate specificity that is broader than those of bacterial and eukaryotic inositol monophosphatases (it can also act as a fructose bisphosphatase). To understand its substrate specificity as well as the poor inhibition by Li(+) (a potent inhibitor of the mammalian enzyme), we have crystallized the enzyme and determined its three-dimensional structure. The overall fold, as expected, is similar to that of the mammalian enzyme, but the details suggest a closer relationship to fructose 1,6-bisphosphatases. Three complexes of the MJ0109 protein with substrate and/or product and inhibitory as well as activating metal ions suggest that the phosphatase mechanism is a three-metal ion assisted catalysis which is in variance with that proposed previously for the human inositol monophosphatase.

Electron flux in photosystem II and photon/electron sinks in wild and domesticated watermelon plants during the progress of drought stress

Responses of activities of the enzymes involved in electron sinks in photosynthesis and of electron flux in photosystem II (PS-II) to drought under illumination at 1000 µmol photons m-2 s-1 were analyzed with wild and domesticated watermelon plants. Although the different responses of both species to water deficit were revealed in the water status and morphological appearance of leaves, the rates of transpiration and net CO2 assimilation gradually decreased to almost zero in the first 3 days. While RuBisCO showed a gradual decrease in activity in the prolonged stress, the activation states of the enzyme and fructose bisphosphatase were kept high during the experimental period in both species. Wild and domesticated watermelon plants induced chloroplast ascorbate peroxidase, but not the cytosolic enzyme, during drought stress. High activities of monodehydroascorbate reductase and other enzymes involved in regeneration of ascorbate might cause the reduction levels of ascorbate and glutathione to be kept at high levels. Although the photochemical efficiency of photosystem II was not damaged in both species under drought for 5 days, the flux of electrons in PS II was regulated in different ways in both species. Wild watermelon leaves repressed the electron flux in PS II to the level, which was persisted after the cessation of the net CO2 assimilation, earlier than the domesticated species. The level of down-regulation of the flux was closely related to the strengths of non-photochemical quenching of both species.

Low Night Temperature-Induced Changes in Photosynthesis and Rubber Accumulation in Guayule (Parthenium Argentatum Gray)

Three-year-old plants of Parthenium argentatum Gray cv. 11591 grown under natural photoperiod were exposed for 60 d to low night temperature (LNT) of 15 °C (daily from 18:00 to 06:00). Effects of the treatment on net photosynthetic rates (PN), rubber accumulation, and associated biochemical traits were examined. LNT initially reduced PN with a parallel decline in the activities of ribulose-1,5-bisphosphate carboxylase, fructose bisphosphatase, and sucrose phosphate synthase for 20-30 d. Later, LNT enhanced PN and the activities of photosynthetic enzymes. Associated with high PN in LNT-treated guayule plants was a two-fold increase in rubber content and rubber transferase activity per unit of protein. The initial decrease in PN in LNT-treated guayule was associated with low content of chlorophyll (a+b), large starch accumulation, and higher ratio of glucose-6-phosphate/fructose-6-phosphate. Photosystem 2 activity in isolated chloroplasts was initially decreased, but increased after 30 d. There was a significant increase in the leaf soluble protein content in LNT-treated plants. Hence the photosynthetic performance of plants grown at 15 °C night temperature for 50 d was superior to those grown under natural photoperiod in all parameters studied. The high photosynthetic capacity may contribute to superior rubber yields under LNT.

CDNA Sequence and Kinetic Properties of Human Lung Fructose(1,6)bisphosphatase

A cDNA encoding fructose(1,6)bisphosphatase was isolated from total human lung RNA. The cDNA contained an open reading frame encoding 337 amino acids. The determined nucleotide sequence of the lung cDNA was significantly different from muscle cDNA and slightly differed from human liver cDNA in a single mutation (Gly-336 for Ala-336) and a T for C substitution in position 648. The human lung fructose(1,6)bisphosphatase [Fru(1,6)Pase] was isolated and its kinetic parameters were compared with liver and muscle isoenzymes. Values ofkcatfor the lung Fru(1,6)Pase were lower than for the liver and muscle enzyme. Like the liver isoenzyme, lung Fru(1,6)Pase is significantly less inhibited by AMP than the muscle enzyme. The values ofI0.5were 9.5, 9.8, and 0.3 μM for the liver, lung, and muscle enzyme, respectively. The lung enzyme was slightly more sensitive to fructose(2,6)bisphosphate [Fru(2,6)P2] inhibition than the liver enzyme.Kiwas 75 μM for the lung and 96 μM for the liver enzyme. The synergistic effect of AMP and Fru(2,6)P2on the lung and liver Fru(1,6)Pase was also observed. In the presence of AMP the corresponding values ofKifor Fru(2,6)P2were 16 μM for the lung and 10 μM for the liver enzyme.

Inhibition of germination by glucose analogues that are hexokinase substrates

The degree of reduction of germination and seedling growth by glucose analogues was related to their phosphorylation coefficient with hexokinase and concentration of added analogue. d-psicose, a substrate for fructo-6-kinase, also inhibited, consistent with an independent role for fructo-6-kinase. K is values for the 6-phosphates of d-mannose and 2-deoxy- d-glucose with a plant hexokinase against ATP were high (52 and 62 mM) and there was negligible inhibition against d-glucose. The degree of reduction of germination and seedling growth of fenugreek, mung beans, white mustard and wheat seeds was related to the phosphorylation coefficient and concentration of the added analogue, and varied with plant species. The significance of these results in inhibition is discussed. 2,5-Anhydro- d-mannitol and d-psicose were exceptions. The bisphosphate of the former potently inhibits fructose bisphosphatase and the latter is a substrate for fructo-6-kinase, consistent with a role for fructo-6-kinase in plants independent of hexokinase.

Cell volume regulates liver phosphoenolpyruvate carboxykinase and fructose-1,6-bisphosphatase genes.

Hypertonic-induced cell shrinkage increases glucose release in H-4-II-E rat hepatoma cells. This is paralleled by a concomitant increase in the mRNA levels of the rate-limiting enzymes of the pathway of gluconeogenesis, phosphoenolpyruvate carboxykinase (PCK) and fructose-1,6-bisphosphatase (FBP), of seven- and fivefold, respectively. In contrast, hypotonic-induced swelling of the cells results in a transient decrease in PCK and FBP mRNAs to 15% and 39% of control levels. The antagonistic effects of hyper- and hypotonicity mimic the counteracting effects of adenosine 3',5'-cyclic monophosphate (cAMP) and insulin on PCK and FBP mRNA levels. The hypertonic-induced increase in mRNA levels is due to an enhanced transcriptional rate, whereas the decrease in mRNAs caused by hypotonicity results from a decrease in transcription as well as mRNA stability. The inductive effect of hypertonicity does not require ongoing protein synthesis and acts independently of the cAMP-dependent protein kinase and protein kinase C pathways. These results suggest that cell volume changes in liver cells may play an important role in regulating hepatic glucose metabolism by altered gene expression.

Inheritance and linkage relationships of isoenzymes in two interspecific cherry progenies

Two interspecific cherry progenies, Prunus avium ‘Napoleon’ × P. incisa E621 and ‘Napoleon’ × P. nipponica F1292, were analysed with polyacrylamide gel electrophoresis for 19 enzyme systems: alanine aminopeptidase (AAP), aldolase (ALD), alkaline phosphatase (AKP), arginine aminopeptidase (ARA), catechol oxidase (CO), diaphorase (DIA), endopeptidase (ENP), esterase (EST), formate dehydrogenase (FDH), fructose-bisphosphatase (FBP), β-galactosidase (GAL), glucose-6-phosphate dehydrogenase (G6PDH), β-glucosidase (GLU), glutamate dehydrogenase (GDH), glyceraldehyde-3-phosphate dehydrogenase (G3PDH), hexokinase (HEX), peroxidase (PRX), phosphorylase (PHO) and triose-phosphate isomerase (TPI). Activity for GAL was identical with GLU, and CO with PRX. In addition, activity for AKP was identical with some regions detected previously as acid phosphatase; and most of the PRX activity was identical with regions detected previously as superoxide dismutase. In all, 16 new segregating loci were identified, Ara-1, Dia-2, Est-1, Est-2, -6, -7 and -9, Glu-1 to -4, Hex-1, Pho-1 and -2, and Prx-8 and -9, together with 10 polymorphic putative loci. Analysis of cosegregations of the segregating loci with each other, and with loci established previously, resulted in 19 more loci being added to the cherry linkage map. Fifteen of these were new, and four had been described previously, but were hitherto unlinked. The additional linkages are: Ara-1– Dia-2– Mdh-2–Est-2; Glu-3/Prx-8–Glu-1/-2/-4–Prx-1/-9; Pgd-2–Pho-1/-2; Idh-2–Est-7–Est-6; (Amy-2)–Est-1–(Adh-4/-6); and (Acp-1/-2/-3)–Hex-1–(Gpi-2). (The bracketed loci had been mapped already). Several cherry linkages resemble linkages reported in apple. Analysis of 14 cultivars of P. avium for the same enzyme systems revealed polymorphism for just four of the 15 loci, and for an additional putative locus that was monomorphic in the interspecific material. This lack of isoenzyme polymorphism among cherry cultivars reduces the utility of these markers for linkage analysis and mapping in progenies of P. avium.

Predicting redox-sensitive cysteines in plant enzymes by homology modeling

One of the unsolved problems in plant biochemistry has been the identification of the regulatory cysteines in the reductively light-activated and -inactivated chloroplast enzymes. Homology modeling based on the sequences of these enzymes and the three-dimensional structures of homologous enzymes has now allowed tentative identification of the redox-sensitive Cys residues in four light-activated chloroplast enzymes. In each case the regulatory disulfides are not positioned in the active site but instead appear to be positioned so as to affect the flexibility or the conformation of the enzyme, and thereby to affect catalysis. In glyceraldehyde-3-P dehydrogenase and malate dehydrogenase inter-domain movement would be restricted. In fructose bisphosphatase and sedoheptulose bisphosphatase the regulatory Cys residues are located on the nucleotide binding domain in a region known to be sensitive to an allosteric effector of other fructose bisphosphatases. Results of site-directed mutagenesis experiments to date are in general agreement with the domain-locking hypothesis. The redox sensitivity of a number of cytosolic enzymes suggests that reductive modulation might occur outside of the chloroplast in leaves, and in the roots, stems and germinating seeds of green plants. Our better understanding of the mechanism of redox regulation may lead to new approaches for the regulation of enzyme activity with biotechnological applications.

The cytosolic fructose bisphosphatase of Brassica napus contains a new potential regulatory disulfide and is redox-sensitive

The deduced amino acid sequences of the Brassica napus and sugar cane ( Saccharum sp.) cytosolic fructose bisphosphatases (EC 3.1.3.11) have appeared recently. When the three-dimensional structure of the Brassica napus bisphosphatase was modeled residue 92, previously identified as a potential redox-sensitive regulatory cysteine in the cytosolic enzymes from potato, sugarbeet and spinach [L.E. Anderson, et al., Planta 196 (1995) 118–124], was a serine. (Numbering according to Protein Data Bank entry 4FBP.) Instead there is a Cys at position 110, close enough to Cys-114, the second member of the potential regulatory cysteine pair in the other cytosolic fructose bisphosphatases, to suggest the possibility of disulfide bond formation and the enzyme is redox-sensitive. The sugar cane enzyme, like the other three cytosolic fructose bisphosphatases, contains Cys-92 and Cys-114. It also is redox-sensitive. Apparently a disulfide anywhere in the region of Cys-114, -92 and -110 can function in the redox-modulation of the activity of this enzyme.

Stimulation of spinach ( Spinacia oleracea) chloroplast fructose-1,6-bisphosphatase by mercuric ions

Chloroplast fructose-1,6-bisphosphatase can exist in an active reduced form or a less active oxidised form. Oxidised fructose bisphosphatase from spinach ( Spinacia oleracea) could be stimulated up to many hundred-fold by 0.1 mM HgCl 2 whereas fructose bisphosphatases from rabbit, yeast, a non-chloroplast enzyme from spinach and the reduced chloroplast enzyme were only inhibited by HgCl 2. Stimulation of the enzyme was maximal at pH 8.0 and low magnesium concentrations where the oxidised enzyme normally has little activity.

Characterization of the allosteric binding pocket of human liver fructose-1,6-bisphosphatase by protein crystallography and inhibitor activity studies.

The structures of three complexes of human fructose-1,6-bisphosphatase (FB) with the allosteric inhibitor AMP and two AMP analogues have been determined and all fully refined. The data used for structure determination were collected at cryogenic temperature (110 K), and with the use of synchrotron radiation. The structures reveal a common mode of binding for AMP and formycine monophosphate (FMP). 5-Amino-4-carboxamido-1 beta-D-5-phosphate-ribofuranosyl-1H-imidazole (AICAR-P) shows an unexpected mode of binding to FB, different from that of the other two ligands. The imidazole ring of AICAR-P is rotated 180 degrees compared to the AMP and FMP bases. This rotation results in a slightly different hydrogen bonding pattern and minor changes in the water structure in the binding pocket. Common features of binding are seen for the ribose and phosphate moieties of all three compounds. Although binding in a different mode, AICAR-P is still capable of making all the important interactions with the residues building the allosteric binding pocket. The IC50 values of AMP, FMP, and AICAR-P were determined to be 1.7, 1.4, and 20.9 microM, respectively. Thus, the approximately 10 times lower potency of AICAR-P is difficult to explain solely from the variations observed in the binding pocket. Only one water molecule in the allosteric binding pocket was found to be conserved in all four subunits in all three structures. This water molecule coordinates to a phosphate oxygen atom and the N7 atom of the AMP molecule, and to similarly situated atoms in the FMP and AICAR-P complexes. This implies an important role of the conserved water molecule in binding of the ligand.

Regulation of photosynthetic enzymes via redox systems

Introduction In the process of photosynthesis complex enzymatic reactions are involved particularly in the Benson-Calvin cycle which occurs in the chloroplasts. Some of these enzymes are inactive in darkness but are activated by light, hence, the cycle functions only during the day. Examples of such enzymes are fructose bisphosphatase (FBPase) and sedoheptulose bisphosphatase (SBPase) which are inactivated in darkness. Several mechanisms have been proposed that account for the activation and deactivation of the Benson-Calvin cycle. For example, upon illumination, the pH of the stroma increases from pH 7 to pH 8 owing to the active pumping of protons into the intrathylakoid space. This is accompanied by a transfer of Mg 2+ ions via the thylakoid membrane into the stroma. As the envelope of the chloroplast is impermeable to Mg 2+ ions, the concentration of Mg 2+ in the stroma increase from 1 to 2-5mm. Most of the enzymes of the Benson-Calvin cycle are sensitive to pH and to changes in Mg 2+ concentration but these changes, by themselves, do not wholly account for the activation of the cycle. Two other types of activation mechanisms have been shown: (1) the photoregulation of these enzymes in which light-regulated redox systems such as ferredoxins are involved, and (2) regulation via energy load and via the concentration of metabolites from the cycle.

Higher-plant chloroplast and cytosolic fructose-1,6-bisphosphatase isoenzymes: origins via duplication rather than prokaryote-eukaryote divergence.

Full-size cDNAs encoding the precursors of chloroplast fructose-1,6-bisphosphatase (FBP), sedoheptulose-1,7-bisphosphatase (SBP), and the small subunit of Rubisco (RbcS) from spinach were cloned. These cDNAs complete the set of homologous probes for all nuclear-encoded enzymes of the Calvin cycle from spinach (Spinacia oleracea L.). FBP enzymes not only of higher plants but also of non-photosynthetic eukaryotes are found to be unexpectedly similar to eubacterial homologues, suggesting a eubacterial origin of these eukaryotic nuclear genes. Chloroplast and cytosolic FBP isoenzymes of higher plants arose through a gene duplication event which occurred early in eukaryotic evolution. Both FBP and SBP of higher plant chloroplasts have acquired substrate specificity, i.e. have undergone functional specialization since their divergence from bifunctional FBP/SBP enzymes of free-living eubacteria.

Highly conserved toxicity of Saccharomyces cerevisiae Rap1p

Budding yeast (Saccharomyces cerevisiae) Rap1p has been expressed in fission yeast (Schizosaccharomyces pombe) under the control of the regulatable fructose bisphosphatase (fbp) promoter. When the fbp promoter was derepressed, cells containing the complete RAP1 gene failed to show any significant growth, suggesting that Rap1p is toxic. A derivative of Rap1p that has a temperature-sensitive mutation in the DNA-binding domain was not toxic in cells grown at 37 degrees C, a temperature at which DNA binding by rap1p(ts) is severely inhibited. Removal of a short region downstream of the DNA-binding domain, including a region previously shown to be essential for Rap1p toxicity in budding yeast, also abolished the toxic effect. The toxic effect of Rap1p has therefore been conserved between two distantly related yeasts. In budding yeast, overexpression of Rap1p also caused changes to the lengths of the telomeric repeats. No effects on telomeres were detected in fission yeast.

Transcriptional regulation of zwf, encoding glucose-6-phosphate dehydrogenase, from the cyanobacterium Nostoc punctiforme strain ATCC 29133.

The gene encoding glucose-6-phosphate dehydrogenase (G6PD), zwf, in Nostoc punctiforme strain ATCC 29133 is part of a four-gene operon that also encodes fructose bisphosphatase (fbp), transaldolase (tal) and a gene product termed OpcA, which is contranscribed with zwf and essential for G6PD activity. The effect of exogenous nitrogen and carbon sources on transcription of these genes was investigated. Growth in the presence of ammonium yielded low levels of transcripts encoding all genes of the operon, while growth under nitrogen-fixing conditions resulted in a large increase of transcripts encoding for fbp and zwf-opcA. When cells are grown in the presence of fructose, levels of transcripts encoding tal and zwf-opcA were increased, relative to levels in ammonium-grown cells. These results indicate that this facultatively heterotrophic cyanobacterium can respond to changes in its environment by altering transcription of genes involved in carbon catabolism. Primer extension identified five 5' ends corresponding to the major regulated transcripts which we conclude arise from independent transcriptional start points.

Fructose-1,6-Bisphosphatase: Genetic and Physical Mapping to Human Chromosome 9q22.3 and Evaluation in Non-Insulin-Dependent Diabetes Mellitus

PCR primers specific to the human liver fructose-1,6-bisphosphatase (FBP) gene were designed and used to isolate a cosmid clone. Physical mapping of the FBP cosmid by FISH, and genetic mapping of an associated GA repeat polymorphism (PIC = 0.35), located the liver FBP gene to chromosome 9q22.3 with no recombination between FBP and the index markers D9S196 ( Z max = 13.2), D9S280 ( Z max = 11.7), D9S287 ( Z max = 15.6), and D9S176 ( Z max = 14.4). Amplification using FBP exon-specific primers with a YAC contig from this region of chromosome 9 further refined the placement of FBP genomic sequences to an approximately 1.7-cM region flanked by D9S280 and D9S287, near the gene for Fanconi anemia group C. Precise localization of the FBP gene enabled evaluation of FBP as a candidate gene for maturity-onset diabetes of the young (MODY) and non-insulin-dependent diabetes (NIDDM) in both Caucasian and African-American families, using the highly informative markers D9S287 and D9S176. Although FBP is a rate-limiting enzyme in gluconeogenesis, using both parametric and nonparametric analysis there was no evidence for linkage of FBP to diabetes in these families.

CO2 Uptake and Electron Transport Rates in Wild-Type and a Starchless Mutant of Nicotiana sylvestris (The Role and Regulation of Starch Synthesis at Saturating CO2 Concentrations).

CO2 uptake rate, chlorophyll fluorescence, and 830-nm absorbance were measured in wild-type (wt) Nicotiana sylvestris (Speg. et Comes) and starchless mutant NS 458 leaves at different light intensities and CO2 concentrations. Initial slopes of the relationships between CO2 uptake and light and CO2 were similar, but the maximum rate at CO2 and light saturation was only 30% in the mutant compared with the wt. O2 enhancement of photosynthesis at CO2 and light saturation was relatively much greater in the mutant than in the wt. In 21% O2, the electron transport rate (ETR) calculated from fluorescence peaked near the beginning of the CO2 saturation of photosynthesis. With the further increase of CO2 concentration ETR remained nearly constant or declined a little in the wt but drastically declined in the mutant. Absorbance measurements at 830 nm indicated photosystem I acceptor side reduction in both plants at saturating CO2 and light. Assimilatory charge (postillumination CO2 uptake) measurements indicated trapping of chloroplast inorganic phosphate, supposedly in hexose phosphates, in the mutant. It is concluded that starch synthesis gradually substitutes for photorespiration as electron acceptor with increasing CO2 concentration in the wt but not in the mutant. It is suggested that starch synthesis is co-controlled by the activity of the chloroplast fructose bisphosphatase.