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1.
J. Kvto 《Biologia Plantarum》2001,44(3):447-450
Three clones of tobacco transformed with the T-DNA of Agrobacterium rhizogenes Ri plasmid A4b cultivated in vitro on a solid agar medium neither showed pronounced morphological diversity nor significantly differed in chlorophyll (Chl) contents from control plants. However, the transformation caused a 27 to 83 % decay in leaf oxygen evolution and in both ribulose-1,5-bisphosphate carboxylase (RuBPC) and phosphoenolpyruvate carboxylase (PEPC) activities. Therefore, the transformation brought about a reduction of active PEPC as well as activable RuBPC amounts in plant tissues. Individual clones did not mutually differ. In tobacco transformed with A. rhizogenes strain TR101 and grown in soil only the mean leaf area tended to reduce. Chl contents, Chl a/b ratio, oxygen evolution, and activities of both RuBPC and PEPC were insignificantly affected by the transformation. 相似文献
2.
Inhibition and Acclimation of Photosynthesis to
Heat Stress
Is Closely Correlated with Activation of
Ribulose-1,5-Bisphosphate
Carboxylase/Oxygenase 总被引:24,自引:2,他引:24 下载免费PDF全文
Increasing the leaf temperature of intact cotton (Gossypium hirsutum L.) and wheat (Triticum aestivum L.) plants caused a progressive decline in the light-saturated CO2-exchange rate (CER). CER was more sensitive to increased leaf temperature in wheat than in cotton, and both species demonstrated photosynthetic acclimation when leaf temperature was increased gradually. Inhibition of CER was not a consequence of stomatal closure, as indicated by a positive relationship between leaf temperature and transpiration. The activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which is regulated by Rubisco activase, was closely correlated with temperature-induced changes in CER. Nonphotochemical chlorophyll fluorescence quenching increased with leaf temperature in a manner consistent with inhibited CER and Rubisco activation. Both nonphotochemical fluorescence quenching and Rubisco activation were more sensitive to heat stress than the maximum quantum yield of photochemistry of photosystem II. Heat stress led to decreased 3-phosphoglyceric acid content and increased ribulose-1,5-bisphosphate content, which is indicative of inhibited metabolite flow through Rubisco. We conclude that heat stress inhibited CER primarily by decreasing the activation state of Rubisco via inhibition of Rubisco activase. Although Rubisco activation was more closely correlated with CER than the maximum quantum yield of photochemistry of photosystem II, both processes could be acclimated to heat stress by gradually increasing the leaf temperature. 相似文献
3.
RubisCO的研究进展 总被引:15,自引:0,他引:15
1,5-二磷酸核酮糖羧化酶/加氧酶(RubisCO)是调节光合和光呼吸,决定净光合作用的一个关键酶;也是植物可溶性蛋白质中含量最高的蛋白质.该酶广泛存在于植物及一些微生物体内.综述了近年来有关RubisCO的一些研究进展. 包括RubisCO的基本性质、结构与功能、酶基因工程、酶活性调节及其活化酶等. 相似文献
4.
The carboxylating activity and content of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO, EC 4.1.1.39), and other soluble proteins in young seedlings and mature leaves of Lutescens-758, a drought-sensitive cultivar of soft spring wheat Triticum aestivum L., were studied under the conditions of drought and subsequent rehydration. Seedlings and mature plants preliminarily treated with the cytokinin-like compound kartolin-4 were compared to untreated plants. Drought-induced decrease in RuBPCO activity should be attributed not only to proteolytic decomposition of the enzyme protein itself but also to a partial inhibition of its catalytic activity. The decrease in RuBPCO activity was larger than that in RuBPCO content. Water stress induced a marked decrease in the soluble protein content. Kartolin-4 increased the resistance to drought. 相似文献
5.
Effects of O2 and CO2 on Nonsteady-State Photosynthesis (Further Evidence for Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Limitation) 下载免费PDF全文
The effects of CO2 and O2 on nonsteady-state photosynthesis following an increase in photosynthetic photon flux density (PPFD) were examined in Spinacia oleracea to investigate the hypotheses that (a) a slow exponential phase (the ribulose-1,5-bisphosphate carboxylase/oxygenase [Rubisco] phase) of nonsteady-state photosynthesis is primarily limited by Rubisco activity and (b) Rubisco activation involves two sequential, light-dependent processes as described in a previous study (I.E. Woodrow, K.A. Mott [1992] Plant Physiol 99: 298-303). Photosynthesis was found to be sensitive to O2 during the Rubisco phase in the approach of photosynthesis to steady state. Analyses of this sensitivity to O2 showed that the control coefficient for Rubisco was approximately equal to 1 during this phase, suggesting that Rubisco was the primary limitation to photosynthesis. O2 had almost no effect on the kinetics (described using a relaxation time, [tau] of the Rubisco phase for leaves starting in darkness or for leaves starting in low PPFD, but [tau] was substantially higher in the former case. CO2 was found to affect both the rate of photosynthesis and the magnitude of [tau] for the Rubisco phase. The [tau] value for the Rubisco phase was found to be negatively correlated with intercellular CO2 concentration (ci), and leaves starting in darkness had higher values of [tau] at any ci than leaves starting in low PPFD. The effects of CO2 and O2 on the Rubisco phase are consistent with the existence of two sequential, light-dependent processes in the activation of Rubisco if neither process is sensitive to O2 and only the second process is sensitive to CO2. The implications of the data for the mechanism of Rubisco activation and for the effects of stomatal conductance on nonsteady-state photosynthesis are discussed. 相似文献
6.
以杂交稻(汕优63)为试验材料,在木村B营养液中培养至三叶期,用草酸5mmol/L预处理水稻2d,再处以氧化胁迫(用0.1mmol/L浓度的活性氧诱发剂甲基紫精处理)。结果表明MV诱发的氧化胁迫下,Rubisco及其它可溶性蛋白快速降解。草酸预处理可明显缓解Rubisco及其它可溶性蛋白的降解,降解速率分别降低1/3和1/2左右。植株经草酸处理后其叶片中几种抗氧化酶如AsA-POD、SOD、CAT活性大大提高,这可能是草酸预处理可缓解氧化胁迫下Rubisco和其它可溶性蛋白降解的重要原因。既然草酸能有效地诱导植物的抗氧化防卫反应,它可能作为一种诱抗剂来提高植物的抗逆性。 相似文献
7.
Regulation of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Activity in Response to Reduced Light Intensity in C4 Plants 下载免费PDF全文
The light-dependent regulation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity was studied in 16 species of C4 plants representing all three biochemical subtypes and a variety of taxonomic groups. Rubisco regulation was assessed by measuring (a) the ratio of initial to total Rubisco activity, which reflects primarily the carbamylation state of the enzyme, and (b) total Rubisco activity per mol of Rubisco catalytic sites, which declines when 2-carboxyarabinitol 1-phosphate (CA1P) binds to carbamylated Rubisco. In all species examined, the activity ratio of Rubisco declined with a reduction in light intensity, although substantial variation was apparent between species in the degree of Rubisco deactivation. No relationship existed between the degree of Rubisco deactivation and C4 subtype. Dicots generally deactivated Rubisco to a greater degree than monocots. The total activity of Rubisco per catalytic site was generally independent of light intensity, indicating that CA1P and other inhibitors are not major contributors to the light-dependent regulation of Rubisco activity in C4 plants. The light response of the activity ratio of Rubisco was measured in detail in Amaranthus retroflexus, Brachiaria texana, and Zea mays. In A. retroflexus and B. texana, the activity ratio declined dramatically below a light intensity of 400 to 500 [mu]mol of photons m-2 s-1. In Z. mays, the activity ratio of Rubisco was relatively insensitive to light intensity compared with the other species. In A. retroflexus, the pool size of ribulose bisphosphate (RuBP) declined with reduced light intensity except between 50 and 500 [mu]mol m-2 s-1, when the activity ratio of Rubisco was light dependent. In Z. mays, by contrast, the pool size of RuBP was light dependent only below 350 [mu]mol m-2 s-1. These results indicate that, in response to changes in light intensity, most C4 species regulate Rubisco by reversible carbamylation of catalytic sites, as commonly observed in C3 plants. In a few species, notably Z. mays, Rubisco is not extensively regulated in response to changes in light intensity, possibly because the activity of the CO2 pump may become limiting for photosynthesis at subsaturating light intensity. 相似文献
8.
9.
Phylogenetic Diversity of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Large-Subunit Genes from Deep-Sea Microorganisms 总被引:5,自引:4,他引:5 下载免费PDF全文
The phylogenetic diversity of the ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO, E.C. 4.1.1.39) large-subunit genes of deep-sea microorganisms was analyzed. Bulk genomic DNA was isolated from seven samples, including samples from the Mid-Atlantic Ridge and various deep-sea habitats around Japan. The kinds of samples were hydrothermal vent water and chimney fragment; reducing sediments from a bathyal seep, a hadal seep, and a presumed seep; and symbiont-bearing tissues of the vent mussel, Bathymodiolus sp., and the seep vestimentiferan tubeworm, Lamellibrachia sp. The RuBisCO genes that encode both form I and form II large subunits (cbbL and cbbM) were amplified by PCR from the seven deep-sea sample DNA populations, cloned, and sequenced. From each sample, 50 cbbL clones and 50 cbbM clones, if amplified, were recovered and sequenced to group them into operational taxonomic units (OTUs). A total of 29 OTUs were recorded from the 300 total cbbL clones, and a total of 24 OTUs were recorded from the 250 total cbbM clones. All the current OTUs have the characteristic RuBisCO amino acid motif sequences that exist in other RuBisCOs. The recorded OTUs were related to different RuBisCO groups of proteobacteria, cyanobacteria, and eukarya. The diversity of the RuBisCO genes may be correlated with certain characteristics of the microbial habitats. The RuBisCO sequences from the symbiont-bearing tissues showed a phylogenetic relationship with those from the ambient bacteria. Also, the RuBisCO sequences of known species of thiobacilli and those from widely distributed marine habitats were closely related to each other. This suggests that the Thiobacillus-related RuBisCO may be distributed globally and contribute to the primary production in the deep sea. 相似文献
10.
Low Activation State of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase in Carboxysome-Defective Synechococcus Mutants 总被引:1,自引:0,他引:1 下载免费PDF全文
The high-CO2-requiring mutant of Synechococcus sp. PCC 7942, EK6, was obtained after extension of the C terminus of the small subunit of ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco). The carboxysomes in EK6 were much larger than in the wild type, but the cellular distribution of the large and small sub-units of Rubisco was not affected. The kinetic parameters of in vitro-activated Rubisco were similar in EK6 and in the wild type. On the other hand, Rubisco appeared to be in a low state of activation in situ in EK6 cells pretreated with an air level of CO2. This was deduced from the appearance of a lag phase when carboxylation was followed with time in cells permeabilized by detergent and subsequently supplied with saturating CO2 and RuBP. Pretreatment of the cells with high CO2 virtually abolished the lag. After low-CO2 treatment, the internal RuBP pool was much higher in mutant cells than in the wild-type cells; pretreatment with high CO2 reduced the pool in mutant cells. We suggest that the high-CO2-requiring phenotype in mutants that possess aberrant carboxysomes arises from the inactivated state of Rubisco when the cells are exposed to low CO2. 相似文献
11.
Diversity of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Large-Subunit Genes from Groundwater and Aquifer Microorganisms 总被引:2,自引:0,他引:2
To test our hypothesis that microbial autotrophic CO2 fixation plays an important role in subsurface systems of two large groundwater remediation projects, several anaerobic/microaerobic aquifer and groundwater samples were taken and used to investigate the distribution and phylogenetic diversity of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) large-subunit genes. Two primer sets were designed for amplifying partial-subunit genes of RubisCO forms I and II from the DNA, directly extracted from the samples. PCR products were used to construct five clone libraries with putative RubisCO form I sequences, and two libraries of DNA amplified by form II primers. Selected clones were screened for variation by restriction fragment length polymorphism analysis, and a total of 28 clone inserts were sequenced and further analyzed. The phylogenies constructed from amino acid sequences derived from the partial RubisCO large-subunit sequences showed a distinct pattern. Diverse sequences affiliated to the cluster of green-like type IA RubisCO sequences were found, representing various obligate and facultative chemolithoautotrophic Proteobacteria, whereas type II RubisCO sequences detected were most closely related to those of thiobacilli species. An isolate obtained from aquifer enrichment culture, which has been provisionally named Halothiobacillus sp. RA13 on the basis of its 16S rDNA sequence, was found to contain both types of RubisCO genes, i.e., forms I and II. Physiological and ecological considerations are discussed in the context of additional microbial data and physicochemical properties. 相似文献
12.
Characterization of Thylakoid-Derived Lipid-Protein Particles Bearing the Large Subunit of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase 下载免费PDF全文
Lipid-protein particles bearing the 55-kD ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) (EC 4.1.1.39) large subunit (RLSU) and no detectable corresponding Rubisco small subunit (RSSU) were isolated from the stroma of intact chloroplasts by flotation centrifugation. Stromal RLSU-bearing particles appear to originate from thylakoids because they can also be generated in vitro by illumination of isolated thylakoids. Their formation in vitro is largely heat denaturable and is facilitated by light or ATP. RLSU-containing lipid-protein particles range from 0.05 to 0.10 [mu]m in radius, contain the same fatty acids as thylakoids, but have a 10- to 15-fold higher free-to-esterified fatty acid ratio than thylakoids. RLSU-bearing lipid-protein particles with no detectable RSSU were also immunopurified from the populations of both stromal lipid-protein particles and those generated in vitro from illuminated thylakoids. Protease shaving indicated that the RLSU is embedded in the lipid-protein particles and that there is also a protease-protected RLSU in thylakoids. These observations collectively indicate that the RLSU associated with thylakoids is released into the stroma by light-facilitated blebbing of lipid-protein particles. The release of RLSU-containing particles may in turn be coordinated with the assembly of Rubisco holoenzyme because chaperonin 60 is also associated with lipid-protein particles isolated from stroma. 相似文献
13.
Photosynthetic parameters were measured in triticale and its parents wheat and rye. Soluble protein content in leaves, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content per fresh mass, total chlorophyll content, biomass yield, leaf area, leaf mass and specific leaf mass were higher but Rubisco content expressed as percentage of soluble protein, carboxylase activity, photosynthetic rate and stomatal conductance were significantly lower in rye than in wheat. Native-PAGE of Rubisco revealed that rye carboxylase was different from that of wheat. The difference was not related to either the small or large subunit of Rubisco but, may be, to the ionic and/or other properties of the Rubisco protein moiety. Triticale Rubisco was similar to wheat. For most of the studied physiological parameters, triticale showed much more similarity with wheat than with rye. 相似文献
14.
Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Specific Proteolysis in Barley Chloroplasts During Dark Induced Senescence 总被引:2,自引:0,他引:2
Intact chloroplasts were isolated from dark-senescing primary barley (Hordeum vulgare L.) leaves in order to study selective ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) degradation by the stromal and membrane fractions. RuBPCO specific degradation was estimated and characterised applying sensitive avidin-biotin ELISA method with non-modified or oxidatively modified biotinylated RuBPCO (BR) as substrates. Distinct proteolytic activities were detected. They differed in ATP and divalent metal ion dependence, protease inhibitory profile, and dynamics in the time-course of dark-induced senescence. The results supported involvement of ATP- and metal ion-dependent serine type proteolytic activity against non-modified BR early in induced senescence and appearance of ATP-independent activity at later stage. Active oxygen-modified BR was degraded by ATP-independent serine-type protease probably containing essential SH-groups and requiring divalent metal ions. 相似文献
15.
The Two Forms of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Activase Differ in Sensitivity to Elevated Temperature 总被引:10,自引:2,他引:10 下载免费PDF全文
Ribulose-1,5-bisphosphate carboxylase/oxygenase activase often consists of two polypeptides that arise from alternative splicing of pre-mRNA. In this study recombinant versions of the spinach (Spinacea oleracea L.) 45- and 41-kD forms of activase were analyzed for their response to temperature. The temperature optimum for ATP hydrolysis by the 45-kD form was 45[deg]C, approximately 13[deg]C higher than the 41-kD form. When the two forms were mixed, the temperature response of the hybrid enzyme was similar to the 45-kD form. In the absence of adenine nucleotide, preincubation of either activase form at temperatures above 25[deg}C inactivated ATPase activity. Adenosine 5[prime]-([gamma]-thio)triphosphate, but not ADP, significantly enhanced the thermostability of the 45-kD form but was much less effective for the 41-kD form. Intrinsic fluorescence showed that the adenosine 5[prime]-([gamma]-thio)triphosphate-induced subunit aggregation was lost at a much lower temperature for the 41-kD than for the 45-kD form. However, the two activase forms were equally susceptible to limited proteolysis after heat treatment. The results indicate that (a) the 45-kD form is more thermostable than, and confers increased thermal stability to, the 41-kD form, and (b) a loss of subunit interactions, rather than enzyme denaturation, appears to be the initial cause of temperature inactivation of activase. 相似文献
16.
Liang C Xiao W Hao H Xiaoqing L Chao L Lei Z Fashui H 《Biological trace element research》2008,121(3):249-257
Mg2+ in various concentrations was added to purified Rubisco in vitro to gain insight into the mechanism of molecular interactions
between Mg2+ and Rubisco. The enzyme activity assays showed that the reaction between Rubisco and Mg2+ was two order, which means that the enhancement of Rubisco activity was accelerated by low concentration of Mg2+ and slowed by high concentration of Mg2+. The kinetics constant (K
m) and V
max was 1.91 μM and 1.13 μmol CO2 mg−1 protein∙min−1, respectively, at a low concentration of Mg2+, and 3.45 μM and 0.32 μmol CO2∙mg−1 protein∙min−1, respectively, at a high concentration of Mg2+. By UV absorption and fluorescence spectroscopy assays, the Mg2+ was determined to be directly bound to Rubisco; the binding site of Mg2+ to Rubisco was 0.275, the binding constants (K
A) of the binding site were 6.33 × 104 and 5.5 × 104 l·mol−1. Based on the analysis of the circular dichroism (CD) spectra, it was concluded that the binding of Mg2+ did not alter the secondary structure of Rubisco, suggesting that the observed enhancement of Rubisco carboxylase activity
was caused by a subtle structural change in the active site through the formation of the complex with Mg2+. 相似文献
17.
Exchange Properties of the Activator CO(2) of Spinach Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase 下载免费PDF全文
The exchange properties of the activator CO2 of spinach ribulose-1,5-bisphosphate carboxylase/oxygenase were characterized both in vitro with the purified enzyme, and in situ within isolated chloroplasts. Carboxyarabinitol-1,5-bisphosphate, a proposed reaction intermediate analog for the carboxylase activity of the enzyme, was used to trap the activator CO2 on the enzyme both in vitro and in situ. Modulation of ribulose-1,5-bisphosphate carboxylase/oxygenase activity in intact chloroplasts during a light/dark cycle was associated with a similar modulation in carboxyarabinitol-1,5-bisphosphate-trapped CO2. The exchange kinetics of the activator CO2 were monitored by activation of the enzyme to steady state in the presence of 12CO2, followed by addition of 14CO2 and determination of the amount of labeled CO2 trapped on the enzyme by carboxyarabinitol-1,5-bisphosphate. Rate constants (Kobs) for exchange with both the purified enzyme (0.45 min−1) and in illuminated chloroplasts (0.18 min−1) were comparable to the observed rate constants for enzyme activation under the two conditions. A similar exchange of the activator CO2 was not observed in chloroplasts in the dark. Kinetic analysis of the exchange properties of the purified enzyme were consistent with an equilibrium between active and inactive forms of the enzyme during steady state activation. 相似文献
18.
Photosynthetic Acclimation to Elevated CO2 Occurs in Transformed Tobacco with Decreased Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Content 总被引:2,自引:1,他引:2 下载免费PDF全文
Inhibition of net carbon assimilation rates during growth at elevated CO2 was studied in transgenic tobacco (Nicotiana tabacum L.) plants containing zero to two copies of antisense DNA sequences to the small subunit polypeptide (rbcS) gene of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). High- and low-Rubisco tobacco plants were obtained from the selfed progeny of the original line 3 transformant (S.R. Rodermel, M.S. Abbott, L. Bogorad [1988] Cell 55: 673-681). Assimilation rates of high- and low-Rubisco tobacco plants increased 22 and 71%, respectively, when transferred from 35- to 70-Pa CO2 chamber air at 900 [mu]mol m-2 s-1 photon flux density. However, CO2-dependent increases of net carbon assimilation rates of high- and low-Rubisco plants virtually disappeared after 9 d of growth in elevated CO2 chamber air. Total above-ground dry matter production of high- and low-Rubisco plants was 28 and 53% greater, respectively, after 9 d of growth at 70 Pa compared with 35 Pa CO2. Most of this dry weight gain was due to increased specific leaf weight. Rubisco activity, Rubisco protein, and total chlorophyll were lower in both high- and low-Rubisco plants grown in enriched compared with ambient CO2 chamber air. Soluble leaf protein also decreased in response to CO2 enrichment in high- but not in low-Rubisco tobacco plants. Decreased Rubisco activities in CO2-adapted high- and low-Rubisco plants were not attributable to changes in activation state of the enzyme. Carbonic anhydrase activities and subunit levels measured with specific antibodies were similar in high- and low-Rubisco tobacco plants and were unchanged by CO2 enrichment. Collectively, these findings suggested that photosynthetic acclimation to enriched CO2 occurred in tobacco plants either with or without transgenically decreased Rubisco levels and also indicated that the down-regulation of Rubisco in CO2-adapted tobacco plants was related to decreased specific activity of this enzyme. 相似文献
19.
Oxidative Stress Induces Partial Degradation of the Large Subunit of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase in Isolated Chloroplasts of Barley 总被引:18,自引:2,他引:16 下载免费PDF全文
The effects of oxidative stress on the degradation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) were studied in isolated chloroplasts from barley (Hordeum vulgare L. cv Angora). Active oxygen (AO) was generated by varying the light intensity, the oxygen concentration, or the addition of herbicides or ADP-FeCl3-ascorbate to the medium. Oxidative treatments stimulated association of Rubisco with the insoluble fraction of chloroplasts and partial proteolysis of the large subunit (LSU). The most prominent degradation product of the LSU of Rubisco showed an apparent molecular mass of 36 kD. The data suggest that an increase in the amount of AO photogenerated by O2 reduction at photosystem I triggers Rubisco degradation. A possible relationship between AO-mediated denaturation of Rubisco and proteolysis of the LSU is discussed. 相似文献
20.
The Association of d-Ribulose- 1,5-Bisphosphate Carboxylase/Oxygenase with Phosphoribulokinase 总被引:1,自引:0,他引:1 下载免费PDF全文
When Ribulose- 1,5-bisphosphate carboxylase/oxygenase was purified from spinach leaves (Spinacia oleracea) using precipitation with polyethylene glycol and MgCl2 followed by DEAE cellulose chromatography, 75% of phosphoribulokinase and 7% of phosphoriboisomerase activities copurified with ribulose- 1,5-bisphosphate carboxylase/oxygenase. This enzyme preparation showed ribose-5-phosphate and ribulose-5-phosphate dependent carboxylase and oxygenase activities which were nearly equivalent to its corresponding ribulose- 1,5-bisphosphate dependent activity. The ribose-5-phosphate and ribulose-5-phosphate dependent reaction rates were stable and linear for much longer time periods than the ribulose- 1,5-bisphosphate dependent rates. When sucrose gradients were used to purify ribulose- 1,5-bisphosphate carboxylase/oxygenase from crude stromal extracts, phosphoribulokinase was found to cosediment with ribulose- 1,5-bisphosphate carboxylase. Under these conditions most of the phosphoriboisomerase activity remained with the slower sedimenting proteins. Ammonium sulfate precipitation resulted in separation of the ribulose- 1,5-bisphosphate carboxylase peak from phosphoribulokinase peak. Crude extracts of peas Pisum sativum and spinach contained 0.725 to 0.730 milligram of phosphoribulokinase per milligram of chlorophyll, respectively, based on an enzyme-linked immunosorbent assay. 相似文献