The Path of Carbon Flow during NO(3)-Induced Photosynthetic Suppression in N-Limited Selenastrum minutum

Nitrate addition to nitrate-limited cultures of Selenastrum minutum Naeg. Collins (Chlorophyta) resulted in a 70% suppression of photosynthetic carbon fixation. In ¹⁴CO₂ pulse/chase experiments nitrate resupply increased radiolabel incorporation into amino and organic acids and decreased radiolabel incorporation into insoluble material. Nitrate resupply increased the concentration of phosphoenolpyruvate and increased the radiolabeling of phosphoenolpyruvate, pyruvate and tricarboxylic acid cycle intermediates, notably citrate, fumarate, and malate. Furthermore, nitrate also increased the pool sizes and radiolabeling of most amino acids, with alanine, aspartate, glutamate, and glutamine showing the largest changes. Nitrate resupply increased the proportion of radiolabel in the C-4 position of malate and increased the ratios of radiolabel in aspartate to phosphoenolpyruvate and in pyruvate to phosphoenolpyruvate, indicative of increased phosphoenolpyruvate carboxylase and pyruvate kinase activities. Analysis of these data showed that the rate of carbon flow through glutamate (10.6 μmoles glutamate per milligram chlorophyll per hour) and the rate of net glutamate production (7.9 μmoles glutamate per milligram chlorophyll per hour) were both greater than the maximum rate of carbon export from the Calvin cycle which could be maintained during steady state photosynthesis. These results are consistent with the hypothesis that nitrogen resupply to nitrogen-limited microalgae results in a transient suppression of photosynthetic carbon fixation due, in part, to the severity of competition for carbon skeletons between the Calvin cycle and nitrogen assimilation (IR Elrifi, DH Turpin 1986 Plant Physiol 81: 273-279).     

Nitrate and Ammonium Induced Photosynthetic Suppression in N-Limited Selenastrum minutum: II. Effects of NO(3) and NH(4) Addition to CO(2) Efflux in the Light

The effects of nitrate and ammonium addition on net and gross photosynthesis, CO₂ efflux and the dissolved inorganic carbon compensation point of nitrogen-limited Selenastrum minutum Naeg. Collins (Chlorophyta) were studied. Cultures pulsed with nitrate or ammonium exhibited a marked decrease in both net and gross photosynthetic carbon fixation. During this period of suppression the specific activity of exogenous dissolved inorganic carbon decreased rapidly in comparison to control cells indicating an increase in the rate of CO₂ efflux in the light. The nitrate and ammmonium induced rates of CO₂ efflux were 31.0 and 33.8 micromoles CO₂ per milligram chlorophyll per hour, respectively, and represented 49 and 48% of the rate of gross photosynthesis. Nitrate addition to cells at dissolved inorganic carbon compensation point caused an increase in compensation point while ammonium had no effect. In the presence of the tricarboxylic acid cycle inhibitor fluoroacetate, the nitrate-induced change in compensation point was greatly reduced suggesting the source of this CO₂ was the tricarboxylic acid cycle. These results are consistent with the mechanism of N-induced photosynthetic suppression outlined by Elrifi and Turpin (1986 Plant Physiol 81: 273-279).     

Short-Term Metabolite Changes during Transient Ammonium Assimilation by the N-Limited Green Alga Selenastrum minutum

In this study, we measured the total pool sizes of key cellular metabolites from nitrogen-limited cells of Selenastrum minutum before and during ammonium assimilation in the light. This was carried out to identify the sites at which N assimilation is acting to regulate carbon metabolism. Over 120 seconds following NH₄⁺ addition we found that: (a) N accumulated in glutamine while glutamate and α-ketoglutarate levels fell; (b) ATP levels declined within 5 seconds and recovered within 30 seconds of NH₄⁺ addition; (c) ratios of pyruvate/phosphoenolpyruvate, malate/phosphoenolpyruvate, Glc-1-P/Glc-6-P and Fru-1,6-bisphosphate/Fru-6-P increased; and (d) as previously seen, photosynthetic carbon fixation was inhibited. Further, we monitored starch degradation during N assimilation over a longer time course and found that starch breakdown occurred at a rate of about 110 micromoles glucose per milligram chlorophyll per hour. The results are consistent with N assimilation occurring through glutamine synthetase/glutamate synthase at the expense of carbon previously stored as starch. They also indicate that regulation of several enzymes is involved in the shift in metabolism from photosynthetic carbon assimilation to carbohydrate oxidation during N assimilation. It seems likely that pyruvate kinase, phosphoenolpyruvate carboxylase, and starch degradation are all activated, whereas key Calvin cycle enzyme(s) are inactivated within seconds of NH₄⁺ addition to N-limited S. minutum cells. The rapid changes in glutamate and triose phosphate, recently shown to be regulators of cytosolic pyruvate kinase, are consistent with them contributing to the short-term activation of this enzyme.     

Steady-State Chlorophyll a Fluorescence Transients during Ammonium Assimilation by the N-Limited Green Alga Selenastrum minutum

The assimilation of ammonium by the N-limited green alga Selenastrum minutum results in the suppression of photosynthetic electron flow from H₂O to CO₂ (6, 7, 18). In this study, results are presented which describe the correponding change in steady-state chlorophyll a fluorescence. The addition of ammonium resulted in a transient decline in fluorescence followed by a marked increase. Fluorescence did not return to control levels until the added ammonium had been assimilated. Analysis of the fluorescence transients showed that ammonium assimilation resulted in a rapid increase in nonphotochemical quenching (Q_e) peaking 10 to 15 seconds after ammonium addition. Q_e then decreased dramatically reaching a minimum value approximately 45 seconds following ammonium addition and returned to the control level only after the added ammonium had been assimilated. There were no effects of ammonium addition on photochemical quenching (Q_q) for approximately 10 to 15 seconds at which time both gross O₂ evolution (as measured by mass spectrometry) and Q_q declined. In the presence of d,l-glyceraldehyde or when cells were held at the CO₂ compensation point, the addition of ammonium resulted in a decline in Qe 10 to 15 seconds after addition. The Q_e peak and the Q_q decline were absent. These results imply that the transient increase in Q_e and the subsequent decline in Q_q may be attributed to the decline in Calvin cycle activity during ammonium assimilation. The decline in Q_e is apparently a direct result of ammonium assimilation. The observation that the Q_e peak precedes the Q_q decline would be consistent with the decreases in Calvin cycle carbon flow occurring at the kinase reactions prior to glyceraldehyde-3-phosphate dehydrogenase.     

Anaerobic Metabolism in the N-Limited Green Alga Selenastrum minutum: II. Assimilation of Ammonium by Anaerobic Cells

The green alga Selenastrum minutum (Naeg.) Collins is able to assimilate NH₄⁺ in the dark under anaerobic conditions (GC Vanlerberghe, AK Horsey, HG Weger, DH Turpin [1989] Plant Physiol 91: 1551-1557). In the present study, analysis of metabolites following addition of NH₄⁺ to cells acclimated to anaerobic conditions has shown the following. There was a transient decline in adenylate energy charge from 0.6 to 0.4 followed by a recovery back to ~0.6. This was accompanied by a rapid increase in pyruvate/phosphoenolpyruvate and fructose-1,6-bisphosphate/fructose-6-phosphate ratios indicating activation of pyruvate kinase and 6-phosphofructokinase, respectively. There was also an increase in fructose-2,6-bisphosphate, which, since this alga lacks pyrophosphate dependent 6-phosphofructokinase can be inferred to inhibit gluconeogenic fructose-1,6-bisphosphatase. These changes resulted in an increase in the rate of anaerobic starch breakdown. Anaerobic NH₄⁺ assimilation also resulted in a two-fold increase in the rate of production of the major fermentative end-products in this alga, d-lactate and ethanol. There was no change in the rate of accumulation of the fermentative end product succinate but malate accumulated under anoxia during NH₄⁺ assimilation. A rapid increase in Gln and decline in Glu indicates that primary NH₄⁺ assimilation under anoxia was via glutamine synthetase-glutamate synthase. Almost all N assimilated under these conditions was sequestered in alanine. These results allow us to propose a model for the regulation of carbon metabolism during anaerobic NH₄⁺ assimilation.     

Anaerobic Metabolism in the N-Limited Green Alga Selenastrum minutum: III. Alanine Is the Product of Anaerobic Ammonium Assimilation

We have determined the flow of ¹⁵N into free amino acids of the N-limited green alga Selenastrum minutum (Naeg.) Collins after addition of ¹⁵NH₄⁺ to aerobic or anaerobic cells. Under aerobic conditions, only a small proportion of the N assimilated was retained in the free amino acid pool. However, under anaerobic conditions almost all assimilated NH₄⁺ accumulates in alanine. This is a unique feature of anaerobic NH₄⁺ assimilation. The pathway of carbon flow to alanine results in the production of ATP and reductant which matches exactly the requirements of NH₄⁺ assimilation. Alanine synthesis is therefore an excellent strategy to maintain energy and redox balance during anaerobic NH₄⁺ assimilation.     

Significance of Phosphoenolpyruvate Carboxylase during Ammonium Assimilation: Carbon Isotope Discrimination in Photosynthesis and Respiration by the N-Limited Green Alga Selenastrum minutum

The effect of N-assimilation on the partitioning of carbon fixation between phosphoenolpyruvate carboxylase (PEPcase) and ribulose bisphosphate carboxylase/oxygenase (Rubisco) was determined by measuring stable carbon isotope discrimination during photosynthesis by an N-limited green alga, Selenastrum minutum (Naeg.) Collins. This was facilitated by a two process model accounting for simultaneous CO₂ fixation and respiratory CO₂ release. Discrimination by control cells was consistent with the majority of carbon being fixed by Rubisco. During nitrogen assimilation however, discrimination was greatly reduced indicating an enhanced flux through PEPcase which accounted for upward of 70% of total carbon fixation. This shift toward anaplerotic metabolism supports a large increase in tricarboxylic acid cycle activity primarily between oxaloacetate and α-ketoglutarate thereby facilitating the provision of carbon skeletons for amino acid synthesis. This provides an example of a unique set of conditions under which anaplerotic carbon fixation by PEPcase exceeds photosynthetic carbon fixation by Rubisco in a C₃ organism.     

Anaerobic Metabolism in the N-Limited Green Alga Selenastrum minutum: I. Regulation of Carbon Metabolism and Succinate as a Fermentation Product

The onset of anaerobiosis in darkened, N-limited cells of the green alga Selenastrum minutum (Naeg.) Collins elicited the following metabolic responses. There was a rapid decrease in energy charge from 0.85 to a stable lower value of 0.6 accompanied by rapid increases in pyruvate/phosphoenolpyruvate and fructose-1,6-bisphosphate/fructose-6-phosphate ratios indicating activation of pyruvate kinase and 6-phosphofructokinase, respectively. There was also a large increase in fructose-2,6-bisphosphate, which, since this alga lacks pyrophosphate dependent 6-phosphofructokinase, can be inferred to inhibit gluconeogenic fructose-1,6-bisphosphatase activity. These changes resulted in an approximately twofold increase in the rate of starch breakdown indicating a Pasteur effect. The Pasteur effect was accompanied by accumulation of d-lactate, ethanol and succinate as fermentation end-products, but not malate. Accumulation of succinate was facilitated by reductive carbon metabolism by a partial TCA cycle (GC Vanlerberghe, AK Horsey, HG Weger, DH Turpin [1989] Plant Physiol 91: 1551-1557). An initial stoichiometric decline in aspartate and increases in succinate and alanine suggests that aspartate catabolism provides an initial source of carbon for reduction to succinate under anoxic conditions. These observations allow us to develop a model for the regulation of anaerobic carbon metabolism and a model for short-term and long-term strategies for succinate accumulation in a green alga.     

Dark Ammonium Assimilation Reduces the Plastoquinone Pool of Photosystem II in the Green Alga Selenastrum minutum

The impact of dark NH₄⁺ and NO₃⁻ assimilation on photosynthetic light harvesting capability of the green alga Selenastrum minutum was monitored by chlorophyll a fluorescence analysis. When cells assimilated NH₄⁺, they exhibited a large decline in the variable fluorescence/maximum fluorescence ratio, the fluorescence yield of photosystem II relative to that of photosystem I at 77 kelvin, and O₂ evolution rate. NH₄⁺ assimilation therefore poised the cells in a less efficient state for photosystem II. The analysis of complementary area of fluorescence induction curve and the pattern of fluorescence decay upon microsecond saturating flash, indicators of redox state of plastoquinone (PQ) pool and dark reoxidation of primary quinone electron acceptor (Q_A), respectively, revealed that the PQ pool became reduced during dark NH₄⁺ assimilation. NH₄⁺ assimilation also caused an increase in the NADPH/NADP⁺ ratio due to the NH₄⁺ induced increase in respiratory carbon oxidation. The change in cellular reductant is suggested to be responsible for the reduction of the PQ pool and provide a mechanism by which the metabolic demands of NH₄⁺ assimilation may alter the efficiency of photosynthetic light harvesting. NO₃⁻ assimilation did not cause a reduction in PQ and did not affect the efficiency of light harvesting. These results illustrate the role of cellular metabolism in the modulating photosynthetic processes.     

Ammonium Assimilation Requires Mitochondrial Respiration in the Light : A Study with the Green Alga Selenastrum minutum

Mass spectrometric analysis of O₂ and CO₂ exchange in the green alga Selenastrum minutum (Naeg. Collins) provides evidence for the occurrence of mitochondrial respiration in light. Stimulation of amino acid synthesis by the addition of NH₄Cl resulted in nearly a 250% increase in the rate of TCA cycle CO₂ efflux in both light and dark. Ammonium addition caused a similar increase in cyanide sensitive O₂ consumption in both light and dark. Anaerobiosis inhibited the CO₂ release caused by NH₄Cl. These results indicated that the cytochrome pathway of the mitochondrial electron transport chain was operative and responsible for the oxidation of a large portion of the NADH generated during the ammonium induced increase in TCA cycle activity. In the presence of DCMU, ammonium addition also stimulated net O₂ consumption in the light. This implied that the Mehler reaction did not play a significant role in O₂ consumption under our conditions. These results show that both the TCA cycle and the mitochondrial electron transport chain are capable of operation in the light and that an important role of mitochondrial respiration in photosynthesizing cells is the provision of carbon skeletons for biosynthetic reactions.     

Regulation of Carbon Partitioning to Respiration during Dark Ammonium Assimilation by the Green Alga Selenastrum minutum

The assimilation of NH₄⁺ causes a rapid increase in respiration to provided carbon skeletons for amino acid synthesis. In this study we propose a model for the regulation of carbon partitioning from starch to respiration and N assimilation in the green alga Selenastrum minutum. We provide evidence for both a cytosolic and plastidic fructose-1,6-bisphosphatase. The cytosolic form is inhibited by AMP and fructose-1,6-bisphosphate and the plastidic form is inhibited by phosphate. There is only one ATP dependent phosphofructokinase which, based on immunological cross reactivity, has been identified as being localized in the plastid. It is inhibited by phosphoenolpyruvate and activated by phosphate. No pyrophosphate dependent phosphofructokinase was found. The initiation of dark ammonium assimilation resulted in a transient increase in ADP which releases pyruvate kinase from adenylate control. This activation of pyruvate kinase causes a rapid 80% drop in phosphoenolpyruvate and a 2.7-fold increase in pyruvate. The pyruvate kinase mediated decrease in phosphoenolpyruvate correlates with the activation of the ATP dependent phosphofructokinase increasing carbon flow through the upper half of glycolysis. This increased the concentration of triosephosphate and provided substrate for pyruvate kinase. It is suggested that this increase in triosephosphate coupled with the glutamine synthetase mediated decline in glutamate, serves to maintain pyruvate kinase activation once ADP levels recover. The initiation of NH₄⁺ assimilation causes a transient 60% increase in fructose-2,6-bisphosphate. Given the sensitivity of the cytosolic fructose-1,6-bisphosphatase to this regulator, its increase would serve to inhibit cytosolic gluconeogenesis and direct the triosephosphate exported from the plastid down glycolysis to amino acid biosynthesis.     

Effects of Phosphorus Limitation on Respiratory Metabolism in the Green Alga Selenastrum minutum

The effects of phosphorus nutrition on several physiological and biochemical parameters of the green alga, Selenastrum minutum, have been examined. Algal cells were cultured in chemostats under conditions of either Pi limitation or nutrient sufficiency. Pi limitation resulted in: (a) a 5-fold lower rate of respiration, (b) a 3-fold decline in rates of photosynthetic carbon dioxide fixation and oxygen evolution, (c) a 3-fold higher rate of dark carbon dioxide fixation, (d) significant increases in activities of phosphoenolpyruvate (PEP) carboxylase and PEP phosphatase (128% and 158% of nutrient sufficient activities, respectively), (e) significant reductions in activities of nonphosphorylating NADP-glyceraldehyde-3-phosphate dehydrogenase and NAD malic enzyme, and (f) no change in levels of ATP:fructose-6-phosphate 1-phosphotransferase, phosphorylating NAD-glyceraldehyde-3-phosphate dehydrogenase, 3-phosphoglycerate kinase, and pyruvate kinase. The intracellular concentrations of Pi, ATP, AMP, soluble protein, and chlorophyll were also significantly reduced in response to Pi limitation. As well, the level of ADP was about 11-fold lower in the Pi-limited cells as compared to the nutrient sufficient controls. It was predicted that because of this low level of ADP, pyruvate kinase catalyzed conversion of PEP to pyruvate may be restricted in Pi-limited cells. During Pi limitation, PEP carboxylase and PEP phosphatase may function to “bypass” the ADP dependent pyruvate kinase, as well as to recycle Pi for its reassimilation into cellular metabolism.     

Mitochondrial Respiration Can Support NO(3) and NO(2) Reduction during Photosynthesis : Interactions between Photosynthesis, Respiration, and N Assimilation in the N-Limited Green Alga Selenastrum minutum

Mass spectrometric analysis shows that assimilation of inorganic nitrogen (NH₄⁺, NO₂⁻, NO₃⁻) by N-limited cells of Selenastrum minutum (Naeg.) Collins results in a stimulation of tricarboxylic acid cycle (TCA cycle) CO₂ release in both the light and dark. In a previous study we have shown that TCA cycle reductant generated during NH₄⁺ assimilation is oxidized via the cytochrome electron transport chain, resulting in an increase in respiratory O₂ consumption during photosynthesis (HG Weger, DG Birch, IR Elrifi, DH Turpin [1988] Plant Physiol 86: 688-692). NO₃⁻ and NO₂⁻ assimilation resulted in a larger stimulation of TCA cycle CO₂ release than did NH₄⁺, but a much smaller stimulation of mitochondrial O₂ consumption. NH₄⁺ assimilation was the same in the light and dark and insensitive to DCMU, but was 82% inhibited by anaerobiosis in both the light and dark. NO₃⁻ and NO₂⁻ assimilation rates were maximal in the light, but assimilation could proceed at substantial rates in the light in the presence of DCMU and in the dark. Unlike NH₄⁺, NO₃⁻ and NO₂⁻ assimilation were relatively insensitive to anaerobiosis. These results indicated that operation of the mitochondrial electron transport chain was not required to maintain TCA cycle activity during NO₃⁻ and NO₂⁻ assimilation, suggesting an alternative sink for TCA cycle generated reductant. Evaluation of changes in gross O₂ consumption during NO₃⁻ and NO₂⁻ assimilation suggest that TCA cycle reductant was exported to the chloroplast during photosynthesis and used to support NO₃⁻ and NO₂⁻ reduction.     

Demonstration of Both a Photosynthetic and a Nonphotosynthetic CO(2) Requirement for NH(4) Assimilation in the Green Alga Selenastrum minutum

Nitrogen-limited and nitrogen-sufficient cell cultures of Selenastrum minutum (Naeg.) Collins (Chlorophyta) were used to investigate the dependence of NH₄⁺ assimilation on exogenous CO₂. N-sufficient cells were only able to assimilate NH₄⁺ maximally in the presence of CO₂ and light. Inhibition of photosynthesis with 3-(3,4-dichlorophenyl)-1,1-dimethylurea, diuron also inhibited NH₄⁺ assimilation. These results indicate that NH₄⁺ assimilation by N-sufficient cells exhibited a strict requirement for photosynthetic CO₂ fixation. N-limited cells assimilated NH₄⁺ both in the dark and in the light in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, diuron, indicating that photosynthetic CO₂ fixation was not required for NH₄⁺ assimilation. Using CO₂ removal techniques reported previously in the literature, we were unable to demonstrate CO₂-dependent NH₄⁺ assimilation in N-limited cells. However, employing more stringent CO₂ removal techniques we were able to show a CO₂ dependence of NH₄⁺ assimilation in both the light and dark, which was independent of photosynthesis. The results indicate two independent CO₂ requirements for NH₄⁺ assimilation. The first is as a substrate for photosynthetic CO₂ fixation, whereas the second is a nonphoto-synthetic requirement, presumably as a substrate for the anaplerotic reaction catalyzed by phosphoenolpyruvate carboxylase.     

Regulation of Phosphoenolpyruvate Carboxylase from the Green Alga Selenastrum minutum: Properties Associated with Replenishment of Tricarboxylic Acid Cycle Intermediates during Ammonium Assimilation

Two isoforms of phosphoenolpyruvate carboxylase (PEPC) with very different regulatory properties were partially purified from the green alga Selenastrum minutum. They were designated PEPC₁ and PEPC₂. PEPC₁ showed sigmoidal kinetics with respect to phosphoenolpyruvate (PEP) whereas PEPC₂ exhibited a typical Michaelis-Menten response. The S_0.5(PEP) of PEPC₁ was 2.23 millimolar. This was fourfold greater than the S_0.5(PEP) of PEPC₂, which was 0.57 millimolar. PEPC₁ was activated more than fourfold by 2.0 millimolar glutamine and sixfold by 2.0 millimolar dihydroxyacetone phosphate (DHAP) at a subsaturating PEP concentration of 0.625 millimolar. In contrast, PEPC₂ showed only 8% and 52% activation by glutamine and DHAP, respectively. The effects of glutamine and DHAP were additive. PEPC₁ was more sensitive to inhibition by glutamate, 2-oxoglutarate, and aspartate than PEPC₂. Both isoforms were equally inhibited by malate. All of these metabolites affected only the S_0.5(PEP) not the V_max. The regulatory properties of S. minutum PEPC in vitro are discussed in terms of (a) increased rates of dark carbon fixation (shown to be catalyzed predominantly by PEPC) and (b) changes in metabolite levels in vivo during enhanced NH₄₊ assimilation. Finally, a model is proposed for the regulation of PEPC in vivo in relation to its role in replenishing tricarboxylic acid cycle intermediates consumed in NH₄₊ assimilation.     

Photosynthetic Effect in Selenastrum capricornutum Progeny after Carbon-Ion Irradiation

A large proportion of mutants with altered pigment features have been obtained via exposure to heavy-ion beams, a technique that is efficient for trait improvement in the breeding of plants and algae. However, little is known about the underlying mechanisms by which the photosynthetic pigments are altered by heavy-ion irradiation. In our study, the photosynthetic characteristics of progenies from carbon-ion irradiated Selenastrum capricornutum were investigated. Five progenies deficient in chlorophyll a were isolated after carbon-ion exposure. Photosynthetic characteristics, photoprotection capacity and gene expression of the light-harvesting complex in these progenies were further characterized by the measurement of chlorophyll fluorescence parameters (Fv/Fm, ФPSII, NPQ, ETR), the de-epoxidation state of the xanthophyll cycle, the amount of lutein and quantitative real-time PCR. High maximum quantum yield of photosystem II at day 10 and high thermal dissipation ability were observed in progenies #23 and #37 under normal culture condition. Progenies #18, #19 and #20 showed stronger resistance against high levels of light steps than the control group (612–1077 μmol photons m ^-2 s ^-1, p< 0.05). The progenies #20 and #23 exhibited strong photoprotection by thermal dissipation and quenching of ³Chl* after 24 h of high light treatment. The mRNA levels of Lhcb5, Lhcbm5 and Lhcbm1 of the light-harvesting complex revealed markedly differential expression in the five progenies irradiated by carbon-ion beams. This work indicates that photosynthetic efficiency, photoprotection ability and the expression of light-harvesting antennae in unicellular green algae can be markedly influenced by irradiation. To our knowledge, this is the first report on changes in the photosynthetic pigments of green algae after treatment with carbon-ion beams.     

Cytochrome and Alternative Pathway Respiration during Transient Ammonium Assimilation by N-Limited Chlamydomonas reinhardtii

Mass spectrometric analysis of gas exchange in light and dark by N-limited cells of Chlamydomonas reinhardtii indicated that ammonium assimilation was accompanied by an increase in respiratory carbon flow to provide carbon skeletons for amino acid synthesis. Tricarboxylic acid (TCA) cycle carbon flow was maintained by the oxidation of TCA cycle reductant via the mitochondrial electron transport chain. In wild-type cells, inhibitor studies and ¹⁸O₂ discrimination experiments indicated that respiratory electron flow was mediated entirely via the cytochrome pathway in both the light and dark, despite a large capacity for the alternative pathway. In a cytochrome oxidase deficient mutant, or in wild-type cells in the presence of cyanide, the alternative pathway could support the increase in TCA cycle carbon flow. These different mechanisms of oxidation of TCA cycle reductant were reflected by the much greater SHAM sensitivity of ammonium assimilation by cytochrome oxidase-deficient cells as compared to wild type.     

Purification and Molecular and Immunological Characterization of a Unique Phosphoribulokinase from the Green Alga Selenastrum minutum

A unique phosphoribulokinase (ADP:D-ribulose 5-phosphate 1-phosphotransferase, EC 2.7.1.19) has been purified to homogeneity from the green alga Selenastrum minutum. The enzyme has a native molecular mass of about 83 kilodaltons and a native isoelectric point of 5.1. The enzyme consists of two different-sized subunits of 41 and 40 kilodaltons, implying that it is a heterodimer. This is the first report of a eukaryotic heterodimeric phosphoribulokinase. The in vivo existence of two nonidentical subunits of S. minutum phosphoribulokinase was confirmed by western blot analysis of crude protein extracts from trichloroacetic acid-killed cells. These two subunits were immunologically similar, as rabbit immunoglobulin G affinity purified against the 41 kilodalton subunit of S. minutum phosphoribulokinase (PRK) cross-reacts with the 40 kilodalton subunit and vice versa. Antibodies against S. minutum phosphoribulokinase also cross-react with the spinach enzyme. NH₂-terminal sequencing revealed that the two S. minutum PRK subunits shared a considerable degree of structure homology with each other and with the enzymes from spinach and Chlamydomonas reinhardtii, but not with PRK from Rhodobacter sphaeroides. There are, however, differences between the NH₂-terminal amino acid sequences of the two S. minutum PRK subunits, that imply that they are the products of separate genes or products of two different mRNAs spliced from a single gene.     

RuBP Limitation of Photosynthetic Carbon Fixation during NH(3) Assimilation : Interactions between Photosynthesis, Respiration, and Ammonium Assimilation in N-Limited Green Algae

The effects of ammonium assimilation on photosynthetic carbon fixation and O₂ exchange were examined in two species of N-limited green algae, Chlorella pyrenoidosa and Selenastrum minutum. Under light-saturating conditions, ammonium assimilation resulted in a suppression of photosynthetic carbon fixation by S. minutum but not by C. pyrenoidosa. These different responses are due to different relationships between cellular ribulose bisphosphate (RuBP) concentration and the RuBP binding site density of ribulose bisphosphate carboxylase/oxygenase (Rubisco). In both species, ammonium assimilation resulted in a decrease in RuBP concentration. In S. minutum the concentration fell below the RuBP binding site density of Rubisco, indicating RuBP limitation of carboxylation. In contrast, RuBP concentration remained above the binding site density in C. pyrenoidosa. Compromising RuBP regeneration in C. pyrenoidosa with low light resulted in an ammonium-induced decrease in RuBP concentration below the RuBP binding site density of Rubisco. This resulted in a decrease in photosynthetic carbon fixation. In both species, ammonium assimilation resulted in a larger decrease in net O₂ evolution than in carbon fixation. Mass spectrometric analysis shows this to be a result of an increase in the rate of mitochondrial respiration in the light.     

Pyruvate kinase isozymes from the green alga, Selenastrum minutum. II. Kinetic and regulatory properties

The kinetic and regulatory properties of two pyruvate kinase isozymes, PKp and PKc (apparent chloroplastic and cytosolic isozymes, respectively) from the green alga Selenastrum minutum were studied. The two isozymes differed greatly in several kinetic properties. Although both isozymes showed hyperbolic substrate saturation kinetics, the apparent Michaelis constants for PEP and ADP were about twofold and fourfold lower, respectively, for PKc as compared with PKp. ADP was the preferred nucleotide substrate for both isozymes. However, PKc utilized alternate nucleotides far more effectively than did PKp. PKc and PKp also differed strongly in the effect of activators and inhibitors on the enzymes. Although both isozymes were activated by dihydroxyacetone phosphate (DHAP) with a similar activation constant of about 30 microM, this activator (0.5 mM) caused an approximate 30% increase in the Vmax of PKc, but had no effect on the Vmax of PKp. PKp, but not PKc, was inhibited by ribose 5-phosphate, ribulose 1,5-bisphosphate, 2-phosphoglycerate, phosphoglycolate, and malate. Both isozymes were inhibited by MgATP, Mg2citrate, Mg2oxalate, and Pi. PKc was far more sensitive to inhibition by Pi, as compared with PKp. Pi was a competitive inhibitor of PKc with respect to phosphoenolpyruvate (PEP) (Ki = 1.3 mM). Glutamate was a potent inhibitor of PKc, but had no effect on PKp. In contrast with Pi, glutamate was a mixed-type inhibitor of PKc with respect to PEP (Ki = 0.7 mM). DHAP facilitated the binding of PEP by both isozymes and reversed or relieved the inhibition of PKc by Pi and/or glutamate. The regulatory properties of PKp indicate that it is likely less active in the light and more active in the dark. The in vivo activity of PKc is probably regulated by the relative cytosolic levels of DHAP, Pi, and glutamate; this provides a rationale for the activation of algal cytosolic pyruvate kinase which occurs during periods of enhanced ammonia assimilation.