Interrelated modules in cyanobacterial photosynthesis: the carbon-concentrating mechanism, photorespiration, and light perception.

Here we consider the cyanobacterial carbon-concentrating mechanism (CCM) and photorespiration in the context of the regulation of light harvesting, using a conceptual framework borrowed from engineering: modularity. Broadly speaking, biological 'modules' are semi-autonomous functional units such as protein domains, operons, metabolic pathways, and (sub)cellular compartments. They are increasingly recognized as units of both evolution and engineering. Modules may be connected by metabolites, such as NADPH, ATP, and 2PG. While the Calvin-Benson-Bassham Cycle and photorespiratory salvage pathways can be considered as metabolic modules, the carboxysome, the core of the cyanobacterial CCM, is both a structural and a metabolic module. In photosynthetic organisms, which use light cues to adapt to the external environment and which tune the photosystems to provide the ATP and reducing power for carbon fixation, light-regulated modules are critical. The primary enzyme of carbon fixation, RuBisCO, uses CO2 as a substrate, which is accumulated via the CCM. However RuBisCO also has a secondary reaction in which it utilizes O2, a by-product of the photochemical modules, which leads to photorespiration. A complete understanding of the interplay among CCM and photorespiration is predicated on uncovering their connections to the light reactions and the regulatory factors and pathways that tune these modules to external cues. We probe this connection by investigating light inputs into the CCM and photorespiratory pathways in the chromatically acclimating cyanobacterium Fremyella diplosiphon.

The regulatory interplay between photorespiration and photosynthesis.

The Calvin-Benson cycle and the photorespiratory pathway form the photosynthetic-photorespiratory supercycle that is responsible for nearly all biological CO2 fixation on Earth. In essence, supplementation with the photorespiratory pathway is necessary because the CO2-fixing enzyme of the Calvin-Benson cycle, ribulose 1,5-bisphosphate carboxylase (Rubisco), catalyses several side reactions including the oxygenation of ribulose 1,5-bisphosphate, which produces the noxious metabolite phosphoglycolate. The photorespiratory pathway recycles the phosphoglycolate to 3-phosphoglycerate and in this way allows the Calvin-Benson cycle to operate in the presence of molecular oxygen generated by oxygenic photosynthesis. While the carbon flow through the individual and combined subprocesses is well known, information on their regulatory interaction is very limited. Regulatory feedback from the photorespiratory pathway to the Calvin-Benson cycle can be presumed from numerous inhibitor experiments and was demonstrated in recent studies with transgenic plants. This complexity illustrates that we are not yet ready to rationally engineer photosynthesis by altering photorespiration since despite massive understanding of the core photorespiratory pathway our understanding of its interaction with other pathways and processes remains fragmentary.

Characterization of the nature of photosynthetic recovery of wheat seedlings from short-term dark heat exposures and analysis of the mode of acclimation to different light intensities.

The nature of photosynthetic recovery was investigated in 10-d-old wheat (Triticum aestivum L., cv. Moskovskaya-35) seedlings exposed to temperatures of 40 and 42 degrees C for 20 min and to temperature 42 degrees C for 40 min in the dark. The aftereffect of heat treatment was monitored by growing the heat-treated plants in low/moderate/high light at 20 degrees C for 72h. The net photosynthetic rates (P(N)) and the fluorescence ratios F(v)/F(m) were evaluated in intact primary leaves and the rates of cyclic and non-cyclic photophosphorylation were measured in the isolated thylakoids. At least two temporally separated steps were identified in the path of recovery from heat stress at 40 and 42 degrees C in the plants growing in high and moderate/high light, respectively. Both photochemical activity of the photosystem II (PSII) and the activity of CO(2) assimilation system were lowered during the first step in comparison with the corresponding activities immediately after heat treatment. During the second step, the photosynthetic activities completely or partly recovered. Recovery from heat stress at 40 degrees C was accompanied by an appreciably higher rate of cyclic photophosphorylation in comparison with control non-heated seedlings. In pre-heated seedlings, the tolerance of the PSII to photoinhibition was higher than in non-treated ones. The mode of acclimation to different light intensities after heat exposures is analyzed.

How energy funnels from the phycoerythrin antenna complex to photosystem I and photosystem II in cryptophyte Rhodomonas CS24 cells.

We report an investigation of energy migration dynamics in intact cells of the photosynthetic cryptophyte Rhodomonas CS24 using analyses of steady-state and time-resolved fluorescence anisotropy measurements. By fitting a specific model to the fluorescence data, we obtain three time scales (17, 58, and 113 ps) by which the energy is transferred from phycoerythrin 545 (PE545) to the membrane-associated chlorophylls (Chls). We propose that these time scales reflect both an angular distribution of PE545 around the photosystems and the relative orientations of the donor dihydrobiliverdin (DBV) bilin and the acceptor Chl. Contrary to investigations of the isolated antenna complex, it is demonstrated that energy transfer from PE545 does not occur from a single-emitting bilin, but rather both the peripheral dihydrobiliverdin (DBV) chromophores in PE545 appear to be viable donors of excitation energy to the membrane-bound proteins. The model shows an almost equal distribution of excitation energy from PE545 to both photosystem I (PSI) and photosystem II (PSII), whose trap times correspond well to those obtained from experiments on isolated photosystems.

Phytochrome phosphorylation in plant light signaling.

Reversible protein phosphorylation is a switching mechanism used in eukaryotes to regulate various cellular signalings. In plant light signaling, sophisticated photosensory receptor systems operate to modulate growth and development. The photoreceptors include phytochromes, cryptochromes and phototropins. Despite considerable progresses in defining the photosensory roles of these photoreceptors, the primary biochemical mechanisms by which the photoreceptor molecules transduce the perceived light signals into cellular responses remain to be elucidated. The signal-transducing photoreceptors in plants are all phosphoproteins and/or protein kinases, suggesting that light-dependent protein phosphorylation and dephosphorylation play important roles in the function of the photoreceptors. This review focuses on the role of phytochromes' reversible phosphorylation involved in the light signal transduction in plants.

Nitrate assimilation in plant shoots depends on photorespiration.

Photorespiration, a process that diminishes net photosynthesis by approximately 25% in most plants, has been viewed as the unfavorable consequence of plants having evolved when the atmosphere contained much higher levels of carbon dioxide than it does today. Here we used two independent methods to show that exposure of Arabidopsis and wheat shoots to conditions that inhibited photorespiration also strongly inhibited nitrate assimilation. Thus, nitrate assimilation in both dicotyledonous and monocotyledonous species depends on photorespiration. This previously undescribed role for photorespiration (i) explains several responses of plants to rising carbon dioxide concentrations, including the inability of many plants to sustain rapid growth under elevated levels of carbon dioxide; and (ii) raises concerns about genetic manipulations to diminish photorespiration in crops.

Identification and differential accumulation of two isoforms of the CF1-beta subunit under high light stress in Brassica rapa.

The chloroplast ATP synthase coupling factor CF1 complex contains five nonidentical subunits, alpha, beta, gamma, delta, and epsilon, with a stoichiometry of 3:3:1:1:1. The beta subunit contains the catalytic site for ATP synthesis during photooxidative phosphorylation in the chloroplast. In this study, we have identified two isoforms of the CF1-beta subunit at 56 and 54 kDa in the chloroplast of Brassica rapa, through isolation/purification, proteolytic digestion and internal peptide sequencing. Examining their accumulation pattern demonstrates that both isoforms coexist during chloroplast biogenesis and in mature thylakoid membranes, but the 54 kDa isoform is more apparently upregulated by light or under light stress. LiDS-PAGE shows that the 56 kDa is a major isoform of the CF1-beta subunit under normal light conditions, and its amount was not influenced during high light or other light stress treatments. The 54 kDa isoform is a minor band at normal conditions; however, it significantly increased under excess light stresses, such as high or low light with drought and/or high temperature. Particularly, a ninefold increase was observed after 8-10 h of high light treatment with drought and high temperature. The results suggest that light stress induction of the 54 kDa CF1-beta isoform may present a positive response during chloroplast photoacclimation.

Light-induced proton slip and proton leak at the thylakoid membrane.

A treatment of leaves of Spinacia oleracea L. with light or with the thiol reagent dithiothreitol in the dark led to partly uncoupled thylakoids. After induction in intact leaves, the partial uncoupling was irreversible at the level of isolated thylakoids. We distinguish between uncoupling by proton slip, which means a decrease of the H+/e(-) -ratio due to less efficient proton pumping, and proton leak as defined by enhanced kinetics of proton efflux. Proton slip and proton leak made about equal contributions to the total uncoupling. The enhanced proton efflux kinetics corresponded to reduction of subunit CF1-gamma of the ATP synthase as shown by fluorescence labeling of thylakoid proteins with the sulfhydryl probe 5-iodoacetamido fluorescein. The maximum value of the fraction of reduced CF1-gamma was only 36%, which indicates that in vivo the reduction of CF1-gamma could be limited by fast reoxidation and/or restricted accessibility of CF1-gamma to thioredoxin. Measurements of the ratio ATP/2e indicated that only the uncoupling related to less efficient proton pumping led to a decrease in the ATP yield.

Cyclic, pseudocyclic and noncyclic photophosphorylation: new links in the chain.

Photosynthetic electron transport is coupled to ATP synthesis. This process - photosynthetic phosphorylation - proceeds by several alternative electron-transport pathways in isolated chloroplasts. The question: 'Which of these works in real life?' has long occupied students of photosynthesis. Recent results from structural biology and genomics suggest that the answer is 'All of them'. The interplay between the pathways might explain the flexibility of photosynthesis in meeting different metabolic demands for ATP.

Guard-cell chloroplasts provide ATP required for H(+) pumping in the plasma membrane and stomatal opening.

To elucidate the role of guard-cell chloroplasts (GCCs) in stomatal movement, we investigated the effects of oligomycin, an inhibitor of oxidative phosphorylation, and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of photosystem II, on fusicoccin (FC)-induced H(+) pumping and stomatal opening. FC was found to induce H(+ )pumping in guard-cell protoplasts (GCPs) from Vicia faba and stomatal opening in the epidermis of Commelina benghalensis; and, red light (RL) slightly stimulated these responses. Oligomycin strongly inhibited the pumping and stomatal opening in the dark. RL partially reversed the inhibitions, and DCMU decreased the effect of RL. FC activated the plasma membrane H(+)-ATPase (EC 3.6.1.35) in GCPs similarly irrespective of these treatments, indicating that the H(+)-ATPase activity was not the limiting step in H(+) pumping. Oligomycin significantly decreased the ATP content in GCPs in the dark. RL partially reversed this effect, and DCMU eliminated the effect of RL. A significant part of the ATP produced by photophosphorylation to H(+) pumping was indicated under RL. These results suggest that GCCs supply ATP to the cytosol under RL, and that the ATP is utilized by the plasma membrane H(+)-ATPase for H(+) pumping.

Localization and light-dependent phosphorylation of white collar 1 and 2, the two central components of blue light signaling in Neurospora crassa.

In Neurospora crassa only two white collar (wc) mutants, wc-1 and wc-2, have been described that seem to be insensitive to light. The pleiotropic phenotypes of these mutants suggest that they represent two central components of blue light signal transduction. The WC proteins have several characteristics of transcription factors consistent with an involvement in transcriptional control of light-regulated genes. Here, we present a biochemical analysis of WC1 and WC2 polypeptides in N. crassa. Using specific antisera against WC1 and WC2, respectively, the subcellular localization of the WC polypeptides was investigated. The WC1 protein was localized exclusively in the nucleus, whereas WC2 was detected in both the nuclear and cytoplasmic fractions. The nuclear localization of WC1 and WC2 was shown to be independent of light and dimerization between the two proteins. In addition, WC1 and WC2 are phosphorylated in response to light. The phosphorylation of WC1 and WC2 was dependent on functional WC1 and WC2 proteins, respectively, which clearly indicated a correlation between the light-dependent phosphorylation and the function of WC1 and WC2 in blue light signaling. However, the light-specific phosphorylation of the WC proteins revealed different kinetics. The phosphorylation of WC1 was transient whereas the WC2 phosphorylation was shown to be stable under constant light conditions. The analysis of the light-dependent phosphorylation of WC1 and WC2 in wc-2 and wc-1 mutants revealed an epistatic relationship for WC1 and WC2 with WC2 acting downstream of WC1 in the signal transduction pathway of blue light.

The effect of microgravity on proton permeability of thylakoid membranes and contribution of II and I photosystems in photosynthetic electron transport in pea chloroplasts.

According to a number investigations microgravity conditions affect membrane apparatus of photosynthesis in cells of higher plants and alga [for review, see Kordyum et al., 1994; Kordyum, 1997]. (see for review). Chloroplasts of space-grown pea plants showed disintegration of grana, shrinkage of the membrane constituting the grana stacks and other structural perturbance of the photosynthetic membranes. However there have been no studies on the effect of microgravity on proton permeability of thylakoid membranes and closely connected with this parameter their photochemical characteristics. The aim of the study is investigation of microgravity effects on protonic permeability of photosynthetic membrane and contribution of photosystem II (PSII) and photosystem I (PSI) in electron transfer from water to potassium ferrycianide (FeCy) in isolated pea chloroplasts. Pea.