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PubMed2003

Pulsed high-frequency EPR study on the location of carotenoid and chlorophyll cation radicals in photosystem II.

When the primary electron-donation pathway from the water-oxidation complex in photosystem II (PS II) is inhibited, chlorophyll (Chl(Z) and Chl(D)), beta-carotene (Car) and cytochrome b(559) are alternate electron donors that are believed to function in a photoprotection mechanism. Previous studies have demonstrated that high-frequency EPR spectroscopy (at 130 GHz), together with deuteration of PS II, yields resolved Car(+) and Chl(+) EPR signals (Lakshmi et al. J. Phys. Chem. B 2000, 104, 10 445-10 448). The present study describes the use of pulsed high-frequency EPR spectroscopy to measure the location of the carotenoid and chlorophyll radicals relative to other paramagnetic cofactors in Synechococcus lividus PS II. The spin-lattice relaxation rates of the Car(+) and Chl(+) radicals are measured in manganese-depleted and manganese-depleted, cyanide-treated PS II; in these samples, the non-heme Fe(II) is high-spin (S = 2) and low-spin (S = 0), respectively. The Car(+) and Chl(+) radicals exhibit dipolar-enhanced relaxation rates in the presence of high-spin (S = 2) Fe(II) that are eliminated when the Fe(II) is low-spin (S = 0). The relaxation enhancements of the Car(+) and Chl(+) by the non-heme Fe(II) are smaller than the relaxation enhancement of Tyr(D)(*) and P(865)(+) by the non-heme Fe(II) in PS II and in the reaction center from Rhodobactersphaeroides, respectively, indicating that the Car(+)-Fe(II) and Chl(+)-Fe(II) distances are greater than the known Tyr(D)(*)-Fe(II) and P(865)(+)-Fe(II) distances. The Car(+) radical exhibits a greater relaxation enhancement by Fe(II) than the Chl(+) radical, consistent with Car being an earlier electron donor to P(680)(+) than Chl. On the basis of the distance estimates obtained in the present study and by analogy to carotenoid-binding sites in other pigment-protein complexes, possible binding sites are discussed for the Car cofactors in PS II. The relative location of Car(+) and Chl(+) radicals determined in this study provides valuable insight into the sequence of electron transfers in the alternate electron-donation pathways of PS II.

Binding Sites↗
PubMed1998

Carbon dioxide as a regulator of gene expression in microorganisms.

CO2 regulates gene expression across a diverse group of microorganisms including fungi, and both photosynthetic and non photosynthetic bacteria. The processes that CO2 regulates are diverse. Several CO2-responsive random promoter lacZ fusions of unknown function have been isolated from a marine Synechococcus and a Pseudoalteromonas sp., highlighting the wide effect of CO2 control in these organisms. Regulatory proteins have been described that mediate the CO2 response at transcription level in Bacillus anthracis, the group A streptococci and two Rhodobacter spp. These regulatory proteins include: AcpA and AtxA that are involved in CO2 control of B. anthracis capsule and toxin production; Mga that regulates surface associated virulence factors in the group A streptococci; and RegB/A, a two component signal transduction system that responds to environmental stimuli including CO2, to regulate photosynthetic apparatus and CO2 fixation enzyme synthesis in Rhodobacter spp.

Bacteria↗
PubMed1997

Nitrogen availability and electron transport control the expression of glnB gene (encoding PII protein) in the cyanobacterium Synechocystis sp. PCC 6803.

The glnB gene from Synechocystis sp. PCC 6803 that encodes the PII protein has been cloned by heterologous hybridization using the corresponding glnB gene from Synechococcus sp. PCC 7942. An ORF of 336 nucleotides appeared that potentially coded for a protein of 112 amino acid residues (M(r) 12,397). The deduced amino acid sequence revealed a high identity (higher than 80%) with its cyanobacterial counterparts and a basal level of identity (close to 60%) with other PII proteins. A single mRNA of about 680 nucleotides was found under all growth conditions studied. glnB gene expression was specifically activated under nitrogen deprivation (a 10-fold increase respect to nitrogen-replete conditions). No differences in glnB mRNA levels were observed when using nitrate or ammonium as nitrogen sources. Amount of glnB mRNA decreased to undetectable levels when transferring cells to the dark, but effect was avoided by adding glucose to the culture medium. Primer extension analysis and band-shift assays indicated that expression of the glnB gene, elevated under nitrogen deprivation, might lie under the control of the nitrogen transcriptional regulator NtcA, although constitutive levels of expression were also detected from a sigma 70-dependent Escherichia coli-like promoter.

Amino Acid Sequence↗
PubMed1999

Phycoerythrins of the oxyphotobacterium Prochlorococcus marinus are associated to the thylakoid membrane and are encoded by a single large gene cluster.

An intrinsic divinyl-chlorophyll a/b antenna and a particular form of phycobiliprotein, phycoerythrin (PE) III, coexist in the marine oxyphotobacterium Prochlorococcus marinus CCMP 1375. The genomic region including the cpeB/A operon of P. marinus was analysed. It encompasses 10,153 nucleotides that encode three structural phycobiliproteins and at least three (possibly five) different polypeptides analogous to cyanobacterial or red algal proteins involved either in the linkage of subunits or the synthesis and attachment of chromophoric groups. This gene cluster is part of the chromosome and is located within a distance of less than 110 kb from a previously characterized region containing the genes aspA-psbA-aroC. Whereas the Prochlorococcus phycobiliproteins are characterized by distinct deletions and amino acid replacements with regard to analogous proteins from other organisms, the gene arrangement resembles the organization of phycobiliprotein genes in some other cyanobacteria, in particular marine Synechococcus strains. The expression of two of the Prochlorococcus polypeptides as recombinant proteins in Escherichia coli allowed the production of individual homologous antisera to the Prochlorococcus alpha and beta PE subunits. Experiments using these sera show that the Prochlorococcus PEs are specifically associated to the thylakoid membrane and that the protein level does not significantly vary as a function of light irradiance or growth phase.

Amino Acid Sequence↗
PubMed2000

Cell turgor: A critical factor for the proliferation of cyanobacteria at unfavorable salinity.

We employed chlorophyll a fluorometry in order to measure the evolution of turgor threshold (intracellular osmolality) during the adaptation of two genetic transformants of the freshwater cyanobacterium Synechococcus sp. PCC7942 to unfavorable external salinity: PAMCOD cells which oxidize imported choline and accumulate approx. 0.06-0.08 M glycine betaine; and PAM cells which do not oxidize choline [Deshnium et al. (1995a) Plant Mol Biol 29: 897-909]. Turgor thresholds increased linearly (a) with the NaCl concentration in the culture, and (b) with the molar sucrose/chlorophyll a ratio in the cell. PAMCOD cells could proliferate in culture medium containing 0.4 M NaCl (external osmolality, 0.815 Osm kg(-1)), after a lag period, during which intracellular sucrose rose to 10 mol (mol Chl a)(-1), or more, and turgor threshold (cytoplasmic osmolality) exceeded 1 Osm kg(-1). At comparative conditions, PAM cells accumulated approx. half as much sucrose, and attained approx. half as high turgor thresholds as the PAMCOD cells, but they did not proliferate. These results indicate that glycine betaine improved the salinity tolerance of the PAMCOD cells synergistically, by means of two effects that implicate sucrose, the main organic osmolyte of Synechocccus: enhancement of sucrose biosynthesis, and/or alleviation of sucrose toxicity.

Journal Article↗
PubMed2000

Rapid evolutionary divergence of Photosystem I core subunits PsaA and PsaB in the marine prokaryote Prochlorococcus.

The nucleotide sequences of the genes coding for the subunits of the Photosystem I (PS I) core, PsaA and PsaB were determined for the marine prokaryotic oxyphototrophs Prochlorococcus sp. MED4 (CCMP1378), P. marinus SS120 (CCMP1375) and Synechococcus sp. WH7803. Divergence of these sequences from those of both freshwater cyanobacteria and higher plants was remarkably high, given the conserved nature of PsaA and PsaB proteins. In particular, the PsaA of marine prokaryotes showed several specific insertions and deletions with regard to known PsaA sequences. Even in between the two Prochlorococcus strains, which correspond to two genetically different ecotypes with shifted growth irradiance optima, the sequence identity was only 80.2% for PsaA and 88.9% for PsaB. Possible causes and implications of the fast evolution rates of these two PS I core subunits are discussed.

Journal Article↗
PubMed2001

Comparative study of thermal degradation of iron-sulfur proteins in spinach chloroplasts and membranes of thermophilic cyanobacteria: mössbauer spectroscopy.

Mössbauer spectra of chloroplasts isolated from spinach plants grown in a mineral medium enriched with 57Fe and Mössbauer spectra of native membranes of the thermophilic cyanobacterium Synechococcus elongatus contain a broad asymmetric doublet typical of the iron-sulfur proteins of Photosystem (PS) I. Exposure of chloroplasts to temperatures of 20-70 degrees C significantly modifies the central part of the spectra. This spectral change is evidence of decreased magnitude of the quadrupole splitting. However, the thermally induced doublet (DeltaQ = 3.10 mm/sec and delta = 1.28 mm/sec) typical of hydrated forms of reduced (divalent) inorganic iron is not observed in spinach chloroplasts. This doublet is usually associated with degradation of active centers of ferredoxin, a surface-exposed protein of PS I. The Mössbauer spectra of photosynthetic membranes of spinach chloroplasts and cyanobacteria were compared using the probability distribution function of quadrupole shift (1/2 quadrupole splitting DeltaQ) of trivalent iron. The results of calculation of these functions for the two preparations showed that upon increasing the heating temperature there was a decrease in the probability of the presence of native iron-sulfur centers FX, FA, and FB (quadrupole shift range, 0.43-0.67 mm/sec) in heated preparations. This process was also accompanied by an increase in the probability of appearance of clusters of trivalent iron. This increase was found to be either gradual and continuous or abrupt and discrete in photosynthetic membranes of cyanobacteria or spinach chloroplasts, respectively. The probability of the presence of the iron-sulfur centers FX, FA, and FB in chloroplasts abruptly decreases to virtually to zero within the temperature range critical for inhibition of electron transport through PS I to oxygen. In cyanobacteria, both thermal destruction of iron-sulfur centers of PS I and functional degradation of PS I are shifted toward a higher temperature. The results of this study suggest that the same mechanism of thermal destruction of the PS I core occurs in both thermophilic and mesophilic organisms: destruction of iron-sulfur centers FX, FA, and FB, release of oxidized (trivalent) iron, and its accumulation in membrane-bound iron-oxo clusters.

Chloroplasts↗
PubMed2002

Correlation of the cytochrome c (550) content of cyanobacterial Photosystem II with the EPR properties of the oxygen-evolving complex.

The Mn(4) cluster of PS II advances through a series of oxidation states (S states) that catalyze the breakdown of water to dioxygen in the oxygen-evolving complex. The present study describes the engineering and purification of highly active PS II complexes from mesophilic His-tagged Synechocystis PCC 6803 and purification of PS II core complexes from thermophilic wild-type Synechococcus lividus with high levels of the extrinsic polypeptide, cytochrome c (550). The g = 4.1 S(2) state EPR signal, previously not characterized in untreated cyanobacterial PS II, is detected in high yields in these PS II preparations. We present a complete characterization of the g = 4.1 state in cyanobacterial His-tagged Synechocystis PCC 6803 PS II and S. lividus PS II. Also presented are a determination of the stoichiometry of cytochrome c (550) bound to His-tagged Synechocystis PCC 6803 PS II and analytical ultracentrifugation results which indicate that cytochrome c (550) is a monomer in solution. The temperature-dependent multiline to g = 4.1 EPR signal conversion observed for the S(2) state in cyanobacterial PS II with high cytochrome c (550) content is very similar to that previously found for spinach PS II. In spinach PS II, the formation of the S(2) state g = 4.1 EPR signal has been found to correlate with the binding of the extrinsic 17 and 23 kDa polypeptides. The finding of a similar correlation in cyanobacterial PS II with the binding of cytochrome c (550) suggests a functional homology between cytochrome c (550) and the 17 and 23 kDa extrinsic proteins of spinach PS II.

Journal Article↗
PubMed2003

Massive light-dependent cycling of inorganic carbon between oxygenic photosynthetic microorganisms and their surroundings.

Membrane inlet mass spectrometry indicated massive light-dependent cycling of inorganic carbon between the medium and the cells of various phytoplankton species representing the main groups of aquatic primary producers. These included diatoms, symbiotic and free living dinoflagellates, a coccolithophorid, a green alga and filamentous and single cell cyanobacteria. These organisms could maintain an ambient CO(2) concentration substantially above or below that expected at chemical equilibrium with HCO(3) (-). The coccolithophorid Emiliania huxleyi shifted from net CO(2) uptake to net CO(2) efflux with rising light intensity. Differing responses of CO(2) uptake and CO(2) fixation to changing light intensity supported the notion that these two processes are not compulsorily linked. Simultaneous measurements of CO(2) and O(2) exchange and of the fluorescence parameters in Synechococcus sp. strain PCC 7942, showed that CO(2) uptake can serve as a sensitive probe of the energy status of the photosynthetic reaction centers. However, during transitions in light intensity, changes in CO(2) uptake did not accord with those expected from fluorescence change. Quantification of the net fluxes of CO(2), HCO(3) (-) and of photosynthesis at steady-state revealed that substantial HCO(3) (-) efflux accompanied CO(2) uptake and fixation in the case of 'CO(2) users'. On the other hand, 'HCO(3) (-) users' were characterized by a rate of net CO(2) uptake below that of CO(2) fixation. The results support the notion that entities associated with the CCM function not only in raising the CO(2) concentration at the site of Rubisco; they may also serve as a means of diminishing photodynamic damage by dissipating excess light energy.

Journal Article↗
PubMed2004

Impact of 10 dietary sterols on growth and reproduction of Daphnia galeata.

In crustaceans, cholesterol is an essential nutrient, which they must directly obtain from their food or by bioconversion from other dietary sterols. Eukaryotic phytoplankton contain a great variety of sterols that differ from cholesterol in having additional substituents or different positions and/or number of double bonds in the side chain or in the sterol nucleus. In this study, we investigated to what extent these structural features affect the growth and reproduction of Daphnia galeata in standardized growth experiments with the cyanobacterium Synechococcus elongatus supplemented with single sterols as food source. The results indicated that delta 5 (sitosterol, stigmasterol, desmosterol) and delta 5.7 (7-dehydrocholesterol, ergosterol) sterols meet the nutritional requirements of the daphnids, while the delta 7 sterol lathosterol supports somatic growth and reproduction to a significantly lower extent than cholesterol. Dihydrocholesterol (delta 0) and lanosterol (delta 8) did not improve the growth of D. galeata, and growth was adversely affected by the delta 4 sterol allocholesterol. Sterols seem to differ in their allocation to somatic growth and reproduction. Thus, structural differences of dietary sterols have pronounced effects on life-history traits of D. galeata.

Animal Nutritional Physiological Phenomena↗
PubMed2004

Steps on the Way to Building Blocks, Topologies, Crystals and X-ray Structural Analysis of Photosystems I and II of Water-oxidizing Photosynthesis.

Basic structural elements of the two photosystems and their component electron donors, acceptors, and carriers were revealed by newly developed spectroscopic methods in the 1960s and subsequent years. The spatial organization of these constituents within the functional membrane was elucidated by electrochromic band shift analysis, whereby the membrane-spanning chlorophyll-quinone couple of Photosystem (PS) II emerged as reaction center and as a model relevant also to other photosystems. A further step ahead for improved structural information was realized with the use of thermophilic cyanobacteria instead of plants which led to isolation of supramolecular complexes of the photosystems and their identification as PS I trimers and PS II dimers. The preparation of crystals of the PS I trimer, started in the late 1980s. Genes encoding the 11 subunits of PS I from Synechococcus elongatus were isolated and the predicted sequences of amino acid residues formed a basis for the interpretation of X-ray structure analysis of the PS I crystals. The crystallization of PS I was optimized by introduction of the 'reverse of salting in' crystallization with water as precipitating agent. On this basis the PS I structure was successively established from 6 A resolution in the early 1990s up to a model at 2.5 A resolution in 2001. The first crystals of the PS II dimer, capable of water oxidation, were prepared in the late 1990s; a PS II model at 3.8-3.6 A resolution was presented in 2001. Implications of the PS II structure for the mechanism of transmembrane charge separation are discussed. With the availability of PS I and PS II crystals, new directional structural results became possible also by application of different magnetic resonance techniques through measurements on single crystals in different orientations.

Journal Article↗
PubMed2001

Three-dimensional structure of cyanobacterial photosystem I at 2.5 A resolution.

Life on Earth depends on photosynthesis, the conversion of light energy from the Sun to chemical energy. In plants, green algae and cyanobacteria, this process is driven by the cooperation of two large protein-cofactor complexes, photosystems I and II, which are located in the thylakoid photosynthetic membranes. The crystal structure of photosystem I from the thermophilic cyanobacterium Synechococcus elongatus described here provides a picture at atomic detail of 12 protein subunits and 127 cofactors comprising 96 chlorophylls, 2 phylloquinones, 3 Fe4S4 clusters, 22 carotenoids, 4 lipids, a putative Ca2+ ion and 201 water molecules. The structural information on the proteins and cofactors and their interactions provides a basis for understanding how the high efficiency of photosystem I in light capturing and electron transfer is achieved.

Carotenoids↗
PubMed1992

Multiple evolutionary origins of prochlorophytes within the cyanobacterial radiation.

The taxonomic group Prochlorales (Lewin 1977) Burger-Wiersma, Stal and Mur 1989 was established to accommodate a set of prokaryotic oxygenic phototrophs which, like plant, green algal and euglenoid chloroplasts, contain chlorophyll b instead of phycobiliproteins. Prochlorophytes were originally proposed (with concomitant scepticism) to be a monophyletic group sharing a common ancestry with these 'green' chloroplasts. Results from molecular sequence phylogenies, however, have suggested that Prochlorothrix hollandica is not on a lineage that leads to plastids. Our results from 16S ribosomal RNA sequence comparisons, which include new sequences from the marine picoplankter Prochlorococcus marinus and the Lissoclinum patella symbiont Prochloron sp., indicate that prochlorophytes are polyphyletic within the cyanobacterial radiation, and suggest that none of the known species is specifically related to chloroplasts. This implies that the three prochlorophytes and the green chloroplast ancestor acquired chlorophyll b and its associated structural proteins in convergent evolutionary events. We report further that the 16S rRNA gene sequence from Prochlorococcus is very similar to those of open ocean Synechococcus strains (marine cluster A), and to a family of 16S rRNA genes shotgun-cloned from plankton in the north Atlantic and Pacific Oceans.

Base Sequence↗
PubMed2005

Photosynthesis genes in marine viruses yield proteins during host infection.

Cyanobacteria, and the viruses (phages) that infect them, are significant contributors to the oceanic 'gene pool'. This pool is dynamic, and the transfer of genetic material between hosts and their phages probably influences the genetic and functional diversity of both. For example, photosynthesis genes of cyanobacterial origin have been found in phages that infect Prochlorococcus and Synechococcus, the numerically dominant phototrophs in ocean ecosystems. These genes include psbA, which encodes the photosystem II core reaction centre protein D1, and high-light-inducible (hli) genes. Here we show that phage psbA and hli genes are expressed during infection of Prochlorococcus and are co-transcribed with essential phage capsid genes, and that the amount of phage D1 protein increases steadily over the infective period. We also show that the expression of host photosynthesis genes declines over the course of infection and that replication of the phage genome is a function of photosynthesis. We thus propose that the phage genes are functional in photosynthesis and that they may be increasing phage fitness by supplementing the host production of these proteins.

Bacteriophages↗
PubMed1996

Photosystem I at 4 A resolution represents the first structural model of a joint photosynthetic reaction centre and core antenna system.

The 4 A X-ray structure model of trimeric photosystem I of the cyanobacterium Synechococcus elongatus reveals 31 transmembrane, nine surface and three stromal alpha-helices per monomer, assigned to the 11 protein subunits: PsaA and PsaB are related by a pseudo two-fold axis normal to the membrane plane, along which the electron transfer pigments are arranged. 65 antenna chlorophyll a (Chl a) molecules separated by < or = 16 A form an oval, clustered net continuous with the electron transfer chain through the second and third Chl a pairs of the electron transfer system. This suggests a dual role for these Chl a both in excitation energy and electron transfer. The architecture of the protein core indicates quinone and iron-sulphur type reaction centres to have a common ancestor.

Cell Membrane↗
PubMed2005

Excitation energy transfer from phycobiliprotein to chlorophyll d in intact cells of Acaryochloris marina studied by time- and wavelength-resolved fluorescence spectroscopy.

The fluorescence decay spectra and the excitation energy transfer from the phycobiliproteins (PBP) to the chlorophyll-antennae of intact cells of the chlorophyll (Chl) d-dominated cyanobacterium Acaryochloris marina were investigated at 298 and 77 K by time- and wavelength-correlated single photon counting fluorescence spectroscopy. At 298 K it was found that (i) the fluorescence dynamics in A. marina is characterized by two emission peaks located at about 650 and 725 nm, (ii) the intensity of the 650 nm fluorescence depends strongly on the excitation wavelength, being high upon excitation of phycobiliprotein (PBP) at 632 nm but virtually absent upon excitation of chlorophyll at 430 nm, (iii) the 650 nm fluorescence band decayed predominantly with a lifetime of 70 +/- 20 ps, (iv) the 725 nm fluorescence, which was observed independent of the excitation wavelength, can be described by a three-exponential decay kinetics with lifetimes depending on the open or the closed state (F(0) or F(m)) of the reaction centre of Photosystem II (PS II). Based on the results of this study, it is inferred that the excitation energy transfer from phycobiliproteins to Chl d of PS II in A. marina occurs with a time constant of about 70 ps, which is about three times faster than the energy transfer from the phycobilisomes to PS II in the Chl a-containing cyanobacterium Synechococcus 6301. A similar fast PBP to Chl d excitation energy transfer was also observed at 77 K. At 77 K a small long-lived fluorescence decay component with a lifetime of 14 ns was observed in the 640-700 nm spectral range. However, it has a rather featureless spectrum, not typical for Chl a, and was only observed upon excitation at 400 nm but not upon excitation at 632 and 654 nm. Thus, this long-lived fluorescence component cannot be used as an indicator that the primary PS II donor of Acaryochloris marina contains Chl a.

Chlorophyll↗
PubMed1973

The characterization of C-phycocyanin from an extremely halo-tolerant blue-green alga, Coccochloris elabens.

C-Phycocyanin was isolated and purified from a uni-algal culture of an extremely halo-tolerant blue-green alga, Coccochloris elabens. This alga can be grown under laboratory conditions in 25% (w/v) NaCl. Purified halophile phycocyanin was characterized by amino acid analysis and the measurement of sedimentation velocity, fluorescence polarization and immunodiffusion as a function of protein concentration, pH and ionic strength. The results were compared with those of studies of phycocyanin isolated from Plectonema calothricoides and from several other sources. The states of aggregation previously characterized as being present in other C-phycocyanins, monomer, trimer and hexamer, were present in halophile phycocyanin and were characterized as antigenically related to all C-phycocyanins tested. The equilibrium between 3S monomer and 11S hexamer at low concentrations in halophile phycocyanin was quantitatively similar to that for other phycocyanins. The effect of pH and ionic strength on the 6S (trimer) and 11S (hexamer) aggregation of halophile phycocyanin was markedly salt-dependent and the relative amount of each aggregate in the presence of 2m-NaCl was like that of C-phycocyanin from mesophiles, in the absence of additional salt. In antigenic relationship and aggregation properties, the phycocyanin from C. elabens appeared to be most closely related to that isolated from the thermophilic blue-green alga, Synechococcus lividus. Amino acid content of the halophile phycocyanin indicated the presence of a significantly larger number of acidic residues than that found in mesophiles. Explanations of the properties of the halophile protein require consideration of a strong contribution of hydrophobic forces and utilize both charge-shielding and salting-out effects.

Amino Acids↗