Two Class I Aldolases in the Green Alga Chara foetida (Charophyceae)

Aldolase activity of Chara foetida (Braun) could be separated into a minor (peak I) and a major peak (peak II) by ion-exchange chromatography on DEAE-cellulose. Affinity chromatography on P-cellulose resulted in highly purified aldolase preparations with specific activities of 3.2 and 4.8 units per milligram protein and molecular subunit masses of 37 and 35 kilodalton, as shown by SDS-PAGE, for the aldolase of peak I and peak II, respectively. Both aldolases belong to class I aldolase since the activity is not inhibited by 1 millimolar EDTA. The K_m (fructose-1,6-bisphosphate) values were 0.64 and 13.4 micromolar, respectively. The aldolase of peak I showed a 6.7 times stronger crossreaction with a specific antiserum against the cytosol aldolase of spinach than with an antiserum against the chloroplast aldolase of spinach. On the other hand the aldolase of peak II showed a 5.1 times stronger cross-reaction with the α-plastidaldolase antiserum than with the α-cytosol-aldolase antiserum. For algae this is the first separation of two class I aldolases. They are similar to the cytosol and chloroplast aldolases in higher plants, but different from a reported class I (Me²⁺ independent) and class II (Me²⁺ dependent) aldolase in other algae.     

Glutamine and Glutamate Transport in Cyanophora paradoxa

The present investigation showed that isolated cyanelles from Cyanophora paradoxa selectively enriched glutamine from the external medium, whereas glutamate poorly penetrated into these organelles. Glutamine uptake proceeded in two phases, presumably involving a low and a high affinity system. The uptake of glutamine was significantly enhanced by 2-oxoglutarate and light. Inhibitor experiments indicated that glutamine and 2-oxoglutarate were converted to glutamate by a ferredoxin-dependent glutamate synthase (GOGAT) reaction inside the cyanelles, and the glutamate formed at best slowly left these organelles. Such results were obtained independently of each other by measuring either the ¹⁴C-glutamine uptake or the 2-oxoglutarate and glutamine-dependent O₂ evolution. Glutamine is suggested to be the N-compound which is supplied to the eukaryotic host. Glutamine could be exported jointly with 2-oxoglutarate, possibly employing a common carrier. Cyanelles have apparently evolved glutamine (and oxoglutarate) carrier(s) with properties not yet described for any other organism.     

Identification and Characterization of Glycolate Oxidase and Related Enzymes from the Endocyanotic Alga Cyanophora paradoxa and from Pea Leaves

Glycolate oxidase (GO) has been identified in the endocyanom Cyanophora paradoxa which has peroxisome-like organelles and cyanelles instead of chloroplasts. The enzyme used or formed equimolar amounts of O₂ or H₂O₂ and glyoxylate, respectively. Aerobically, the enzyme did not reduce the artificial electron acceptor dichlorophenol indophenol. However, after an inhibitor of glycolate dehydrogenase, KCN (2 millimolar), was added to the assay medium, considerable aerobic glycolate:dichlorophenol indophenol reductase activity was detectable. The leaf GO inhibitor 2-hydroxybutynoate (30 micromolar), which binds irreversibly to the flavin moiety of the active site of leaf GO, inhibited Cyanophora GO and pea (Pisum sativum L.) GO to the same extent. This suggests that the active sites of both enzymes are similar. Cyanophora GO and pea GO cannot oxidize d-lactate. In contrast to GO from pea or other organisms, the affinity of Cyanophora GO for l-lactate is very low (K_m 25 millimolar). Another important difference is that Cyanophora GO produced sigmoidal kinetics with O₂ as varied substrate, whereas pea GO produced normal Michaelis-Menten kinetics. It is concluded that there is considerable inhomogeneity among the glycolate-oxidizing enzymes from Cyanophora, pea, and other organisms. The specific catalase activity in Cyanophora was only one-tenth of that in leaves. NADH-and NADPH-dependent hydroxypyruvate reductase (HPR) and glyoxylate reductase activities were detected in Cyanophora. NADH-HPR was markedly inhibited by hydroxypyruvate above 0.5 millimolar. Variable substrate inhibition was observed with glyoxylate in homogenates from different algal cultures. It is proposed that Cyanophora has multiple forms of HPR and glyoxylate reductase, but no enzyme clearly resembling leaf peroxisomal HPR was identified in these homogenates. Moreover, no serine:glyoxylate aminotransferase activity was detected. These results collectively indicate the possibility that the glycolate metabolism in Cyanophora deviates from that in leaves.     

Metabolic activities in Cyanophora paradoxa and its cyanelles

Isolated cyanelles of Cyanophora paradoxa perform photosystem I and II dependent Hill reactions. The photosynthetic electron transport of the cyanelles does not show special features uncommon in cyanobacteria or chloroplasts of red algae. A preparation of cyanelles performs photosynthetic O₂-evolution with approximately 1/3 of the rate of intact Cyanophora, in only, however, the first three minutes of the experiment. All attempts to stabilize the CO₂-fixation activity of isolated cyanelles failed. Isolated cyanelles do not perform KCN-sensitive O₂-uptake, indicating that respiratory cytochrome oxidase is lacking in cyanelles. O₂-consumption by crude extracts from Cyanophora is inhibited by KCN when N-tetramethyl-p-phenylenediamine/ascorbate or NADH but not NADPH are supplied as the electron donors in contrast to the situation in cyanobacteria. These findings suggest that cyanelles do not respire. It is concluded that cyanelles are not so much related to cyanobacteria as formerly believed, but share many properties with chloroplasts of eukaryotic cells.Abbreviations Chl chlorophyll - DCPIP dichlorophenol-indophenol - TMPD N-tetramethyl-p-phenylenediamine To whom correspondence should be addressed     

Pathway of cytosolic starch synthesis in the model glaucophyte Cyanophora paradoxa

The nature of the cytoplasmic pathway of starch biosynthesis was investigated in the model glaucophyte Cyanophora paradoxa. The storage polysaccharide granules are shown to be composed of both amylose and amylopectin fractions, with a chain length distribution and crystalline organization similar to those of green algae and land plant starch. A preliminary characterization of the starch pathway demonstrates that Cyanophora paradoxa contains several UDP-glucose-utilizing soluble starch synthase activities related to those of the Rhodophyceae. In addition, Cyanophora paradoxa synthesizes amylose with a granule-bound starch synthase displaying a preference for UDP-glucose. A debranching enzyme of isoamylase specificity and multiple starch phosphorylases also are evidenced in the model glaucophyte. The picture emerging from our biochemical and molecular characterizations consists of the presence of a UDP-glucose-based pathway similar to that recently proposed for the red algae, the cryptophytes, and the alveolates. The correlative presence of isoamylase and starch among photosynthetic eukaryotes is discussed.     

Exchange of Metabolites in Cyanophora paradoxa and its Cyanelles*

The flagellate Cyanophora paradoxa contains blue-greenish, organelle-like inclusions termed cyanelles which perform photosynthetic CO₂-fixation in place of chloroplasts. By the use of the HPLC-technique, Cyanophora was shown to form glucose, sucrose, maltose, mannitol, ribose, glycerol and trehalose. Extracts from the whole organism and from the eucaryotic host, but not from the cyanelles, convert ¹⁴C-labelled UDP-glucose to polyglucan. Synthesis of sucrose from UDP-glucose and fructose-6-P or fructose could not be demonstrated in any extract from Cyanophora. The transfer of metabolites into cyanelles was monitored by the silicone oil filtering technique. The solute spaces for ¹⁴C-labelled sorbitol and ³H₂O were the same indicating that sorbitol freely penetrated the plasma membrane of cyanelles in contrast to the situation found in chloroplasts. The measurements of the solute spaces for the different compounds showed that maltose and sucrose were not accumulated by isolated cyanelles. Other compounds like fructose, fucose, glutamine or glycine had intermediate sizes of their solute spaces. Cyanelles apparently possess a rapidly transporting glucose carrier and not a malate/oxaloacetate shuttle and also not an ATP/ADP translocator. The carrier composition at the plasma membrane of cyanelles and at the inner envelope membrane of chloroplasts seems to be totally different.     

Properties of the Photosynthetic System and DNA of Cyanophora paradoxa Cyanelles

The cyanelle from the photosynthetic biflagellate protist Cyanophora paradoxa has been studied in terms of its photosynthetic properties. Structurally, the cyanelle resembles unicellular cyanobacteria. The cyanelle is readily released from the host cell by means of the French press. The isolated cyanelle shows typical photosystem I and photosystem II activities as well as phenazine methosulfate-mediated photophosphorylation. The kinetic parameters K_m and V_max were determined for CO₂ fixation in the cyanelle and cells of C. paradoxa and compared to a cyanobacterium. The determined values were not much different, although the cyanobacterium had a significantly greater rate of CO₂ fixation, and the cyanelle was least active in this regard. Photosystem I chlorophyll-protein complex is readily isolated from the thylakoid membrane. In all these respects, the photosynthetic apparatus of the cyanelle resembles that of cyanobacteria. No nitrogen fixation activity was observed. Attempts to regenerate the isolated cyanelle were not successful, but in some cases, an unidentified cyanobacterium grew up in standing cultures of C. paradoxa cyanelles. Buoyant density data indicate that the strain of C. paradoxa we have investigated differs from that employed by others, since our strain shows a value of 1.716 grams per cubic centimeter and others report values of 1.695 and 1.691.     

UV effects on photosynthesis and phycobiliprotein composition in the flagellate Cyanophora paradoxa

Abstract The effects of solar and artificial ultraviolet radiation on photosynthetic oxygen production and phycobiliprotein composition were investigated in the freshwater flagellate, Cyanophora paradoxa . The phycobiliproteins of the cell acting as photosynthetic accessory pigments were found to be readily affected by even short exposure to ultraviolet radiation, while the membrane-bound chlorophyll protein complexes were hardly impaired as demonstrated by SDS polyacrylamide gel electrophoresis, isoelectric focussing and fast protein liquid chromatography (FPLC). The phycobilisomes are dissembled from the outside inwards and go from higher molecular weight components to hexamers ( αβ )₆, trimers ( αβ )₃ and finally monomers (αβ). Fluorescence spectra indicated that the energy transfer from accessory pigments to the photosystems was impaired by ultraviolet radiation. Photosynthetic oxygen production was affected on a much faster timescale than changes in the absorption spectra or at the protein level.     

On the developmental dependence between Cyanophora paradoxa and Cyanocyta korschikoffiana in symbiosis

The prokaryote Cyanocyta korschikoffiana was isolated from the eukaryote Cyanophora paradoxa. The synthesis of several thylakoid proteins in these cyanelles is influenced by light and darkness and is sensitive to cycloheximide, the inhibitor of the eukaryotic host's translation. The possibility of a direct coordination between the translations of the host and of the cyanelles is discussed.Abbreviations CHM treatment addition of cycloheximide - CPN chlorophylline - PBN phycobiline - SDS-PAGE sodium-dodecylsulphate-polyacrylamide gelelectrophoresis     

Novel supercomplex organization of photosystem I in Anabaena and Cyanophora paradoxa

The supercomplex organization of photosystem complexes was studied in various cyanobacteria, a glaucocystophyte and a primitive rhodophyte by blue-native PAGE with a wide range of detergent concentrations. In contrast to known cyanobacteria that produced the PSI trimer, a filamentous N(2)-fixing cyanobacterium Anabaena sp. PCC 7120 and a glaucocystophyte Cyanophora paradoxa NIES 547 had a PSI tetramer and dimer but no trimer at all. This was confirmed by sucrose density gradient centrifugation. A primitive rhodophyte Cyanidioschyzon merolae had two species of PSI monomeric complex with a light-harvesting Chl complex of a different composition. These results are discussed with regard to the evolution of the PSI supercomplex.     

UV effects on photosynthesis and phycobiliprotein composition in the flagellate Cyanophora paradoxa

Abstract The effects of solar and artificial ultraviolet radiation on photosynthetic oxygen production and phycobiliprotein composition were investigated in the freshwater flagellate, Cyanophora paradoxa . The phycobiliproteins of the cell acting as photosynthetic accessory pigments were found to be readily affected by even short exposure to ultraviolet radiation, while the membrane-bound chlorophyll protein complexes were hardly impaired as demonstrated by SDS polyacrylamide gel electrophoresis, isoelectric focussing and fast protein liquid chromatography (FPLC). The phycobilisomes are dissembled from the outside inwards and go from higher molecular weight components to hexamers (αβ)₆, trimers (αβ)₃ and finally monomers (αβ). Fluorescence spectra indicated that the energy transfer from accessory pigments to the photosystems was impaired by ultraviolet radiation. Photosynthetic oxygen production was affected on a much faster timescale than changes in the absorption spectra or at the protein level.     

Isolation and characterization of oxygen-evolving thylakoid membranes and photosystem II particles from a glaucocystophyte, Cyanophora paradoxa

Conditions for preparing oxygen-evolving thylakoid membranes and PSII complexes, and those for observing the PSII activity were investigated in a glaucocystophyte, Cyanophora paradoxa. The active thylakoid membranes were isolated either with a medium containing glycerol or with that containing high concentrations of sucrose, phosphate, and citrate. Active PSII particles were solubilized by octyl-beta-D-glucoside from thylakoid membranes and were separated by sucrose density gradient centrifugation. The thylakoid membranes and PSII particles showed an oxygen-evolving activity only in high-ionic-strength media. The extrinsic 33 kDa protein (PsbO) and the cytochrome c(550) (PsbV) were found to be present in the PSII particles as in cyanobacteria or red algae, but no 12 kDa protein (PsbU) was detected. The PsbO protein was classified as a land-plant type by its N-terminal amino acid sequence.     

Effects of solar and artificial radiation on motility and pigmentation in Cyanophora paradoxa

The effects of solar radiation and artificial UV irradiation on motility and pigmentation were studied in the flagellate system Cyanophora paradoxa. Both percentage of motile cells and average velocity decreased drastically after a solar exposure of a few hours. This effect was not due to an overheating since the cells were exposed under temperaturecontrolled conditions. Partial reduction of the UV-B radiation by cut-off filters or by insertion of an artificial ozone layer increased the tolerated exposure times. Artificial UV radiation also induced the same effects. Under both solar and artificial UV irradiation the photosynthetic pigments within the cyanelles were bleached also within short exposure times. Kinetics of pigment destruction showed that the accessory phycobilins are lost with a half life of 1.3 h while the chlorophylls had a half life of 33 h and a carotenoid with an absorption maximum at 480 nm of 17.3 h.     

Immunocytochemical Localization of Phosphoribulose Kinase in the Cyanelles of Cyanophora paradoxa and Glaucocystis nostochinearum

The distribution of phosphoribulose kinase (PRK) in the cyanelles of Cyanophora paradoxa Korschikoff and Glaucocystis nostochinearum Itzigsohn was studied by protein A-gold immunoelectron microscopy. In both endocyanomes, antiserum against PRK heavily labeled the thylakoid region of the cyanelles, whereas little or no label was present over the carboxysomes. Antiserum against ribulose 1,5-bisphosphate carboxylase/oxygenase by contrast heavily labeled the carboxysomes of each endocyanome. In vitro studies of PRK distribution in cell-free extracts of C. paradoxa showed that 93% of the enzyme was in the soluble fraction. Quantitative immunoelectron microscopy showed that more than 99% of the PRK in the cyanelle of C. paradoxa was localized in the thylakoid region. We conclude that the carboxysomes of cyanelles like the carboxysomes of autotrophic prokaryotes and the pyrenoids of green algal chloroplasts do not contain phosphoribulose kinase.     

RNase P of the Cyanophora paradoxa cyanelle: a plastid ribozyme

Ribonuclease P (RNase P) is a ribonucleoprotein enzyme that generates the mature 5' ends of tRNAs. Ubiquitous across all three kingdoms of life, the composition and functional contributions of the RNA and protein components of RNase P differ between the kingdoms. RNA-alone catalytic activity has been reported throughout bacteria, but only for some archaea, and only as trace activity for eukarya. Available information for RNase P from photosynthetic organelles points to large differences to bacterial as well as to eukaryotic RNase P: for spinach chloroplasts, protein-alone activity has been discussed; for RNase P from the cyanelle of the glaucophyte Cyanophora paradoxa, a type of organelle sharing properties of both cyanobacteria and chloroplasts, the proportion of protein was found to be around 80% rather than the usual 10% in bacteria. Furthermore, the latter RNase P was previously found catalytically inactive in the absence of protein under a variety of conditions; however, the RNA could be activated by a cyanobacterial protein, but not by the bacterial RNase P protein from Escherichia coli. Here we demonstrate that, under very high enzyme concentrations, the RNase P RNA from the cyanelle of C. paradoxa displays RNA-alone activity well above the detection level. Moreover, the RNA can be complemented to a functional holoenzyme by the E. coli RNase P protein, further supporting its overall bacterial-like architecture. Mutational analysis and domain swaps revealed that this A,U-rich cyanelle RNase P RNA is globally optimized but conformationally unstable, since changes as little as a single point mutation or a base pair identity switch at positions that are not part of the universally conserved catalytic core led to a complete loss of RNA-alone activity. Likely related to this low robustness, extensive structural changes towards an E. coli-type P5-7/P15-17 subdomain as a canonical interaction site for tRNA 3'-CCA termini could not be coaxed into increased ribozyme activity.     

Structure of the Thylakoids and Envelope Membranes of the Cyanelles of Cyanophora paradoxa

The cyanelles of Cyanophora paradoxa Korsch. are photosynthetically active obligate endosymbionts in which phycobiliproteins serve as the major accessory pigments. Freeze-fracture electron micrographs of thylakoids in isolated cyanelles reveal long parallel rows of particles covering most of the E-face, while a more random particle arrangement is evident in some areas. The center-to-center spacing of particles within these rows is about 10 nanometers. Their mean diameter was measured at 9.4 nanometers. The particles on the P-face have a mean diameter of 7.2 nanometers. Thylakoids that retained nearly the full complement of phycobiliproteins (determined spectrophotometrically and by gel electrophoresis) were isolated from the cyanelles. In thin sections of these preparations, rows of disc-shaped phycobilisomes are evident on the surface of the thylakoids. The spacing of the rows of phycobilisomes corresponds to that of the rows of E-face particles (approximately 45 nanometers, center to center). The periodicity of the disc-shaped phycobilisomes within a row is 10 nanometers suggesting a one-to-one association between phycobilisomes and E-face particles.

In addition, visualization of the protoplasmic surface (PS) of isolated thylakoids by freeze-etch electron microscopy shows that rows of disc-shaped phycobilisomes are aligned directly above rows of particles exhibiting two subunits, presumably the P-surface projections of the 10-nanometer intramembrane particles. These observations, together with earlier studies indicating that the 10-nanometer E-face particles probably represent photosystem II (PSII) complexes, suggest that phycobilisomes are positioned on the thylakoid surface in direct contact with PSII centers within the thylakoid membrane.

The inner envelope membrane of the cyanelles, observed in freeze-fracture replicas, resembles cyanobacterial plasma membranes and is dissimilar to the chloroplast envelope membranes of red or green algae. The envelope of isolated cyanelles exhibits two additional layers: (a) a 5- to 7-nanometer-thick layer that lies adjacent to the inner membrane and which seems to correspond to the peptidoglycan layer of cyanobacteria; and (b) a layer external to the purported peptidoglycan layer that exhibits fracture faces similar to those of the lipopolysaccharide layer of gram negative bacteria. Our findings indicate that the supramolecular architecture of cyanelles differs only slightly from free-living cyanobacteria to which they are presumably related.

     

Conservative sorting in a primitive plastid. The cyanelle of Cyanophora paradoxa

Higher plant chloroplasts possess at least four different pathways for protein translocation across and protein integration into the thylakoid membranes. It is of interest with respect to plastid evolution, which pathways have been retained as a relic from the cyanobacterial ancestor ('conservative sorting'), which ones have been kept but modified, and which ones were developed at the organelle stage, i.e. are eukaryotic achievements as (largely) the Toc and Tic translocons for envelope import of cytosolic precursor proteins. In the absence of data on cyanobacterial protein translocation, the cyanelles of the glaucocystophyte alga Cyanophora paradoxa for which in vitro systems for protein import and intraorganellar sorting were elaborated can serve as a model: the cyanelles are surrounded by a peptidoglycan wall, their thylakoids are covered with phycobilisomes and the composition of their oxygen-evolving complex is another feature shared with cyanobacteria. We demonstrate the operation of the Sec and Tat pathways in cyanelles and show for the first time in vitro protein import across cyanobacteria-like thylakoid membranes and protease protection of the mature protein.