Showing papers in "Photosynthesis Research in 2004"

New Fluorescence Parameters for the Determination of QA Redox State and Excitation Energy Fluxes.

Abstract: A number of useful photosynthetic parameters are commonly derived from saturation pulse-induced fluorescence analysis. We show, that qP, an estimate of the fraction of open centers, is based on a pure ‘puddle’ antenna model, where each Photosystem (PS) II center possesses its own independent antenna system. This parameter is incompatible with more realistic models of the photosynthetic unit, where reaction centers are connected by shared antenna, that is, the so-called ‘lake’ or ‘connected units’ models. We thus introduce a new parameter, qL, based on a Stern–Volmer approach using a lake model, which estimates the fraction of open PS II centers. We suggest that qL should be a useful parameter for terrestrial plants consistent with a high connectivity of PS II units, whereas some marine species with distinct antenna architecture, may require the use of more complex parameters based on intermediate models of the photosynthetic unit. Another useful parameter calculated from fluorescence analysis is ΦII, the yield of PS II. In contrast to qL, we show that the ΦII parameter can be derived from either a pure ‘lake’ or pure ‘puddle’ model, and is thus likely to be a robust parameter. The energy absorbed by PS II is divided between the fraction used in photochemistry, ΦII, and that lost non-photochemically. We introduce two additional parameters that can be used to estimate the flux of excitation energy into competing non-photochemical pathways, the yield induced by downregulatory processes, ΦNPQ, and the yield for other energy losses, ΦNO.

A simple alternative approach to assessing the fate of absorbed light energy using chlorophyll fluorescence.

Abstract: We propose a simplified alternative method for quantifying the partitioning of excitation energy between photochemistry, fluorescence and thermal dissipation. This alternative technique uses existing well-defined quantum efficiencies such as Phi(PS II), leaving no 'excess' efficiency unaccounted for, effectively separates regulated and constitutive thermal dissipation processes, does not require the use of F(o) and F'(o) measurements and gives very similar results to the method proposed by Kramer et al. [(2004) Photosynth Res 79: 209-218]. We demonstrate the use of the technique using chlorophyll fluorescence measurements in grapevine leaves and observe a high dependence on thermal dissipation processes (up to 75%) at both high light and low temperature.

ADP-Glucose Pyrophosphorylase: A Regulatory Enzyme for Plant Starch Synthesis.

Abstract: In plants, the synthesis of starch occurs by utilizing ADP-glucose as the glucosyl donor for the elongation of α-1,4-glucosidic chains. In photosynthetic bacteria the synthesis of glycogen follows a similar pathway. The first committed step in these pathways is the synthesis of ADP-glucose in a reaction catalyzed by ADP-glucose pyrophosphorylase (ADPGlc PPase). Generally, this enzyme is allosterically regulated by intermediates of the major carbon assimilatory pathway in the respective organism. In oxygenic photosynthesizers, ADPGlc PPase is mainly regulated by 3-phosphoglycerate (activator) and inorganic orthophosphate (inhibitor), interacting in four different patterns. Recent reports have shown that in higher plants, some of the enzymes could also be redox regulated. In eukaryotes, the enzyme is a heterotetramer comprised of two distinct subunits, a catalytic and a modulatory subunit. The latter has been proposed as related to variations in regulation of the enzyme in different plant tissues. Random and site-directed mutagenesis experiments of conserved amino acids revealed important residues for catalysis and regulation. Prediction of the ADPGlc PPase secondary structure suggests that it shares a common folding pattern to other sugar-nucleotide pyrophosphorylases, and they evolved from a common ancestor.

The super-excess energy dissipation in diatom algae: comparative analysis with higher plants.

Abstract: When grown at intermittent light regime, diatom alga Phaeodactylum tricornutum is able to form photoprotective non-photochemical chlorophyll fluorescence quenching (NPQ) three to five times larger than that observed in the higher plants. This quenching is sustained in the dark for 5 to 10 min, reverses completely within approximately 1 h and seems to be very tightly related to the presence of the zeaxanthin analogue, diatoxanthin. Addition of the uncoupler NH4Cl before illumination can completely abolish formation of NPQ, revealing the ΔpH-dependency of the xanthophyll cycle activity. Once established, NPQ can also be almost completely reversed by the uncoupler. However, the higher NPQ is formed the more time is required for its reversal. At the point when the fluorescence was approximately 90% recovered the level of illumination-induced diatoxanthin was found to be only partially reduced. This indicates that the proton gradient is a key triggering factor of NPQ. It was also noticed that NPQ in Phaeodactylum cells was absent even when majority of reaction centers were closed and the plastoquinone pool was significantly reduced. The absence of NPQ at these conditions could be due to very low levels of ΔpH. It is likely that in diatoms alternative sources of protons such as the PS I cyclic electron transfer and/or chlororespiration are important in generating the proton gradient sufficient to trigger NPQ. Absorption changes associated with the xanthophyll cycle activity were found to be larger than those for higher plants. The position of the positive maximum in the difference spectrum illuminated-minus-dark was 512–514 nm in comparison to the 505–508 nm for leaves. The 535 nm band associated with NPQ in plants is absent in Phaeodactylum. An uncoupler-sensitive absorption change at 522 nm was discovered. Kinetics of NPQ showed linear correlation with the 522 nm absorption change.

Sulfur assimilation and the role of sulfur in plant metabolism: a survey.

Abstract: Sulfur occurs in two major amino-acids, cysteine (Cys) and methionine (Met), essential for the primary and secondary metabolism of the plant. Cys, as the first carbon/nitrogen-reduced sulfur product resulting from the sulfate assimilation pathway, serves as a sulfur donor for Met, glutathione, vitamins, co-factors, and sulfur compounds that play a major role in the growth and development of plant cells. This sulfur imprinting occurs in a myriad of fundamental processes, from photosynthesis to carbon and nitrogen metabolism. Cys and Met occur in proteins, with the former playing a wide range of functions in proteins catalysis. In addition, the sulfur atom in proteins forms part of a redox buffer, as for glutathione, through specific detoxification/protection mechanisms. In this review, a survey of sulfur assimilation from sulfate to Cys, Met and glutathione is presented with highlights on open questions on their respective biosynthetic pathways and regulations that derived from recent findings. These are addressed at the biochemical and molecular levels with respect to the fate of Cys and Met throughout the plant-cell metabolism.

Thinking about the evolution of photosynthesis.

Abstract: Photosynthesis is an ancient process on Earth. Chemical evidence and recent fossil finds indicate that cyanobacteria existed 2.5–2.6 billion years (Ga) ago, and these were certainly preceded by a variety of forms of anoxygenic photosynthetic bacteria. Carbon isotope data suggest autotrophic carbon fixation was taking place at least a billion years earlier. However, the nature of the earliest photosynthetic organisms is not well understood. The major elements of the photosynthetic apparatus are the reaction centers, antenna complexes, electron transfer complexes and carbon fixation machinery. These parts almost certainly have not had the same evolutionary history in all organisms, so that the photosynthetic apparatus is best viewed as a mosaic made up of a number of substructures each with its own unique evolutionary history. There are two schools of thought concerning the origin of reaction centers and photosynthesis. One school pictures the evolution of reaction centers beginning in the prebiotic phase while the other school sees reaction centers evolving later from cytochrome b in bacteria. Two models have been put forth for the subsequent evolution of reaction centers in proteobacteria, green filamentous (non-sulfur) bacteria, cyanobacteria, heliobacteria and green sulfur bacteria. In the selective loss model the most recent common ancestor of all subsequent photosynthetic systems is postulated to have contained both RC1 and RC2. The evolution of reaction centers in proteobacteria and green filamentous bacteria resulted from the loss of RC1, while the evolution of reaction centers in heliobacteria and green sulfur bacteria resulted from the loss of RC2. Both RC1 and RC2 were retained in the cyanobacteria. In the fusion model the most recent common ancestor is postulated to have given rise to two lines, one containing RC1 and the other containing RC2. The RC1 line gave rise to the reaction centers of heliobacteria and green sulfur bacteria, and the RC2 line led to the reaction centers of proteobacteria and green filamentous bacteria. The two reaction centers of cyanobacteria were the result of a genetic fusion of an organism containing RC1 and an organism containing RC2. The evolutionary histories of the various classes of antenna/light-harvesting complexes appear to be completely independent. The transition from anoxygenic to oxygenic photosynthesis took place when the cyanobacteria learned how to use water as an electron donor for carbon dioxide reduction. Before that time hydrogen peroxide may have served as a transitional donor, and before that, ferrous iron may have been the original source of reducing power.

X-Ray Structure of Rhodobacter Capsulatus Cytochrome bc (1): Comparison with its Mitochondrial and Chloroplast Counterparts.

Abstract: Ubihydroquinone: cytochrome (cyt)c oxidoreductase, or cyt bc 1, is a widespread, membrane integral enzyme that plays a crucial role during photosynthesis and respiration. It is one of the major contributors of the electrochemical proton gradient, which is subsequently used for ATP synthesis. The simplest form of the cyt bc 1 is found in bacteria, and it contains only the three ubiquitously conserved catalytic subunits: the Fe–S protein, cyt b and cyt c 1. Here we present a preliminary X-ray structure of Rhodobacter capsulatus cyt bc 1 at 3.8 A and compare it to the available structures of its homologues from mitochondria and chloroplast. Using the bacterial enzyme structure, we highlight the structural similarities and differences that are found among the three catalytic subunits between the members of this family of enzymes. In addition, we discuss the locations of currently known critical mutations, and their implications in terms of the cyt bc 1 catalysis.

Alternative Photosystem I-Driven Electron Transport Routes: Mechanisms and Functions

Abstract: In addition to the linear electron transport, several alternative Photosystem I-driven (PS I) electron pathways recycle the electrons to the intersystem electron carriers mediated by either ferredoxin:NADPH reductase, NAD(P)H dehydrogenase, or putative ferredoxin:plastoquinone reductase. The following functions have been proposed for these pathways: adjustment of ATP/NADPH ratio required for CO2 fixation, generation of the proton gradient for the down-regulation of Photosystem II (PS II), and ATP supply the active transport of inorganic carbon in algal cells. Unlike ferredoxin-dependent cyclic electron transport, the pathways supported by NAD(P)H can function in the dark and are likely involved in chlororespiratory-dependent energization of the thylakoid membrane. This energization may support carotenoid biosynthesis and/or maintain thylakoid ATPase in active state. Active operation of ferredoxin-dependent cyclic electron transport requires moderate reduction of both the intersystem electron carriers and the acceptor side of PS I, whereas the rate of NAD(P)H-dependent pathways under light depends largely on NAD(P)H accumulation in the stroma. Environmental stresses such as photoinhibition, high temperatures, drought, or high salinity stimulated the activity of alternative PS I-driven electron transport pathways. Thus, the energetic and regulatory functions of PS I-driven pathways must be an integral part of photosynthetic organisms and provides additional flexibility to environmental stress.

Novel Insights into the Enzymology, Regulation and Physiological Functions of Light-dependent Protochlorophyllide Oxidoreductase in Angiosperms.

Abstract: The reduction of protochlorophyllide (Pchlide) is a key regulatory step in the biosynthesis of chlorophyll in phototrophic organisms. Two distinct enzymes catalyze this reduction; a light-dependent NADPH:protochlorophyllide oxidoreductase (POR) and light-independent Pchlide reductase (DPOR). Both enzymes are widely distributed among phototrophic organisms with the exception that only POR is found in angiosperms and only DPOR in anoxygenic photosynthetic bacteria. Consequently, angiosperms become etiolated in the absence of light, since the reduction of Pchlide in angiosperms is solely dependent on POR. In eukaryotic phototrophs, POR is a nuclear-encoded single polypeptide and post-translationally imported into plastids. POR possesses unique features, its light-dependent catalytic activity, accumulation in plastids of dark-grown angiosperms (etioplasts) via binding to its substrate, Pchlide, and cofactor, NADPH, resulting in the formation of prolamellar bodies (PLBs), and rapid degradation after catalysis under subsequent illumination. During the last decade, considerable progress has been made in the study of the gene organization, catalytic mechanism, membrane association, regulation of the gene expression, and physiological function of POR. In this review, we provide a brief overview of DPOR and then summarize the current state of knowledge on the biochemistry and molecular biology of POR mainly in angiosperms. The physiological and evolutional implications of POR are also discussed.

The glutaredoxin family in oxygenic photosynthetic organisms.

Abstract: Glutaredoxins (GRXs) are small redox proteins of the thioredoxin (TRX) superfamily. Compared to TRXs, much less information on the GRX family is available, especially in photosynthetic organisms since GRXs have been mainly studied in E. coli, yeast and mammal cells. The analysis of the TRX family in oxygenic photosynthetic organisms revealed an unsuspected multiplicity of TRXs but it is not known if the same situation holds for GRXs. Despite the availability of genome sequences from different oxygenic photosynthetic organisms, the number of GRXs and the different groups present in these organisms are still undescribed. This paper presents a comparative analysis of the GRX families present in Arabidopsis, Chlamydomonas and Synechocystis which were found to contain 30, 6 and 3 GRX genes, respectively. The putative subcellular localization of each GRX and its relative expression level, based on EST data, have been investigated. This analysis reveals the presence of three major classes of GRXs, the CPYC type, the CGFS type and a previously undescribed type, called the CC type that appears specific to higher plants. These data are discussed in view of recent results suggesting a complex cross-regulation between the TRX and GRX systems.

The Chlamydomonas genome reveals its secrets: chaperone genes and the potential roles of their gene products in the chloroplast

Abstract: The first draft of the Chlamydomonas nuclear genome was searched for genes potentially encoding members of the five major chaperone families, Hsp100/Clp, Hsp90, Hsp70, Hsp60, the small heat shock proteins, and the Hsp70 and Cpn60 co-chaperones GrpE and Cpn10/20, respectively. This search yielded 34 potential (co-)chaperone genes, among them those 8 that have been reported earlier inChlamydomonas. These 34 genes encode all the (co-)chaperones that have been expected for the different compartments and organelles from genome searches in Arabidopsis, where 74 genes have been described to encode basically the same set of (co-)chaperones. Genome data from Arabidopsis and Chlamydomonas on the five major chaperone families are compared and discussed, with particular emphasis on chloroplast chaperones.

Structure and function of plant-type ferredoxins

Abstract: The plant-type ferredoxins (Fds) are the [2Fe-2S] proteins that function primarily in photosynthesis; they transfer electrons from photoreduced Photosystem I to ferredoxin NADP(+) reductase in which NADPH is produced for CO(2) assimilation. In addition, Fds partition electrons to various ferredoxin-dependent enzymes not only for assimilations of inorganic nitrogen and sulfur and N(2) fixation but also for regulation of CO(2) assimilation cycle. Although Fds are small iron-sulfur proteins with molecular weight of 11 KDa, they are expected to interact with surprisingly many enzymes. Several Fd isoforms were found in non-photosynthetic cells as well as Fds in photosynthetic cells, leading to the recognition that they have differentiated physiological roles. In a quarter of century, X-ray crystallography and NMR spectroscopy provided wealth of structural data, which shed light on the structure-function relationship of the plant-type Fds and gave structural basis for the biochemical and spectroscopic properties so far accumulated. Thus the structural studies of Fds have come to a new era in which different roles of Fds and interactions with various enzymes are clarified on the basis of the tertiary and quaternary structures, although they are premature at present. This article reviews briefly the structures of the plant-type Fds together with their functions, properties, and interactions with Fd related enzymes. Lastly the folding motif of Fd, that has grown to be a large family by including many functionally unrelated proteins, is noted.

Structure, function and assembly of Photosystem II and its light-harvesting proteins.

Abstract: Photosystem II (PSII) is a multisubunit chlorophyll-protein complex that drives electron transfer from water to plastoquinone using energy derived from light. In green plants, the native form of PSII is surrounded by the light-harvesting complex (LHCII complex) and thus it is called the PSII-LHCII supercomplex. Over the past several years, understanding of the structure, function, and assembly of PSII and LHCII complexes has increased considerably. The unicellular green alga Chlamydomonas reinhardtii has been an excellent model organism to study PSII and LHCII complexes, because this organism grows heterotrophically and photoautotrophically and it is amenable to biochemical, genetic, molecular biological and recombinant DNA methodology. Here, the genes encoding and regulating components of the C. reinhardtii PSII-LHCII supercomplex have been thoroughly catalogued: they include 15 chloroplast and 20 nuclear structural genes as well as 13 nuclear genes coding for regulatory factors. This review discusses these molecular genetic data and presents an overview of the structure, function and assembly of PSII and LHCII complexes.

Analysis of the Structure of the PsbO Protein and its Implications.

Abstract: The PsbO protein is a ubiquitous extrinsic subunit of Photosystem II (PS II), the water splitting enzyme of photosynthesis. A recently determined 3D X-ray structure of a cyanobacterial protein bound to PS II has given an opportunity to conduct complete analyses of its sequence and structural characteristics using bioinformatic methods. Multiple sequence alignments for the PsbO family are constructed and correlated with the cyanobacterial structure. We identify the most conserved regions of PsbO and the mapping of their positions within the structure indicates their functional roles especially in relation to interactions of this protein with the lumenal surface of PS II. Homologous models for eukaryotic PsbO were built in order to compare with the prokaryotic protein. We also explore structural homology between PsbO and other proteins for which 3D structures are known and determine its structural classification. These analyses contribute to the understanding of the function and evolutionary origin of the PS II manganese stabilising protein.

The FMO Protein.

Abstract: In this article I review the history of research on the Fenna-Matthews-Olson (FMO) protein with emphasis on my contributions. The FMO protein, which transfers energy from the chlorosome to the reaction center in green sulfur bacteria, was discovered in 1962 and shown to contain bacteriochlorophyll a. From the absorption and circular dichroism spectra, it was clear that there was an exciton interaction between the bacteriochlorophyll molecules. Low temperature spectra indicated a seven-fold exciton splitting of the Qy band. The FMO protein was crystallized in 1964, and the X-ray structure determined in 1979 by B.W. Matthews, R.E. Fenna, M.C. Bolognesi, M.F. Schmidt and J.M. Olson. The structure showed that the protein consisted of three subunits, each containing seven bacteriochlorophyll molecules. The optical spectra were satisfactorily simulated in 1997. In living cells the FMO protein is located between the chlorosome and the reaction centers with the C3 symmetry axis perpendicular to the membrane. The FMO protein may be related to PscA in the reaction center.

Discoveries in oxygenic photosynthesis (1727-2003): a perspective.

Abstract: We present historic discoveries and important observations, related to oxygenic photosynthesis, from 1727 to 2003. The decision to include certain discoveries while omitting others has been difficult. We are aware that ours is an incomplete timeline. In part, this is because the function of this list is to complement, not duplicate, the listing of discoveries in the other papers in these history issues of Photosynthesis Research. In addition, no one can know everything that is in the extensive literature in the field. Furthermore, any judgement about significance presupposes a point of view. This history begins with the observation of the English clergyman Stephen Hales (1677–1761) that plants derive nourishment from the air; it includes the definitive experiments in the 1960–1965 period establishing the two-photosystem and two-light reaction scheme of oxygenic photosynthesis; and includes the near-atomic resolution of the structures of the reaction centers of these two Photosystems, I and II, obtained in 2001–2002 by a team in Berlin, Germany, coordinated by Horst Witt and Wolfgang Saenger. Readers are directed to historical papers in Govindjee and Gest [(2002a) Photosynth Res 73: 1–308], in Govindjee, J. Thomas Beatty and Howard Gest [(2003a) Photosynth Res 76: 1–462], and to other papers in this volume for a more complete picture. Several photographs are provided here. Their selection is based partly on their availability to the authors (see Figures 1-15). Readers may view other photographs in Part 1 (Volume 73, Photosynth Res, 2002), Part 2 (Volume 76, Photosynth Res, 2003) and Part 3 (Volume 80, Photosynth Res, 2004) of the history issues of Photosynthesis Research. Photographs of most of the Nobel-laureates are included in Govindjee, Thomas Beatty and John Allen, this volume. For a complementary time line of anoxygenic photosynthesis, see H. Gest and R. Blankenship (this volume).

Loss of Functional Photosystem II Reaction Centres in Zooxanthellae of Corals Exposed to Bleaching Conditions: Using Fluorescence Rise Kinetics.

Abstract: Mass coral bleaching is linked to elevated sea surface temperatures, 1–2 °C above average, during periods of intense light. These conditions induce the expulsion of zooxanthellae from the coral host in response to photosynthetic damage in the algal symbionts. The mechanism that triggers this release has not been clearly established and to further our knowledge of this process, fluorescence rise kinetics have been studied for the first time. Corals that were exposed to elevated temperature (33 °C) and light (280 μmol photons m−2 s−1), showed distinct changes in the fast polyphasic induction of chlorophyll-a fluorescence, indicating biophysical changes in the photochemical processes. The fluorescence rise over the first 2000ms was monitored in three species of corals for up to 8 h, with a PEA fluorometer and an imaging-PAM. Pocillopora damicornis showed the least impact on photosynthetic apparatus, while Acropora nobilis was the most sensitive, with Cyphastrea serailia intermediate between the other two species. A. nobilis showed a remarkable capacity for recovery from bleaching conditions. For all three species, a steady decline in the slope of the initial rise and the height of the J-transient was observed, indicating the loss of functional Photosystem II (PS II) centres under elevated-temperature conditions. A significant loss of PS II centres was confirmed by a decline in photochemical quenching when exposed to bleaching stress. Non-photochemical quenching was identified as a significant mechanism for dissipating excess energy as heat under the bleaching conditions. Photophosphorylation could explain this decline in PS II activity. State transitions, a component of non-photochemical quenching, was a probable cause of the high non-photochemical quenching during bleaching and this mechanism is associated with the phosphorylation-induced dissociation of the light harvesting complexes from the PS II reaction centres. This reversible process may account for the coral recovery, particularly in A. nobilis.

Repair of DNA Damage Induced by Solar UV

Abstract: Solar UV radiation induces significant levels of DNA damage in living things. This damage, if left unrepaired, is lethal in humans. Recent work has demonstrated that plants possess several repair pathways for UV-induced DNA damage, including pathways for the photoreactivation of both 6-4 products and cyclobutane pyrimidine dimers (CPDs), the two lesions most frequently induced by UV. Plants also possess the more general nucleotide excision repair (NER) pathway as well as bypass polymerases that enable the plant to replicate its DNA in the absence of DNA repair.

Rings, ellipses and horseshoes: how purple bacteria harvest solar energy.

Abstract: This Review summarises the current state of research on the structure and function of light-harvesting apparatus in purple photosynthetic bacteria. Particular emphasis is placed on the major open questions still outstanding in this field in addition to what is already known.

Trails of green alga hydrogen research - from hans gaffron to new frontiers.

Abstract: This paper summarizes aspects of the history of photosynthetic hydrogen research, from the pioneering discovery of Hans Gaffron over 60 years ago to the potential exploitation of green algae in commercial H2-production. The trail started as a mere scientific curiosity, but promises to be a most important discovery, one that leads photosynthesis research to important commercial applications. Progress achieved in the field of photosynthetic hydrogen production by green algae includes elucidation of the mechanism, the ability to modify photosynthesis by physiological means and to produce bulk amounts of H2 gas, and cloning of the [Fe]-hydrogenase genes in several green algal species.

Fe Resupply to Fe-deficient Sugar Beet Plants Leads to Rapid Changes in the Violaxanthin Cycle and other Photosynthetic Characteristics without Significant de novo Chlorophyll Synthesis.

Abstract: The effects of Fe resupply to Fe-deficient plants have been investigated in hydroponically-grown sugar beet. In the short-term (24 h) after Fe resupply, major changes were observed, although de novo chlorophyll (Chl) synthesis had not begun yet. Approximately 50% of the zeaxanthin was converted into violaxanthin, whereas the actual Photosystem II (PS II) efficiency increased by 69% and non-photochemical quenching (NPQ) and the amount of thermally dissipated energy decreased markedly (by 47% and 40%, respectively). At the same time, photosynthetic rate increased approximately by 50%. From one to two days after Fe resupply, there was a gradual increase in the leaf concentrations of Chl and other photosynthetic pigments, accompanied by a further conversion of zeaxanthin into violaxanthin, increases in actual PS II efficiency and photosynthetic rates and decreases in NPQ and the amount of thermally dissipated energy. At 72–96 h after Fe resupply, leaf pigment concentrations, photosynthetic rates and actual PS II efficiency had increased further, although both photosynthetic rate and leaf pigment concentrations were still lower than those found in Fe-sufficient leaves. Good correlations were observed between the amount of light thermally dissipated by the PS II antenna, NPQ and the antheraxanthin + zeaxanthin concentration after Fe resupply, confirming the photoprotective role of the xanthophyll cycle in Fe-deficient sugar beet leaves. Similar correlations were observed for lutein, suggesting a possible role of this pigment in photoprotection.

The Q-cycle - A Personal Perspective.

Abstract: In this Minireview, I provide an overview of the developments over the period 1970 to 1990 that led to the current view of the Q-cycle mechanism of the cytochrome bc 1 complex. The perspective is necessarily personal, and places some emphasis on research on the complex in the photosynthetic bacteria, where the kinetics could be studied in situ and with better time resolution than in mitochondria. Peter Mitchell’s original Q-cycle underwent several early revisions. The version of the Q-cycle currently accepted in most labs owed much to a perceptive critique by Peter Garland, who proposed a modified Q-cycle that allowed the complex to act independently. This was among several variants discussed by Mitchell in a seminal review from 1976. Six years later, despite significant advances in both mitochondrial and bacterial work, discrimination between the half-dozen or so variants that remained in active contention had proved elusive, and the kinetic data from both mitochondrial and photosynthetic systems was refractory. This was the basis of my own opposition to the Q-cycle. While trying to explain this opposition to an undergraduate student in the lab I was led to a re-evaluation of the kinetic data in the light of the substantial advances in our understanding of the biochemistry and thermodynamic properties of the complex. From this it became apparent that one version of the Q-cycle could account with satisfactory economy for the data from the photosynthetic bacteria, and for most results from work with mitochondrial complexes. The resulting model was highly constrained, and, since it incorporated Garland’s suggestions for an independent mechanism, was called the modified Q-cycle. The modified Q-cycle has stood the test of time well, and the recent structural information has both confirmed the general mechanism, and allowed extension to a more detailed understanding of the molecular architecture, and the relation between structure and function.

Proteomics Uncovers Proteins Interacting Electrostatically with Thioredoxin in Chloroplasts

Abstract: The ability of thioredoxin f to form an electrostatic (non-covalent) complex, earlier found with fructose-1,6-bisphosphatase, was extended to include 27 previously unrecognized proteins functional in 11 processes of chloroplasts. The proteins were identified by combining thioredoxin f affinity chromatography with proteomic analysis using tandem mass spectrometry. The results provide evidence that an association with thioredoxin enables the interacting protein to achieve an optimal conformation, so as to facilitate: (i) the transfer of reducing equivalents from the ferredoxin/ferredoxin—thioredoxin reductase complex to a target protein; (ii) in some cases, to enable the channeling of metabolite substrates; (iii) to function as a subunit in the formation of multienzyme complexes.

Structure and function of the water-soluble carotenoid-binding proteins of cyanobacteria.

Abstract: The orange carotenoid protein (OCP) and its derivative, the red carotenoid protein (RCP), appear to play important photoprotective roles in cyanobacteria. Structural and functional characterization is gradually elucidating the specific details of how carotenoid-protein interactions, including the role of six methionine residues oriented toward the pigment, contribute to the spectral and functional properties of these proteins.

Interaction of Ferredoxin–NADP+ Reductase with its Substrates: Optimal Interaction for Efficient Electron Transfer

Abstract: Electron transfer (ET) reactions in systems involving proteins require an oriented interaction between electron donor and acceptor in order to accommodate their respective redox centres in optimal orientation for efficient ET. Such type of reactions are critical for the maintenance of the physiological functions of living organisms, since they are implicated in vital actions, as is, for example, in the photosynthetic ET chain that leads to NADPH reduction. In this particular case, a small redox protein ET chain is responsible for ET from Photosystem I (PS I) to NADP(+). In this system the enzyme responsible for NADP(+) reduction is ferredoxin-NADP(+) reductase (FNR), a FAD-containing NADP(+) dependent reductase. In order to produce such reduction, this enzyme receives electrons from a [2Fe-2S] plant-type ferredoxin (Fd), which is previously reduced by PS I. Moreover, in the case of some algae and cyanobacteria, an FMN-dependent protein, flavodoxin (Fld), has been shown to replace Fd in this function. The processes of interaction and ET between FNR and all of its substrates involved in the photosynthetic ET chain, namely Fd, Fld and NADP(+)/H have been extensively investigated in recent years using a large number of techniques, including the introduction of site-specific mutations in combination with kinetic and structural studies of the produced mutants. The present manuscript summarises the information so far reported for an efficient interaction between FNR and its substrates, compares such information with that revealed by other systems for which the FNR structure is a prototype and, finally, discusses the implications of the processes of association in ET between FNR and its substrates.

Nitrogen-fixing symbiosis between photosynthetic bacteria and legumes

Abstract: Rhizobia having photosynthetic systems form nitrogen-fixing nodules on the stem and/or root of some species of the legumes Aeschynomene and Lotononis. This review is focused on the recent knowledge about the physiology, genetics and role of the photosystem in these bacteria. Photosynthetic electron transport seems to involve reaction centers, soluble cytochrome c2 and cytochrome bc1. Anaerobically, the electron transport system becomes over-reduced. The photosynthesis genes have been partially characterized; their organization is classical but their regulation is unusual as it is activated by far-red light via a bacteriophytochrome. This original mechanism of regulation seems well adapted to promote photosynthesis during stem symbiosis. Photosynthesis plays a major role in the efficiency of stem nodulation. It is also observed that infrared light stimulates nitrogen fixation in nodules containing photosynthetic bacteroids, suggesting that photosynthesis may additionally provides energy for nitrogen fixation, allowing for more efficient plant growth. Other aspects of these bacteria are discussed, in particular their taxonomic position and nodulation ability, the role of carotenoids and the potential for application of photosynthetic rhizobia in rice culture.

Chloroplast RNA Processing and Stability

Abstract: Primary chloroplast transcripts are processed in a number of ways, including intron splicing, internal cleavage of polycistronic RNAs, and endonucleolytic or exonucleolytic cleavages at the transcript termini. All chloroplast RNAs are also subject to degradation, although a curious feature of many chloroplast mRNAs is their relative longevity. Some of these processes, e.g., psbA splicing and stability of a number of chloroplast mRNAs, are regulated in response to light–dark cycles or nutrient availability. This review highlights recent advances in our understanding of these processes in the model organism Chlamydomonas reinhardtii, focusing on results since the extensive reviews published in 1998 [Herrin DL et al. 1998 (pp. 183–195), Nickelsen Y 1998 (pp. 151–163), Stern DB and Drager RG 1998 (pp. 164–182), in Rochaix JD et al. (eds) The Molecular Biology of Chloroplasts and Mitochondria in Chlamydomonas. Kluwer Academic Publishers, Dordrecht, The Netherlands]. We also allude to studies with other organisms, and to the potential impact of the Chlamydomonas genome project where appropriate.

Aerobic Phototrophic Bacteria: New Evidence for the Diversity, Ecological Importance and Applied Potential of this Previously Overlooked Group

Abstract: The aerobic phototrophic bacteria are a recently discovered group capable of producing a photosynthetic apparatus similar to that of purple phototrophic bacteria. However, this apparatus, in contrast to that of their anaerobic counterparts, is functional in terms of photoinduced electron transport only under aerobic conditions. Although these bacteria have been widely studied, little is yet known about their ecological importance, and why they differ from other anoxygenic phototrophs with respect to oxygen requirements. In recent years a large number of new genera and species have been described from a wide variety of habitats, and evidence has been presented to support their important ecological role. This minireview focuses on recent discoveries regarding taxonomy, ecology and physiology, as well as the latest advances in the understanding of their photosynthetic apparatus and its genetic regulation.

Photorespiration Contributes to Stomatal Regulation and Carbon Isotope Fractionation: A Study with Barley, Potato and Arabidopsis Plants Deficient in Glycine Decarboxylase

Abstract: The rates of respiration in light and darkness, C i/C a and carbon isotope fractionation were investigated in glycine decarboxylase-deficient plants of barley, potato and Arabidopsis thaliana grown in climate chambers with controlled light intensity, temperature, humidity, irradiation and different CO2 concentrations (360, 700 and 1400 µl l−1) and compared to the wild-type plants All photorespiration-impaired plants exhibited higher C i/C a and corresponding lower apparent water-use efficiencies, which were more expressed under high irradiance and elevated temperature The mutants were depleted in 13C as compared to the wild-type plants, with a difference of up to 6‰ following growth in 360 µl l−1 CO2 We determined the carbon isotope content at different CO2 concentrations to calculate the contribution of both C i/C a and photorespiration for 13C/12C fractionation The direct effect of photorespiration was in the range of 07–10‰, from which we calculated the value of fractionation at the site of glycine decarboxylation as being 10–13‰, which is in agreement with the previously reported carbon isotope discrimination exerted by the glycine decarboxylase Respiratory rates, particularly in the light, were increased in the glycine decarboxylase mutants The necessity of the maintenance of a high CO2 concentration near the site of carboxylation in chloroplasts in plants deficient in photorespiratory enzymes, requires an increased opening of the stomata with a corresponding decrease in water-use efficiency It is concluded that photorespiration participates in the regulation of C i/C a and contributes to carbon isotope fractionation, both via effects on stomata and via discrimination of 13C in the glycine decarboxylase reaction

High-Temperature Induced Chlorophyll Fluorescence Rise in Plants at 40–50 °C: Experimental and Theoretical Approach

Abstract: We studied the temperature dependence of chlorophyll fluorescence intensity in barley leaves under weak and actinic light excitation during linear heating from room temperature to 50 °C. The heat-induced fluorescence rise usually appearing at around 40–50 °C under weak light excitation was also found in leaves treated with 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) or hydroxylamine (NH2OH). However, simultaneous treatment with both these compounds caused a disappearance of the fluorescence rise. We have suggested that the mechanism of the heat-induced fluorescence rise in DCMU-treated leaves is different than that in untreated or NH2OH-treated leaves. In DCMU-treated leaves, the heat-induced fluorescence rise reflects an accumulation of QA − even under weak light excitation due to the thermal inhibition of the S2QA − recombination as was further documented by a decrease in the intensity of the thermoluminescence Q band. Mathematical model simulating this experimental data also supports our interpretation. In the case of DCMU-untreated leaves, our model simulations suggest that the heat-induced fluorescence rise is caused by both the light-induced reduction of QA and enhanced back electron transfer from QB to QA. The simulations also revealed the importance of other processes occurring during the heat-induced fluorescence rise, which are discussed with respect to experimental data.