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At least 973 records · Page 54Linked to original sources
PubMed1971

The C 4 -pathway of photosynthesis. Evidence for an intermediate pool of carbon dioxide and the identity of the donor C 4 -dicarboxylic acid.

1. Leaves were exposed to (14)CO(2) under steady-state conditions for photosynthesis. The kinetics of entry or loss of label in pools of CO(2) and other compounds was examined during the period of the pulse and a ;chase' with (12)CO(2). 2. With maize the kinetics of labelling of the major CO(2) pool and of depletion of label during a ;chase' was consistent with this pool being derived from the C-4 of malate and being the precursor of the C-1 of 3-phosphoglycerate. 3. Similar results were obtained for Amaranthus leaves except that the C-4 of aspartate rather than malate was apparently the primary source of CO(2). 4. The size and turnover time of the CO(2) and C(4) acid pools was calculated. These results provided the basis for estimating the concentration of CO(2) in the bundle-sheath cells or chloroplasts assuming the pool was largely restricted to one or other of these compartments. 5. These findings are considered in relation to current schemes for the C(4)-pathway and the operation of a CO(2) concentrating mechanism to serve ribulose diphosphate carboxylase.

Aspartic Acid↗
PubMed1981

The measurement of membrane potential during photosynthesis and during respiration in intact cells of Rhodopseudomonas capsulata by both electrochromism and by permeant ion redistribution.

1. The membrane potential in intact cells of Rhodopseudomonas capsulata during photosynthesis and during dark respiration has been measured by two independent methods. 2. The light-induced and O2-induced shifts in the carotenoid absorption spectrum were measured in the intact cells. The shift was calibrated with K+-diffusion potentials in chromatophores derived from those cells. The light-induced and O2-induced membrane potentials were -290 mV and -230 mV respectively. 3. The energized uptake of butyltriphenylphosphonium ions was measured in the same batch of cells. The light-induced and O2-induced membrane potentials calculated from the Nernst equation were -160 mV and -120 mV respectively. 4. It is concluded that the two kinds of probe measure the electric potentials across different domains of the cytoplasmic membrane, but it is difficult to reconcile the existence of such domains with simple electrical analogues of the membrane and aqueous phases.

Bacterial Chromatophores↗
PubMed1999

Activation of the cycA P2 promoter for the Rhodobacter sphaeroides cytochrome c2 gene by the photosynthesis response regulator.

The Rhodobacter sphaeroides photosynthesis response regulator, PrrA, positively regulates cycA P2 expression. Deletion analysis has identified sequences within 73 bp upstream of the transcription initiation site that are required for the activation of cycA P2 by PrrA. A mutant form of the Rhodobacter capsulatus PrrA homologue, whose activity is independent of phosphorylation (RegA*), protects an approximately 26 bp region of cycA P2 that is centred at approximately -50 from DNase digestion, and activates transcription of a mutant -14T promoter with increased activity when using either R. sphaeroides RNA polymerase or Escherichia coli Esigma70. A 4 bp target site mutation that eliminated DNA binding and transcription activation by RegA* in vitro also abolished PrrA activation of cycA P2 transcription in vivo, indicating that this region contains a PrrA binding site. By analysing the behaviour of the -14T mutant cycA P2 promoter in vivo, we also found that PrrA uses the same target site to activate expression in both the presence and the absence of O2. However, the extent of transcription activation by PrrA at cycA P2 in vivo is greater under anaerobic conditions.

Bacterial Proteins↗
PubMed1992

Maize C4 photosynthesis involves differential regulation of phosphoenolpyruvate carboxylase genes.

Maize as a C4 plant partitions CO2 fixation in two consecutive, spatially separated steps, thus eliminating photorespiration. The crucial enzyme for primary CO2 fixation is a C4-specific phosphoenolpyruvate carboxylase (PEPC). The differential expression of the unique C4-specific gene pepcZm1 and two non-C4-specific genes, pepcZm2A and pepcZm3B, in leaf, root, and stem is reported here. It is shown, in a transient homologous system, that this tissue-specific regulation is mainly controlled by their distinct promoters. The light induction of the C4-specific pepcZm1 in illuminated etiolated (greening) leaves probably relies on light-dependent developmental changes instead of an immediate responsiveness found for other maize genes. Analyses of deleted, mutated, and hybrid promoters revealed the redundant nature of a 14mer which is repeated four times and a decisive function of the TATA box-like motif, TATTT, and the sequences directly preceding it. No consensus sequences to other photosynthetic gene promoters were uncovered. Although light induces the expression of C4 PEPC and other photosynthetic genes in maize, this co-ordination is apparently mediated through different signal transduction pathways and distinct regulatory elements. This study indicates that the acquisition of a new promoter is at least partially responsible for the C4-specific expression of pepcZm1 essential for C4 photosynthesis.

Amino Acid Sequence↗
PubMed1997

An Arabidopsis mutant showing reduced feedback inhibition of photosynthesis.

Many plant genes are responsive to sugars but the mechanisms used by plants to sense sugars are unknown. A genetic approach has been used in Arabidopsis to identify genes involved in perception and transduction of sugar signals. For this purpose, an in vivo reporter system was established consisting of the light- and sugar-regulated plastocyanin promoter, fused to the luciferase coding sequence (PC-LUC construct). At the seedling stage, expression of the PC-LUC gene is repressed by sucrose, and a number of sucrose-uncoupled (sun) mutants were selected in which sucrose is unable to repress the activity of the PC promoter. Three mutants have been characterized in more detail. The sugar analog 2-deoxy-D-glucose (2DG) was used to repress whole plant photosynthesis, PC-LUC gene expression and total ribulose-1,5-bisphosphate activity. It was found that the sun6 mutation makes plants unresponsive to these 2DG-induced effects. Moreover, unlike wild-type plants, sun6 mutants are insensitive to elevated levels of glucose in the growth medium. These findings suggest that the SUN6 gene is active in a hexose-activated signal transduction pathway.

Arabidopsis↗
PubMed2000

The role of inorganic phosphate in the development of freezing tolerance and the acclimatization of photosynthesis to low temperature is revealed by the pho mutants of Arabidopsis thaliana.

Low temperature inhibits sucrose synthesis, leading to a phosphate-limitation of photosynthesis. We have used the Arabidopsis pho1-2 and pho2-1 mutants with decreased and increased shoot phosphate, respectively, to investigate whether low phosphate triggers cold acclimatization of photosynthetic carbon metabolism. Wild-type Arabidopsis, pho1-2 and pho2-1 were grown at 23 degrees C and transferred to 5 degrees C to investigate acclimatization in pre-existing leaves and in new leaves developing at 5 degrees C. The development of frost tolerance and the accumulation of proline and sugars was unaltered or improved in pho1-2, and impaired in pho2-1. Sucrose phosphate synthase and cytoplasmic fructose-1,6-bisphosphatase activity and protein increase after transfer to 5 degrees C. This increase was accentuated in pho1-2 and attenuated in pho2-1. RBCS and LHCB2 transcript levels decrease in pre-formed wild-type leaves after transfer to 5 degrees C and recover in new leaves that develop at 5 degrees C. The initial decrease was attenuated in pho1-2, and accentuated in pho2-1, where the recovery in new leaves was also suppressed. Rubisco activity increased in wild-type leaves that developed at 5 degrees C. This increase was accentuated in pho1-2 and absent in pho2-1. NADP-glyceraldehyde-3-phosphate dehydrogenase, plastidic fructose-1,6-bisphosphatase and aldolase activity increase relative to phosphoglycerate kinase, transketolase and phosphoribulokinase in wild-type leaves at 5 degrees C. This shift was accentuated in pho1-2 and reversed in pho2-1. Transcript levels for COR genes increase transiently 1 day after transfer to 5 degrees C but were very low in leaves that developed at 5 degrees C in wild-type Arabidopsis, pho1-2 and pho2-1. We conclude that low phosphate plays an important role in triggering cold acclimatization of leaves, leading in particular to an increase of Rubisco expression, changes in other Calvin cycle enzymes to minimize sequestration of phosphate in metabolites, and increased expression of sucrose biosynthesis enzymes.

Acclimatization↗
PubMed2002

Proof of C4 photosynthesis without Kranz anatomy in Bienertia cycloptera (Chenopodiaceae).

Kranz anatomy, with its separation of elements of the C4 pathway between two cells, has been an accepted criterion for function of C4 photosynthesis in terrestrial plants. However, Bienertia cycloptera (Chenopodiaceae), which grows in salty depressions of Central Asian semi-deserts, has unusual chlorenchyma, lacks Kranz anatomy, but has photosynthetic features of C4 plants. Its photosynthetic response to varying CO2 and O2 is typical of C4 plants having Kranz anatomy. Lack of night-time CO2 fixation indicates it is not acquiring carbon by Crassulacean acid metabolism. This species exhibits an independent, novel solution to function of the C4 mechanism through spatial compartmentation of dimorphic chloroplasts, other organelles and photosynthetic enzymes in distinct positions within a single chlorenchyma cell. The chlorenchyma cells have a large, spherical central cytoplasmic compartment interconnected by cytoplasmic channels through the vacuole to the peripheral cytoplasm. This compartment is filled with mitochondria and granal chloroplasts, while the peripheral cytoplasm apparently lacks mitochondria and has grana-deficient chloroplasts. Immunolocalization studies show enzymes compartmentalized selectively in the CC compartment, including Rubisco in chloroplasts, and NAD-malic enzyme and glycine decarboxylase in mitochondria, whereas pyruvate, Pi dikinase of the C4 cycle is localized selectively in peripheral chloroplasts. Phosphoenolpyruvate carboxylase, a cytosolic C4 cycle enzyme, is enriched in the peripheral cytoplasm. Our results show Bienertia utilizes strict compartmentation of organelles and enzymes within a single cell to effectively mimic the spatial separation of Kranz anatomy, allowing it to function as a C4 plant having suppressed photorespiration; this raises interesting questions about evolution of C4 mechanisms.

Amino Acid Oxidoreductases↗
PubMed2003

A quantitative structure-function relationship for the Photosystem II reaction center: supermolecular behavior in natural photosynthesis.

Light-induced charge separation is the primary photochemical event of photosynthesis. Efficient charge separation in photosynthetic reaction centers requires the balancing of electron and excitation energy transfer processes, and in Photosystem II (PSII), these processes are particularly closely entangled. Calculations that treat the cofactors of the PSII reaction center as a supermolecular complex allow energy and electron transfer reactions to be described in a unified way. This calculational approach is shown to be in good agreement with experimentally observed energy and electron transfer dynamics. This supermolecular view also correctly predicts the effect of changing the redox potentials of cofactors by site-directed mutagenesis, thus providing a unified and quantitative structure-function relationship for the PSII reaction center.

Bacteria↗
PubMed2004

Transfer of photosynthesis genes to and from Prochlorococcus viruses.

Comparative genomics gives us a new window into phage-host interactions and their evolutionary implications. Here we report the presence of genes central to oxygenic photosynthesis in the genomes of three phages from two viral families (Myoviridae and Podoviridae) that infect the marine cyanobacterium Prochlorococcus. The genes that encode the photosystem II core reaction center protein D1 (psbA), and a high-light-inducible protein (HLIP) (hli) are present in all three genomes. Both myoviruses contain additional hli gene types, and one of them encodes the second photosystem II core reaction center protein D2 (psbD), whereas the other encodes the photosynthetic electron transport proteins plastocyanin (petE) and ferredoxin (petF). These uninterrupted, full-length genes are conserved in their amino acid sequence, suggesting that they encode functional proteins that may help maintain photosynthetic activity during infection. Phylogenetic analyses show that phage D1, D2, and HLIP proteins cluster with those from Prochlorococcus, indicating that they are of cyanobacterial origin. Their distribution among several Prochlorococcus clades further suggests that the genes encoding these proteins were transferred from host to phage multiple times. Phage HLIPs cluster with multicopy types found exclusively in Prochlorocococus, suggesting that phage may be mediating the expansion of the hli gene family by transferring these genes back to their hosts after a period of evolution in the phage. These gene transfers are likely to play a role in the fitness landscape of hosts and phages in the surface oceans.

Base Sequence↗
PubMed2005

A transcriptional response to singlet oxygen, a toxic byproduct of photosynthesis.

The ability of phototrophs to convert light into biological energy is critical for life on Earth. However, there can be deleterious consequences associated with this bioenergetic conversion, including the production of toxic byproducts. For example, singlet oxygen (1O2) can be formed during photosynthesis by energy transfer from excited triplet-state chlorophyll pigments to O2. By monitoring gene expression and growth in the presence of 1O2, we show that the phototrophic bacterium Rhodobacter sphaeroides mounts a transcriptional response to this reactive oxygen species (ROS) that requires the alternative sigma factor, sigma(E). An increase in sigma(E) activity is seen when cells are exposed to 1O2 generated either by photochemistry within the photosynthetic apparatus or the photosensitizer, methylene blue. Wavelengths of light responsible for the generating triplet-state chlorophyll pigments in the photosynthetic apparatus are sufficient for a sustained increase in sigma(E) activity. Continued exposure to 1O2 is required to maintain this transcriptional response, and other ROS do not cause a similar increase in sigma(E)-dependent gene expression. When a sigma(E) mutant produces low levels of carotenoids, 1O2 is bacteriocidal, suggesting that this response is essential for protecting cells from this ROS. In addition, global gene expression analysis identified approximately 180 genes (approximately 60 operons) whose RNA levels increase > or = 3-fold in cells with increased sigma(E) activity. Gene products encoded by four newly identified sigma(E)-dependent operons are predicted to be involved in stress response, protecting cells from 1O2 damage, or the conservation of energy.

Carotenoids↗
PubMed2006

An internal antisense RNA regulates expression of the photosynthesis gene isiA.

Small regulatory noncoding RNAs exist in both eukaryotic and prokaryotic organisms. Most of these RNA transcripts are trans-encoded RNAs with short and only partial antisense complementarity to their target RNAs, which regulate gene expression by modifying mRNA stability and translation. In contrast, reports on the function of cis-encoded, perfectly complementary antisense RNAs in eubacteria are rare. Cyanobacteria respond to iron deficiency by expressing IsiA (iron stress-induced protein A), which forms a giant ring structure around photosystem I. Here, we show that this process is controlled by IsrR (iron stress-repressed RNA), a cis-encoded antisense RNA transcribed from the isiA noncoding strand. Artificial overexpression of IsrR under iron stress causes a strongly diminished number of IsiA-photosystem I supercomplexes, whereas IsrR depletion results in premature expression of IsiA. The coupled degradation of IsrR/isiA mRNA duplexes appears to be a reversible switch that can respond to environmental changes. IsrR is the only RNA known so far to regulate a photosynthesis component.

Bacterial Proteins↗
PubMed2006

Production of hydrogen peroxide in the atmosphere of a Snowball Earth and the origin of oxygenic photosynthesis.

During Proterozoic time, Earth experienced two intervals with one or more episodes of low-latitude glaciation, which are probable "Snowball Earth" events. Although the severity of the historical glaciations is debated, theoretical "hard Snowball" conditions are associated with the nearly complete shutdown of the hydrological cycle. We show here that, during such long and severe glacial intervals, a weak hydrological cycle coupled with photochemical reactions involving water vapor would give rise to the sustained production of hydrogen peroxide. The photochemical production of hydrogen peroxide has been proposed previously as the primary mechanism for oxidizing the surface of Mars. During a Snowball, hydrogen peroxide could be stored in the ice; it would then be released directly into the ocean and the atmosphere upon melting and could mediate global oxidation events in the aftermath of the Snowball, such as that recorded in the Fe and Mn oxides of the Kalahari Manganese Field, deposited after the Paleoproterozoic low-latitude Makganyene glaciation. Low levels of peroxides and molecular oxygen generated during Archean and earliest Proterozoic non-Snowball glacial intervals could have driven the evolution of oxygen-mediating and -using enzymes and thereby paved the way for the eventual appearance of oxygenic photosynthesis.

Earth, Planet↗
PubMed2001

Plastome-encoded bacterial ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) supports photosynthesis and growth in tobacco.

The efficiency with which crop plants use their resources of light, water, and fertilizer nitrogen could be enhanced by replacing their CO(2)-fixing enzyme, d-ribulose-1,5-bisphosphate carboxylase-oxygenase (RubisCO), with more efficient forms, such as those found in some algae, for example. This important challenge has been frustrated by failure of all previous attempts to substitute a fully functional, foreign RubisCO (efficient or inefficient) into higher plants. This failure could be caused by incompatibility between the plastid-encoded large subunits and the nucleus-encoded small subunits or by inability of the foreign RubisCO subunits to fold or assemble efficiently in the plastid. Mismatch between the regulatory requirements of the foreign RubisCO and conditions in the chloroplast also might render the substituted enzyme inactive but, previously, it has not been possible to test this. To answer the general question of whether a foreign RubisCO can support photosynthesis in a plant, we used plastid transformation to replace RubisCO in tobacco with the simple homodimeric form of the enzyme from the alpha-proteobacterium, Rhodospirillum rubrum, which has no small subunits and no special assembly requirements. The transplastomic plants so obtained are fully autotrophic and reproductive but require CO(2) supplementation, consistent with the kinetic properties of the bacterial RubisCO. This establishes that the activity of a RubisCO from a very different phylogeny can be integrated into chloroplast photosynthetic metabolism without prohibitive problems.

Carbon Dioxide↗
PubMed1969

Synthesis of amodel system for the primary energy conversion reactions in photosynthesis.

A model system is constructed which, like the photosynthetic apparatus of green plants, can convert light to chemical free energy through pigment-sensitized photooxidation of water. The system has two light-harvesting subunits connected electrically in series. Each subunit is made of a multimolecular layer of Zn(II)-tetraphenylporphyrin deposited on a clean aluminum surface and immersed in an aqueous mixture of potassium ferri- and ferrocyanide. Upon illumination by amber light, charge transfer takes place across more than 70 molecular layers of the pigment with a photoelectromotive force of 1.1 to 1.3 volts per subunit. With NADP as the electron acceptor and NADP-reductase as a mediator, the system can photooxidize water to oxygen gas. With these model experiments as a guide, a molecular mechanism for the primary energy conversion reactions in photosynthesis is formulated that offers a unified interpretation of most of the relevant observations reported in the literature.

Electron Transport↗
PubMed1971

Primary reactions of photosynthesis: photoreduction of a bound chloroplast ferredoxin at low temperature as detected by EPR spectroscopy.

An electron paramagnetic resonance signal was observed at 25 degrees K in whole spinach chloroplasts after illumination at 77 degrees K. The light-induced epr spectrum had g-values (g(x) = 1.86, g(y) = 1.94, g(z) = 2.05) and a temperature dependence that were characteristic of the reduced state of a plant-type ferredoxin. The light-induced epr spectrum was also observed in broken spinach chloroplasts from which soluble ferredoxin was removed. Chemical analyses showed that both whole and broken spinach chloroplasts contained amounts of nonheme iron and "acid-labile sulfide" consistent with the presence of a bound iron-sulfur protein, at a level of about one molecule per 75 chlorophyll molecules. These results support the conclusion that chloroplasts contain a bound ferredoxin that may serve as a primary low-potential electron acceptor in photosynthesis.

Chloroplasts↗
PubMed1977

Tripartite model for the photochemical apparatus of green plant photosynthesis.

Equations for fluorescence and the rates of photochemistry of photosystem I and photosystem II are derived from a photochemical model for the photosynthetic apparatus that includes the various interactions of the light-harvesting chlorophyll a/b complex with photosystem I and photosystem II as specific photochemical rate constants. The degree of coupling between photosystem II and the chlorophyll a/b complex which is expressed as a product of two probability terms plays a central role in this three-pigment system. The cycling of excitation energy back and forth between photosystem II and the chlorophyll a/b complex increases the exciton density in both arrays of chlorophyll according to a simple analytical expression in the equations. These equations of the tripartite model provide new and credible insights into the photochemical apparatus of photosynthesis.

Mathematics↗
PubMed1990

Expression of photosynthesis-related gene fusions is restricted by cell type in transgenic plants and in transfected protoplasts.

We have analyzed the expression of chimeric genes in populations of protoplasts isolated from the photosynthetic and nonphotosynthetic tissues within leaves of transgenic tobacco plants and separated by fluorescence-activated cell sorting. Expression of transcriptional gene fusions controlled by promoters from photosynthesis-associated genes showed a striking dependence on cell type. These patterns of expression were preserved when the gene fusions were transfected into normal (nontransgenic) tobacco leaf protoplasts.

Base Sequence↗
PubMed2000

The proton to electron stoichiometry of steady-state photosynthesis in living plants: A proton-pumping Q cycle is continuously engaged.

A noninvasive technique is introduced with which relative proton to electron stoichiometries (H(+)/e(-) ratios) for photosynthetic electron transfer can be obtained from leaves of living plants under steady-state illumination. Both electron and proton transfer fluxes were estimated by a modification of our previously reported dark-interval relaxation kinetics (DIRK) analysis, in which processes that occur upon rapid shuttering of the actinic light are analyzed. Rates of turnover of linear electron transfer through the cytochrome (cyt) b(6)f complex were estimated by measuring the DIRK signals associated with reduction of cyt f and P(700). The rates of proton pumping through the electron transfer chain and the CF(O)-CF(1) ATP synthase (ATPase) were estimated by measuring the DIRK signals associated with the electrochromic shifting of pigments in the light-harvesting complexes. Electron transfer fluxes were also estimated by analysis of saturation pulse-induced changes in chlorophyll a fluorescence yield. It was shown that the H(+)/e(-) ratio, with respect to both cyt b(6)f complex and photosystem (PS) II turnover, was constant under low to saturating illumination in intact tobacco leaves. Because a H(+)/e(-) ratio of 3 at a low light is generally accepted, we infer that this ratio is maintained under conditions of normal (unstressed) photosynthesis, implying a continuously engaged, proton-pumping Q cycle at the cyt b(6)f complex.

Chlorophyll↗