Ribulose-1, 5-diphosphate carboxylase of Chromatium strain D

RuDP carboxylase isolated from autotrophically grown cells ofphotosynthetic sulfur bacterium, Chromatium strain D, was partiallypurified by (NH₄)₂SO₄ precipitation and Sephadex G-200 gel filtration.The molecular size of the bacterial RuDP carboxylase was foundto be large, analogous to that of the plant enzyme, in agreementwith results of previous workers. Sucrose density gradient centrifugationshowed the S_rel to be approximately 18; the omission of Mg⁺⁺caused no dissociation of the enzyme molecule in its subunits.Chromatium RuDP carboxylase showed similarities to the plantenzyme in some of its kinetic properties; (a) a shift of pHoptimum to the neutral side from the alkaline side on the additionof Mg⁺⁺, (b) deviation of the substrate concentration (NaHCO₃)-activityrelationship from the MICHAELIS formula and (c) a marked stimulativeeffect of Mg⁺⁺. A unique sigmoidal saturation curve of the enzymeto RuDP, which had been detected in Rhodospirillum rubrum andRhodopseudomonas spheroides RuDP carboxylase in the absenceof Mg⁺⁺, was not found. Another characteristic feature of ChromatiumRuDP carboxylase is its partial immunological response to therabbit anti-spinach RuDP carboxylase serum as detected by theinhibition of the carboxylation reaction due to the antibody-antigenreaction. ¹Part X, Structure and Function of Chloroplast Proteins. Supportedin part by research grants from the Ministry of Education ofJapan (No. 8719) and USPHS (AM-10792-03) (Received July 4, 1969; )     

Compatible solutes and their effects on phosphoenolpyruvate carboxylase of C4-halophytes

Abstract Phosphoenolpyruvate carboxylase (PEPCase), extracted from two Poaceae (Cynodon dactylon and Sporobolus pungens) grown on saline soil, was affected physiologically by betaine and proline. Its affinity for phosphoenolpyruvate (PEP) was increased and full protection against NaCl inhibition was observed; enzymic activity was also stabilized when assayed at low PEP levels. Betaine has the same effects on PEPCase extracted from a Chenopodiaceae (Salsola soda), whereas proline behaves as a competitive inhibitor, i.e. it does not protect the enzyme against NaCl and it accelerates inactivation at low PEP levels. Betaine was only compatible with PEPCase extracted from Saisola kali, without any effect on activity, protection or stabilization, but proline was again inhibitory. The levels of free proline in the two salt-stressed Poaceae were high, whereas in the Chenopodiaceae the free proline was low, as in non-stressed plants. The above data indicate that osmoregulators could not only be compatible with cytoplasmic enzymes, but they could either promote or inhibit enzyme activity, depending on the source of enzyme. Coevolution of PEPCase with the osmolyte selected for, could also be inferred.     

Salt Tolerance in the Halophyte, Suaeda maritima (L.) Dum.: The Influence of the Salinity of the Culture Solution on the Content of Various Organic Compounds

Plants of the halophyte Suaeda maritima were grown in tap wateror in a culture solution in the presence or absence of sodiumchloride and the levels of sugars, amino acids, organic acidsand quaternary ammonium compounds determined in relation tothe balance between cytoplasmic and vacuolar water potentials.The sugar content (some 7 µmol. g f. wt^–1) was unaffectedby the salinity of the growth medium as was the overall contentof amino acids (about 4 µmol. g f. wt^–1). The organicacid content was maximal in plants kept in tap water alone wherethe dominant acid was malic. Plants grown in culture solutioncontained the same acids, although addition of sodium chlorideto the medium brought about the apparent loss of glycolic acidand the appearance of oxalic acid. Only a single quaternaryammonium compound, glycinebetaine, was apparently present inthe tissues: the content of betaine doubled (to 37·5µrmol. g f. wt^–) when sodium chloride was addedto the culture solution. The content of these various compoundsis discussed in relation to the relative values of the cytoplasmicand vacuolar components of the overall tissue water potential Suaeda maritima, halophyte, salt tolerance, betaine, organic compounds, water potential     

Growth and nitrogen assimilation in nodules in response to nitrate levels in Vicia faba under salt stress

This study analyses the effects of salt on the effective symbiosisof faba bean (Vicia faba L. var. minor cv. Alborea) and salt-tolerantRhizobium leguminosarum biovar. viciae strain GRA19 grown withtwo KNO₃ levels (2 and 8 mM). The addition of 8 mM KNO₃ to thegrowth medium increases plant tolerance to salinity even witha concentration of 100 mM NaCl. This KNO₃ level in control plantsreduced the N₂ fixation. For 2 and 8 mM KNO₃ the plants treatedwith NaCl reduced N₂ fixation to identical values. The activityof the enzymes mediating ammonium assimilation in nodules (GS,NADH-GOGAT and NADH-GDH) was decreased by high KNO₃ levels.The results show that NADH-GOGAT activity was more markedlyinhibited than was GS activity by salinity, therefore NADH-GOGATlimits the ammonium assimilation by nodules in V. faba undersalt stress. The total proline content in the nodule was notrelated to salt tolerance and thus does not serve as a salttoleranceindex for V. faba. Key words: Glutamate synthase, glutamine synthetase, N₂ fixation, nitrate, salinity     

The Effects of Compatible Solutes on the Heat Stability of Glutamine Synthetase from Chickpeas Grown under Different Nitrogen and Temperature Regimes

The activity of glutamine synthetase (GS) extracted from chickpeasgrown under different nitrogen and temperature regimes was examined.Cytosolic GS (GS₁) was much less temperature sensitive thanchloroplastic GS (GS₂). There was no change in the ratio ofthe two isoforms from plants with different growth temperatures.Specific activity of GS appeared to be independent of growthtemperature and was little affected by nitrogen supply. In vitro studies were performed on the heat stability of GS₂in the presence of glycine betaine, proline, and myoinositol.The latter was most effective in preventing loss of enzyme activityat high temperatures. Glycine betaine was also effective asa heat protectant but enzyme activity in the presence of prolinewas lower than the control at temperatures above 30 ?C. Theimportance of myo-inositol protection of metabolism is discussedin relation to the accumulation in chickpeas of another cyclitol,pinitol. Key words: Chickpeas, high temperature stress, glutamine synthetase     

Salinity Stress and the Content of Proline in Roots of Pisum sativum and Tamarix tetragyna

Amino acid composition of the free amino acid pool and the TCA-insolubleprotein fraction were investigated in root tips of pea and Tamarixtetragyna plants grown at various levels of NaCl salinity. Salinitystress induced an increase of proline content, mainly in thefree amino acid pool in both plants, and of proline or hydroxyprolinecontent in the protein. Externally-supplied proline was absorbedand incorporated into protein, by pea roots, more effectivelythan by Tamarix roots. Salinity stress, apparently, stimulatedthe metabolism of externally-supplied labelled proline. Pearoots have a very large pool of free glutamic acid; however,70 per cent of the ¹⁴C from externally-supplied ¹⁴C-U-glutamicacid was released as CO₂. Very small amounts of it were incorporatedinto protein. No measurable amount of radioactivity could bedetected in any one of the individual amino acids, either ofprotein hydrolysate or the free amino acid pool. Proline very effectively counteracted the inhibitory effectof NaCl on pea seed germination and root growth. A similar effectbut to a lesser degree was achieved with phenylalanine and asparticacid. The feasibility of proline being a cytoplasmic osmoticumis discussed.     

The Effects of Silicon on Cucumber Plants Grown in Recirculating Nutrient Solution

Cucumber plants (Cucumis sativus) cv. Corona were grown in recirculatingnutrient solution containing either 10 mg l^–1 SiO₂ (lowSi) which was the level present in the water supply or givenan additional 100 mg l^–1 SiO₂ (high Si). Silicate wasdepleted from the solution when cucumbers were grown, but accumulatedwhen tomatoes were grown. Major effects on cucumber leaves ofadded Si were: increased rigidity of the mature leaves whichhad a rougher texture and were held more horizontally; theywere darker green and senescence was delayed. The mature highSi leaves acquired characteristics of leaves grown in a higherlight intensity, i.e. they had shorter petioles and an increasedfresh weight per unit area, dry weight per unit area, chlorophyllcontent, RuBPcarboxylase activity and soluble protein (all expressedper unit area of interveinal laminar tissue). Addition of Sidid not affect the final leaf area of the mature leaves butroot fresh weight and dry weight were increased. A pronouncedeffect of Si addition was the increased resistance to the powderymildew fungus Sphaerotheca fuliginea. Despite regular applicationsof fungicide, outbreaks of the fungal disease occurred on mostof the mature leaves on the low Si plants, while the high Siplants remained almost completely free of symptoms. The additionof Si could be beneficial to cucumbers grown in areas wherethe local water supply is low in this element, especially whengrown in recirculating solution or in a medium low in Si, e.g.peat. Cucumber, silicon, fungal resistance, powdery mildew, senescence, RuBPcarboxylase     

PEP Carboxylases from Two C4 Species of Panicum with Markedly Different Susceptibilities to Cold Inactivation

Phosphoenolpyruvate carboxylase (PEPC) (EC 4.1.1.31 [EC] ) assayedin extracts of Panicum maximum Jacq. loses up to 50% of itsactivity after incubation for 60 minutes at 0C while the enzymefrom P. miliaceum L. is completely stable under these conditions.Following dilution at room temperature the enzyme from P. maximumis labile, while that from P. miliaceum is stable. The P. maximumenzyme can be largely stabilized against dilution and againstcold-inactivation by D₂O which stabilizes hydrophobic bondsand the compatible solutes proline, betaine and trimethylamine-N-oxide.Mineral ions, previously demonstrated to be protective againstcold inactivation of pyruvate, P_i dikinase from maize, provideno protection of P. maximum PEPC against either cold or dilution.The chaotropic ion SCN^- causes partial inactivation of the enzymefrom P. miliaceum in the cold. The possible interrelationshipbetween inactivation by dilution and inactivation by cold isdiscussed. The enzyme from both species, when assayed withoutpreincubation at low temperature, exhibits similar, slightlycurvilinear Arrhenius plots; and no differences were found betweenthe two species in the temperature dependence of photosynthesis. ¹Present address: Botany Dept., University of California, DavisCA 95616 U.S.A.     

Properties of Phosphoenolpyruvate Carboxylase and Carbohydrate Accumulation in the C3-CAM Intermediate Sedum telephium L. Grown Under Different Light and Watering Regimes

A comparison of the activity and properties of the enzyme phosphoenolpyruvatecarboxylase (PEPC) was made for plants of Sedum telephium L.grown under low (70 µmol m^–2 s^–1) or high(500µmol m^–2 s^–1) PPFD and subjected to varyingdegrees of water stress. Under well-watered conditions onlyplants grown under high PPFD accumulated titratable acidityovernight and the extractable activity of PEPC was almost 2-foldhigher in these plants than in plants grown under low PPFD.Increasing drought stress resulted in a substantial increasein the activity of PEPC extracted both during the light anddark periods and a decrease in the sensitivity to inhibitionby malic acid. The magnitude of these changes was determinedby the severity and duration of drought and by light intensity.A comparison of the kinetic properties of PEPC from severelydroughted plants revealed that plants droughted under high PPFDhad a lower K_m for PEP than plants under low PPFD. Additionof 2·0 mol m^–3 malate resulted in an increase inthe K_m for PEP, with plants draughted under low PPFD havinga significantly higher K_m in the presence of malic acid comparedto those under high _PPFD. Response to the activator glc-6-P,which lowered the K_m for PEP, also varied between plants grownunder the two light regimes. Under well-watered conditions PEPCextracted from plants under high PPFD was more sensitive toactivation by glc-6-P than those under low PPFD. After the severedrought treatment, however, the K_m for PEP in the presence ofglc-6-P was similar for enzyme extracted from plants grown underboth light regimes. Soluble sugars and starch were depletedovernight and were both possible sources of substrate for PEPC.With increasing drought, however, the depletion of starch relativeto soluble sugars increased under both light regimes. The propertiesof PEPC and the characteristics of carbohydrate accumulation/depletionare discussed in relation to the regulation of CAM in S. telephiumgrown under different light and watering regimes. Key words: PEP carboxylase, CAM, carbohydrates, Sedum telephium     

Effect of CO2 Concentration During Growth on a CO2 Concentrating Mechanism in White Clover as Predicted from Differential 14CO2/12CO2Uptake

Lehnherr, B. M?chler, F. and N?sberger, J. 1985. Effect of CO₂concentration during growth on a CO₂ concentrating mechanismin white clover as predicted from differential ¹⁴CO₂/¹²CO² uptake.-J. exp. Bot. 36: 1835-1841. White clover was grown at 20 and100 Pa p(CO₂). The CO₂ response of net photosynthesis and differentialuptake of ¹⁴CO₂ and ¹²CO₂ by leaves were measured at varioustemperatures and at various O₂ and CO₂ partial pressures andcompared with predictions from ribulose bisphosphate carboxylase/oxygenasekinetics. Discrepancies between the observed gas exchange characteristicsfor the leaves and those predicted from the enzyme kineticswere interpreted as being due to a CO₂ concentrating mechanism.Plants grown at 20 Pa p(CO₂) showed a higher affinity for CO₂than plants grown at 100 Pa p(CO₂) when measured at 10 ?C. Nodifference in affinity was found at 30 ?C. The postulated CO₂concentrating effect was greater in plants grown at low CO₂than in plants grown at high CO₂ concentration and occurredonly at low temperature and low CO₂ partial pressure. It issuggested that plants grown at the lower CO₂ partial pressurehave a higher affinity for CO₂ due to a more efficient CO₂ concentratingsystem than plants grown at the higher CO₂ partial pressure. Key words: Photosynthesis, CO₂, concentration, RuBP carboxylase/oxygenase     

Assimilation of carbon dioxide in mesophyll chloroplasts and bundle sheath strands mechanically isolated from corn leaves

Mesophyll chloroplasts capable of assimilating 1.2 µmolesCO₂ per milligram chlorophyll per hour were isolated from 7-day-oldcorn (Zea mays, Nagano No. 1) leaves. Addition of phosphoenolpyruvateincreased the rate of CO₂ fixation in light up to 22 µmolesper milligram chlorophyll per hour, whole exogenously addedribose 5-phosphate and adenosine triphosphate brought aboutonly small increases. The CO₂ fixation products were mostlymalate and aspartate. Bundle sheath strands isolated from the same plants were capableof assimilating 3–26 µmoles CO₂ per milligram chlorophyllper hour. The fixation rate increased 3- to 5-fold on additionof ribose 5-phosphate and adenosine triphosphate, while exogenousphosphoenolpyruvate had no effect. The bulk of early productsof light-induced CO₂ fixation were phosphate esters. These results indicate that corn mesophyll chloroplasts initiallyfix CO₂ by phoenolpyruvate carboxylase and that reductive pentosephosphate cycle occurs in corn bundle sheath cells, but notin the mesophyll chloroplasts. (Received January 25, 1974; )     

Ribulose-1,5-bisphosphate carboxylase from plants adapted to extreme environments

The K_m and V_max of ribulose-1,5-bisphosphate carboxylase (RuBPCase)in selenium absorbing plants (Astragalus flavus Barn., Astragalusrafaelensis Barn. and Stanleya pinnata Bril.) were similar toRuBPCase from tomato (Lycopersicon esculentum L. var. tropic).The pH optima for RuBPCase activity was 8.0 for L. esculentumand A. flavus and 7.0 for A. rafaelensis and S. pinnata. TheActivation Energy (E) values for the enzymes were as follows:A.flavus (21.37), S.pinnata (19.85), A. rafaelensis (19.12)and L. escudentum (18.58). The energy of activation was higherfor the desert plants as compared to the tomato. The Arrheniusplot curves were linear to 50?C far the desert plants as comparedto 45?C for tomato. Enzyme kinetics of RuBPCase from halophytic plants (Salicorniapacifica Stand., var. utahensis (Tidestrom) Munz. and Salicorniarubra Nels.) indicated the enzyme was at least as sensitiveto NaCl concentrations as the enzyme from tomato. (Received November 9, 1976; )     

Affinity of RuBP Carboxylases for Carbon Dioxide and Inhibition of the Enzymes by Oxygen

Ribulose bisphosphate carboxylase (E.C.4.1.1.39) was purifiedfrom leaves of Triticum aestivum, Hordeum vulgare, Spinaceaoleracea, Petroselinum crispum, salad mustard-most likely Brassicanapus, Helianthus annuus, Solanum tuberosum, Beta vulgaris,Lolium perenne, Equisetum arvense, Zea mays, Ginkgo biloba,Pteris aquilina, Salix babylonica, Chamaecyparis lawsonianaand Atrichum undulatum by density gradient centrifugation andgel filtration or by ammonium sulphate fractionation, densitygradient centrifugation, ion-exchange chromatography and gelfiltration. Purified enzymes were freeze-dried and then storedat 0 °C to 4 °C. Portions of each enzyme preparationwere reactivated at 25 °C for 5 h in the presence of 10mM HCO₂ and 20 mM MgCl₂-RuBP carboxylase activities were measuredat four different concentrations of CO₂ at 25 °C and pH8.2 in solutions equilibrated with pure nitrogen or air (21%O₂, 79% N₂). K_m(CO₂), V_max and K₁(O₂) values were computed fromthe results. Significant differences were found in the K_m(CO₂)values for enzymes isolated from different species. Sensitivityof the enzymes to oxygen was less variable.     

The Response of Phaseolus vulgaris L. to Root-zone Anaerobiosis, Waterlogging and High Sodium Chloride

Phaseolus vulgaris L. grown at a range of external concentrationsof NaCl (0 to 80 mM) responded differently to gaseous anaerobiosis(N₂ gas) in nutrient solution or stagnant waterlogging of theroot-zone. With similar patterns of distribution of Na⁺ andCl^- occurring in the plants with comparable NaCl treatments,and similar final concentrations of Na⁺ and Cl^- in plants grownunder both root-zone conditions, rates of uptake of Na⁺ andCl^- were much higher in plants with the stagnant waterloggedrootzones. After 72 h stagnant waterlogging, plant tops fromplants grown at 40 mM NaCl contained 1.42 per cent Na⁺ and 3.44per cent Cl^- (d. wt basis) while after 9 days exposure to NaClwith gaseous anaerobiosis, leaf tissue contained 1.49 per centNa⁺ and 4.28 per cen Cl^- (d. wt basis). Plants exposed to 40mM external NaCl were severely damaged within 72 h when grownwith stagnant waterlogged root-zones; those grown with N₂ anaerobiosiscontinued growth and development over the 9 d period. Plantsgrown in nutrient solution showed changes in distribution andconcentration of Na⁺ and Cl^- when oxygen concentration was reducedbelow 21 per cent O₂ (full aeration). Phaseolus vulgaris. L., bean, mineral salt distribution, anaerobiosis, salinity, waterlogging     

Prolonged Survival of CAM-Mode Mesembryanthemum crystallinum in Darkness and its Possible Dependence on Malate

A remarkable difference was found in the survival of leavesof Mesembryanthemum crystallinum with plants grown in the C₃versus the CAM mode. With excised leaves (petiole in solution)of C₃-mode plants subjected to 6 days of darkness, there wasa large reduction in the chlorophyll content of the leaf andleaf turgor had decreased. By day 9, the chlorophyll had disappeared,except at the major veins, and the leaf tip had dried and turnedbrown. In contrast, the leaf tissue in the CAM mode showed onlya partial loss of chlorophyll during the same period, and evenafter 17 days of darkness, the tissue at the base was stillalive. Similarly, intact plants grown in the C₃ mode deterioratedmuch faster during 20 days of darkness than did plants grownin the CAM mode. Chlorophyll content, chlorophyll a/b ratio,phosphoenolpyruvate carboxylase, NADP-malic enzyme, malate andstarch content were measured. In both C₃- and CAM-mode plants,the starch content decreased rapidly during the dark periodand was nearly depleted after two days. In the CAM-mode tissue,there was a relatively high level of malate during prolongeddarkness (up to 17 days), with a transitory rise early in thedark period. In contrast, the malate content was low and rapidlydepleted in the C₃-mode leaves kept in darkness. These findingssuggest that malate may be an important source of carbon forsustaining leaves of CAM-mode M. crystallinum during prolongeddarkness. (Received May 20, 1987; Accepted October 23, 1987)     

Kinetics of cold inactivation and heat reactivation of the ribulose diphosphate carboxylase activity of crystalline Fraction I proteins isolated from different species and hybrids of Nicotiana

The maximum specific ribulose diphosphate carboxylase activityof crystalline Fraction I proteins isolated from 8 species,and 12 reciprocal, interspecific, F₁ hybrids of Nicotiana changedto minimum specific activity between 16 and 20 hr after theenzymes at 25?C were placed in ice baths. The minimum statewas preserved for more than one year without further changewhen crystals of enzymes were suspended in buffer at 0?C. Atany time during cold storage, the maximum state of specificactivity could be regained by dissolving the crystals in buffercontaining NaCl and heating the solution at temperatures upto 50?C. Maximum activity was attained by heating at 50?C for20 min whereas more than 100 hr was required at 25?C. An equilibrium was demonstrated between maximally active andminimally active enzyme molecules. The equilibrium constantwas a function of the storage temperatures between 0? and 25?C.The molar enthalpy, H, separating maximally active from minimallyactive enzyme molecules was 3.75?0.1 kcal/mole, an energy differenceconsistent with previous findings that the change in specificactivity did not result from dissociation of subunits or depolymerizationof aggregates. The effect of cold storage of Nicotiana enzyme would seem tooffer a molecular basis for explaining the effect of cold temperatureson photosynthetic CO₂ fixation by winter wheat leaves whichwas shown by Sawada and Miyachi (5) to produce an accumulationof ribulose 1–5 diphosphate and diversion of the normalpath of photosynthetic CO₂ fixation away from phosphoglycericacid to the appearance of fixed carbon in serine and glycineby the glycolate path. (Received October 22, 1973; )     

Carbon Dioxide Fixation and Ribulose-1, 5-bisphosphate Carboxylase Activity in Intact Leaves of Sugar Beet Plants Exposed to Salinity and Water Stress

The influence of salinity in the growing media on ribulose-1,5-bisphosphate (RuBP) carboxylase and on CO₂ fixation by intactsugar beet (Beta vulgaris) leaves was investigated. RuBP carboxylase activity was mostly stimulated in young leavesafter exposure of plants for 1 week to 180 mM NaCl in the nutrientsolution. This stimulation was more effective at the higherNaHCO₂ concentrations in the reaction medium. Salinity also enhanced CO₂ fixation in intact leaves mostlyat rate-limiting light intensities. A 60 per cent stimulationin CO₂ fixation rate was obtained by salinity under 450 µEm^–2 s^–1. At quantum flux densities of 150 µEm^–2 s^–1 (400–700 nm) this stimulation was280 per cent. Under high light intensities no stimulation bysalinity was found. In contrast, water stress achieved by directleaf desiccation or by polyethylene glycol inhibited enzymeactivity up to fourfold at –1.2 MPa. Beta vulgaris, sugar beet, ribulose-1, 5-bisphosphate carboxylase, salt stress, water stress, carbon dixoide fixation, salinity     

Variation of Phosphoenolpyruvate Carboxylase Activity in Dunaliella Associated with Changes in Atmospheric CO2 Concentration

In Dunaliella tertiolecta, D. bioculata and D. viridis the activitiesof phosphoenolpyruvate carboxylase and carbonic anhydrase werehigher in the cells grown in ordinary air (low-CO₂ cells) thanin those grown in air enriched with 1–5% CO₂ (high-CO₂cells), whereas in Porphyridium cruentum R-1 there was no differencein phosphoenolpyruvate carboxylase activity between these twotypes of cells. Apparent K_m(NaHCO₃) values for photosynthesisin low-CO₂ cells of all species tested were smaller than thosein high-CO₂ cells. Most of the ¹⁴C was incorporated into 3-phosphoglycerate,sugar mono- and di-phosphates during the initial periods ofphotosynthetic NaH¹⁴CO₃ indicating that both types of cellsin D. tertiolecta are C₃ plants. (Received May 27, 1985; Accepted June 25, 1985)     

Enzymic Activities Associated with the Ability of Aerial and Submerged Forms of Littorella uniflora (L.) Aschers to perform CAM

ABSRACT: Groenhof, A. C, Smirnoff, N. and Bryant, J. A. 1988. Enzymicactivities associated with the ability of aerial and submergedforms of Littorella uniflora (L.) Aschers to perform CAM.—J.exp. Bot. 39: 353-361. The submerged form of Littorella uniflora shows a full CAM modeof photosynthesis as shown by diel acid fluctuations and elevatedactivities (in comparison to non-submerged leaves) of the enzymesphosphoenolpyruvate carboxylase (PEPC) and NADP-malic enzyme.Non-submerged plants exhibit no diel fluctuations of acidityand no changes in activity of NADP-malic enzyme or PEPC. PEPCactivity is low and NADP-malic enzyme is not detectable. Furthercharacterization of PEPC extracted from submerged plants duringthe light and dark periods of a diel cycle shows that the enzymeextracted in the dark is more active. In addition, the enzymeshows a decrease in K_m (PEP) and an increase in V_max in thepresence of glucose-6-phosphate, whilst in the presence of malateK_m (PEP) is increased and V_max decreased; this response to malateis only observed in the light and at pH 7.2. Molecular weightdeterminations using a Sephacryl S-300 column show that theenzyme extracted from plants during the dark period has an apparentmol. wt. of 375 KDa and the enzyme extracted from plants duringthe light period has an apparent mol. wt. of 307 KDa. Key words: Littorella uniflora (shoreweed), Crassulacean acid metabolism, PEP carboxylase, malic enzyme     

Nitrogen-Regulated Accumulation of mRNA and Protein for Photosynthetic Carbon Assimilating Enzymes in Maize

Nitrogen and light effects on kinetic patterns of mRNA and proteinaccumulations for ribulose 1,5-bisphosphate carboxylase (RuBPC),phosphoenolpyruvate carboxylase (PEPC), and pyruvate orthophosphatedikinase (PPDK) were examined by analyzing the transient statesinduced by supplementing nitrogen or light intensity to Zeamays plants grown for 3 weeks under low nitrate (0.8 nM KNO₃)and/or low light (12% full sunlight) conditions. When plantsgrown previously under low nitrate were supplemented with highnitrate (16 mM KNO₃) for 3 days under high sunlight, mRNAs forthese enzymes accumulated progressively as did their proteins.The accumulation of mRNA for these enzymes was stimulated inplants previously grown under high nitrate by increasing lightintensity from low light to high light (48% full sunlight).This stimulation, however, did not occur in plants previouslygrown under low nitrate. These results imply that nitrogen isa regulatory factor essential for the light stimulation of mRNAaccumulation of these enzymes. The kinetic patterns of proteinand mRNA accumulation also indicate that the accumulation ofproteins of PEPC and PPDK is a reflect of the level of respectivemRNA, irrespective of the conditions used for inducing transientchanges in gene expression. For RuBPC, protein and mRNA accumulationsdid not necessarily coincide. ¹Present address: Laboratories of Biology and Pharmacology,Research Laboratories, Nippon Kayaku Co., Ltd., Shimo 3-31-12,Kita-ku, Tokyo 115, Japan. (Received September 26, 1985; Accepted January 22, 1986)