The biogenesis of the cyanellae of Cyanophora paradoxa. III. In vitro synthesis of cyanellar polypeptides using separated cytoplasmic and cyanellar RNA

RNA from Cyanophora paradoxa was separated into cytoplasmic and cyanellar fractions by using a combination of subcellular fractionation and oligo-dT chromatography. In vitro translation of the separated cytoplasmic and cyanellar RNAs in a rabbit reticulocyte lysate system in the presence of [35S]methionine resulted in the incorporation of radiolabel into electrophoretically distinct sets of polypeptides. Monospecific and polyspecific antibodies that react with cyanellar polypeptides were used to probe the in vitro translation products by indirect immunoprecipitation by using Staphylococcus protein A conjugated to Sepharose beads. The results indicate that linker polypeptide L1 of the phycobilisome, the gamma subunit of coupling factor CF1, and subunit II of PS I are synthesized in the cytoplasm as precursor molecules that are 5-8 kDa larger than their mature sizes. Antibodies directed against the psbA gene product (the D1 protein) precipitated a polypeptide found in the translation products of the cyanellar RNA-directed reactions, which is about 1.5 kDa larger than the mature protein.     

The biogenesis of the cyanellae of Cyanophora paradoxa. I. Polypeptide composition of the cyanellae

Based on polypeptide separation, protein purification and immunoblotting techniques using heterologous antibodies, we have been able to identify several photosynthetically important polypeptide components of the cyanellae of Cyanophora paradoxa. Cytochrome c-552 and ferredoxin have been purified to electrophoretic homogeneity and exhibit apparent molecular masses of 10.5 and 9.0 kDa, respectively. Cytochrome c-552 has an isoelectric point of pH 4.2 +/- 0.1. Plastocyanin was immunologically and spectrally undetectable even in cells grown in the presence of Cu2+. Polypeptides for cytochromes f, b-6 and c-552 have been located in electrophoretically resolved thylakoid samples by using the TMBZ-staining procedure. Intact phycobilisomes have been purified and characterized with respect to polypeptide composition and absorption and emission spectra. Photosystems I and II have been isolated and characterized with respect to their photochemical activities, spectral characteristics and polypeptide composition. Photochemically active PS I complexes fluoresce maximally at 720 nm at 77 K and comprise five polypeptide subunits resolved under denaturing conditions with apparent molecular masses of 66, 21, 18, 14 and 11 kDa. PS II core complexes mediate light-dependent 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-sensitive electron transfer between 1,5-diphenylcarbazide (DPC) and 2,6-dichlorophenolindophenol (DPIP) at rates of 140-200 mumol h-1 mg-1 chlorophyll. These complexes exhibit absorption maxima at 436 and 673 nm and show fluorescence emission maxima at 685 and 695 nm at 77 K. Rubisco was separated by two-dimensional electrophoresis and immunologically characterized.     

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.     

Gene map for the Cyanophora paradoxa cyanelle genome.

The genes for the following proteins were localized by hybridization analysis on the cyanelle genome of Cyanophora paradoxa: the alpha and beta subunits of phycocyanin (cpcA and cpcB); the alpha and beta subunits of allophycocyanin (apcA and apcB); the large and small subunits of ribulose-1,5-bisphosphate carboxylase (rbcL and rbcS); the two putative chlorophyll alpha-binding apoproteins of the photosystem I-P700 complex (psaA and psaB); four apoproteins believed to be components of the photosystem II core complex (psbA, psbB, psbC, and psbD); the two apoprotein subunits of cytochrome b-559 which is also found in the core complex of photosystem II (psbE and psbF); three subunits of the ATP synthase complex (atpA and atpBE); and the cytochrome f apoprotein (petA). Eighty-five percent of the genome was cloned as BamHI, BglII, or PstI fragments. These cloned fragments were used to construct a physical map of the cyanelle genome and to localize more precisely some of the genes listed above. The genes for phycocyanin and allophycocyanin were not clustered and were separated by about 25 kilobases. Although the rbcL gene was adjacent to the atpBE genes and the psbC and psbD genes were adjacent, the arrangement of other genes encoding various polypeptide subunits of protein complexes involved in photosynthetic functions was dissimilar to that observed for known chloroplast genomes. These results are consistent with the independent development of this cyanelle from a cyanobacterial endosymbiont.     

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.     

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.     

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.     

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.     

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.     

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.

     

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.     

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.     

The central part of the cyanelle rDNA unit of Cyanophora paradoxa: Sequence comparison with chloroplasts and cyanobacteria

The 287-bp spacer and the flanking 3-end of the 16S- and 5-end of the 23S-rRNA genes of the cyanelles from Cyanophora paradoxa have been sequenced and compared with the corresponding regions of cyanobacteria and chloroplasts. The spacer contains the uninterrupted genes for tRNA^ile and tRNA^ala. All coding regions show high homology to their prokaryotic counterparts. At the 3-end of the 16S-rDNA a CCTCCTTT sequence has been identified which is complementary to putative ribosome binding sites observed immediately upstream of the coding region of cyanelle protein genes.     

The cyanelle genome of Cyanophora paradoxa, unlike the chloroplast genome, codes for the ribosomal L3 protein.

We describe a 1132 bp sequence of the cyanelle genome of Cyanophora paradoxa containing the rpl3 gene. This gene, which is not chloroplast encoded in plants, is the first of a long cyanelle ribosomal operon whose organization resembles that of the S10 operon of E. coli. We have shown that the rpl3 gene is transcribed in cyanelles as a 7500 nucleotide precursor and that the 5'-end of the mRNA starts approximately 90 nucleotides upstream from the initiation codon. However, no typical procaryotic promoter could be found for this gene. We have detected, using anti E. coli L3 antibodies, the cyanelle L3 protein in cyanelle extracts and in E. coli cells transformed with the cyanelle rpl3 gene.