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PubMed2001

CD spectroscopy provides evidence for excitonic interactions involving red-shifted chlorophyll forms in photosystem I.

Selective destruction of the strongly dichroic red-shifted chlorophyll form (C709 nm) in photosystem I (PSI) trimers from Spirulina, by either non-selective high intensity illumination (photobleaching) or incubation with low concentrations of Triton X-100 is accompanied by changes in the circular dichroism spectrum of the same amplitude and of opposite sign at 677 nm. The data are interpreted in terms of a dimeric chlorophyll structure with excitonic bands at these two wavelengths. Similar photobleaching experiments with PSI-200 from maize also suggest the presence of bulk antenna/red form excitonic interactions.

Chlorophyll↗
PubMed2003

Exploring the ability of chlorophyll b to bind to the CP43' protein induced under iron deprivation in a mutant of Synechocystis PCC 6803 containing the cao gene.

Cyanobacteria, unlike plants and green algae, do not contain chlorophyll (Chl) b. This is because of the absence of the cao gene which encodes the enzyme that catalyses a two step oxygenation of chlorophyllide a to chlorophyllide b. Recently, however, the cao gene of higher plants was engineered into Synechocystis PCC 6803 leading to Chl b synthesis in this cyanobacterium [Satoh et al., J. Biol. Chem. 276 (2001) 4293-4297]. Here we use this same cao-plus mutant to show that Chl b can bind to the CP43' protein, expressed in cells exposed to low iron levels, which normally binds Chl a only. In so doing CP43' is changed to a Chl a/Chl b-binding protein and in this respect resembles the closely related Chl a/Chl b-binding Pcb protein of prochlorophytes (green oxyphotobacteria). The results emphasise the possibility of using an in vitro system to elucidate factors which control the binding of these two different forms of chlorophylls to the six transmembrane helical light-harvesting proteins of oxygenic photosynthetic organisms.

Bacterial Proteins↗
PubMed2003

The room temperature emission band shape of the lowest energy chlorophyll spectral form of LHCI.

Selective excitation, at room temperature, in the long wavelength absorption tail of the photosystem I antenna complexes, known as light harvesting complex I, induces pronounced pre-equilibration fluorescence from the directly excited pigment state. This has allowed determination of the fluorescence band shape of this low energy photosystem I chlorophyll antenna state, at room temperature, for the first time. The emission maximum is near 735 nm. The remarkable band width (55 nm) and asymmetry have never been previously reported for chlorophyll a states.

Chlorophyll↗
PubMed1997

Cytochrome b6/f complex from the cyanobacterium Synechocystis 6803: evidence of dimeric organization and identification of chlorophyll-binding subunit.

Fractionation of photosynthetic membranes from the cyanobacterium Synechocystis 6803 by polyacrylamide gel electrophoresis in the presence of Deriphat-160 allowed the isolation of a number of pigmented bands. Two of them, with molecular masses of 240+/-20 and 110+/-15 kDa respectively, showed peroxidase activity and, by means of polypeptide composition, immunoblotting and N-terminal sequencing, were identified as dimeric and monomeric cytochrome b6/f complexes, containing 1.3+/-0.35 chlorophyll molecules per cytochrome f. Further fractionation of monomeric complexes by mild gel electrophoresis in the presence of sodium dodecyl sulfate indicated that it is the cytochrome b6 polypeptide which provides the actual binding site for the chlorophyll molecule observed in the complex.

Bacterial Proteins↗
PubMed1998

Demonstration of thermal dissipation of absorbed quanta during energy-dependent quenching of chlorophyll fluorescence in photosynthetic membranes.

When plant leaves or chloroplasts are exposed to illumination that exceeds their photosynthetic capacity, photoprotective mechanisms such as described by the energy-dependent (non-photochemical) quenching of chlorophyll fluorescence are involved. The protective action is attributed to an increased rate constant for thermal dissipation of absorbed quanta. We applied photoacoustic spectroscopy to monitor thermal dissipation in spinach thylakoid membranes together with simultaneous measurement of chlorophyll fluorescence in the presence of inhibitors of opposite action on the formation of delta pH across the thylakoid membrane (tentoxin and nigericin/valinomycin). A linear relationship between the appearance of fluorescence quenching during formation of the delta pH and the reciprocal variation of thermal dissipation was demonstrated. Dicyclohexylcarbodiimide, which is known to prevent protonation of the minor light-harvesting complexes of photosystem II, significantly reduced the formation of fluorescence quenching and the concurrent increase in thermal dissipation. However, the addition of exogenous ascorbate to activate the xanthophyll de-epoxidase increased non-photochemical fluorescence quenching without affecting the measured thermal dissipation. It is concluded that a portion of energy-dependent fluorescence quenching that is independent of de-epoxidase activity can be readily measured by photoacoustic spectroscopy as an increase in thermal deactivation processes.

Ascorbic Acid↗
PubMed2002

Chlorophyll-deficient propagules of Avicennia marina and apparent longer term deterioration of mangrove fitness in oil-polluted sediments.

A correlation between petroleum hydrocarbon concentrations in sediments and chlorophyll-deficient mutations in mangroves may occur also in Australian mangroves. Earlier reports of such mutations in the Caribbean area were evident in viviparous propagules of the common mangrove genera, Rhizophora, borne on otherwise normal trees. These mutant propagules were termed albinos' since they lacked chlorophyll and normal green coloration, leaving them white, yellow or red. The mutation was considered lethal since newly established albino seedlings appeared unable to survive more than a few months. Our preliminary investigation of mangroves in SE Queensland found a similar mutation in another common mangrove genus, Avicennia, and this was apparently also correlated with oil concentrations in sediments. Although, more evidence is required, an apparently similar relationship shows that whatever caused the mutations may act commonly across a diverse range of plant types in quite separate locations. How widespread might this mutation be in mangroves? How many genera and species are affected? Are all occurrences correlated with oil in sediments? Does oil cause the mutation? We discuss these important questions and the potentially serious implications to coastal management where high mutant densities may be indicative of longer term genetic deterioration of mangrove habitat in oil-polluted wetland environments.

Avicennia↗
PubMed1998

NCC malonyltransferase catalyses the final step of chlorophyll breakdown in rape (Brassica napus).

Of the three final products of chlorophyll breakdown that in senescing cotyledons of oilseed rape are accumulated progressively, the nonfluorescent Bn-NCC-1 is the most abundant catabolite. It represents the malonylester of the minor catabolite Bn-NCC-3. The in vitro malonylation of Bn-NCC-3 into Bn-NCC-1 was investigated. Extracts from senescent as well as from presenescent cotyledons contained corresponding activities in the presence of malonyl-coenzyme A as the co-substrate. Malonyltransferase activity exhibited pH- and activation optima at 8 and 34 degrees, respectively, and it was saturable with an apparent Michaelis constant of 58 microM for Bn-NCC-3. The partially purified enzyme recognized chlorophyll catabolites as substrate specifically, provided that they had a free hydroxyl group in the ethyl side chain of pyrrole B.

Acyl-Carrier Protein S-Malonyltransferase↗
PubMed2002

A risk assessment of pollution: induction of atrazine tolerance in phytoplankton communities in freshwater outdoor mesocosms, using chlorophyll fluorescence as an endpoint.

We investigated the validity and sensitivity of assessments of the induction of atrazine tolerance in freshwater outdoor mesocosmic phytoplankton communities, using the in vivo fluorescence of chlorophyll a as an endpoint, for monitoring ecotoxicology and for risk assessment programs applied to phytoplankton contaminated by photosystem II herbicides. Atrazine inhibits the photosynthetic process, and so the rise in in-vivo fluorescence could be used as a physiological manifestation of acute toxicity. Short-term tests (1 h) were used, in which increasing concentrations of the herbicide were applied to phytoplankton samples taken every two days from the mesocosms, and used to plot dose-response curves. The concentration at which atrazine increased the fluorescence by 25% relative to control samples was used to demonstrate the sensitivity of the phytoplankton, and the values found were compared for samples from different mesocosms (contaminated and non-contaminated). The taxonomic composition of the phytoplankton was also determined. The data showed that chronic exposure (25 days) to 30 microg/L of atrazine significantly increased the apparent tolerance of the phytoplankton to further contamination by the same compound. The use of in vivo fluorescence of chlorophyll a appears to be a reliable and effective parameter for monitoring the effects of atrazine pollution, and detecting the changes in community tolerance driven by pollution selection pressure.

Adaptation, Physiological↗
PubMed2002

Kinetics and efficiency of excitation energy transfer from chlorophylls, their heavy metal-substituted derivatives, and pheophytins to singlet oxygen.

Time-resolved measurements of the singlet oxygen infrared (1269 nm) luminescence were used to follow the kinetics and efficiency of excitation energy transfer (EET) between chlorophyll (Chl) derivatives and oxygen in acetone. The studied pigments were Mg-Chl a and b and their heavy metal (Cu(2+) and Zn(2+))-substituted analogues, as well as pheophytin (Pheo) a and b. The efficiency of EET from chlorophyll to oxygen was highly dependent on the central ion in the pigment. Cu-Chl a and Cu-Chl b had the lowest efficiencies of singlet oxygen production, while Pheo a had a higher one, and Zn-Chl a had a similar one compared to Mg-Chl a. Also the side chain (position C-7, i.e. Chl a vs. Chl b) influenced the efficiency of singlet oxygen formation. In the case of square-planar complexes like Cu-Chl and Pheo, EET was more efficient in the Chl a derivatives than in those of Chl b; the opposite effect was observed in the case of the five- or six-coordinated Mg-Chl and Zn-Chl. As for the lifetime of the Chl triplet state, the most striking difference to Mg-Chl again was found in the case of Cu-Chls, which had much shorter lifetimes. Furthermore, the central ion in Chl affected the physical quenching of singlet oxygen: its efficiency was decreasing from Mg-Chl through Zn-Chl over Cu-Chl to Pheo. The results are discussed in the context of the oxidative stress accompanying heavy metal-induced stress in plants.

Chlorophyll↗
PubMed2002

Changes in the structure of chlorophyll-protein complexes and excitation energy transfer during photoinhibitory treatment of isolated photosystem I submembrane particles.

The activity of light-induced oxygen consumption, absorption spectra, low temperature (77 K) chlorophyll fluorescence emission and excitation spectra were studied in suspensions of photosystem (PS) I submembrane particles illuminated by 2000 microE m(-2) s(-1) strong white light (WL) at 4 degrees C. A significant stimulation of oxygen uptake was observed during the first 1-4 h of photoinhibitory treatment, which rapidly decreased during further light exposure. Chlorophyll (Chl) content gradually declined during the exposure of isolated PSI particles to strong light. In addition to the Chl photobleaching, pronounced changes were found in Chl absorption and fluorescence spectra. The position of the major peak in the red part of the absorption spectrum shifted from 680 nm towards shorter wavelengths in the course of strong light exposure. A 6-nm blue shift of that peak was observed after 5-h illumination. Even more pronounced changes were found in the characteristics of Chl fluorescence. The magnitude of the dominating long-wavelength emission band at 736 nm located in untreated particles was five times reduced after 2-h exposure, whereas the loss in absolute Chl contents did not exceed 10% of its initial value. The major peak in low-temperature Chl fluorescence emission spectra shifted from 736 to 721 nm after 6-h WL treatment. Individual Chl-protein complexes differed in the response of their absorption spectra to strong WL. Unlike light-harvesting complexes (LHC), LHCI-680 and LHC-730, which did not exhibit changes in the major peak position, its maximum was shifted from 678 to 671 nm in CPIa complex after PSI submembrane particles were irradiated with strong light for 6 h. The results demonstrated that excitation energy transfer represents the stage of photosynthetic utilization of absorbed quanta which is most sensitive to strong light in isolated PSI particles.

Chlorophyll↗
PubMed2003

The acclimative response of the main light-harvesting chlorophyll a/b-protein complex of photosystem II (LHCII) to elevated irradiances at the level of trimeric subunits.

The changes in structural organization of the major light-harvesting chlorophyll a/b-protein complex of photosystem II (LHC II) at the level of trimeric subcomplexes were studied in spinach plants grown under low light conditions (50 micromol quanta m(-2) s(-1)) and then acclimated to elevated irradiances. By monitoring photochemical quenching of fluorescence yield (qP), photosystem II (PS II) functional status was assessed in leaves of plants acclimated to a range of elevated irradiances. Three separate acclimative irradiances were selected for the experiments, reflecting: limiting light conditions (150 micromol quanta m(-2) s(-1)), near to the inflexion point on the irradiance curve conditions (300 micromol quanta m(-2) s(-1)) and an excessive light, causing a moderate stress in the form of down regulation of PS II (450 micromol quanta m(-2) s(-1)). An immunoblot analysis showed that there was a clear decline in an abundance on chlorophyll basis of Lhcb1-3 apoproteins as an acclimative irradiance increased from 50 to 450 micromol quanta m(-2) s(-1), with Lhcb1 decreasing to a lesser extent than Lhcb2 and Lhcb3 (only at excessive irradiance). When analyzed by non-denaturing isoelectric focusing BBY membrane fragments (PSIIalpha-enriched, stacked thylakoid membranes) isolated from low light-grown plants were resolved into nine fractions, seven of which (labelled 3-9) were established by us previously [Jackowski and Pielucha, J. Photochem. Photobiol. B: Biol. 64 (2001) 45] to be LHC II subcomplexes representing mixed populations of closely similar trimers, comprising permutations of Lhcb1 and Lhcb2 (subcomplexes 3-7) or Lhcb1-3 (subcomplexes 8 and 9). A heterogeneity with regard to accumulation behaviour of LHC II subcomplexes in response to elevated irradiances was revealed. The subcomplexes 5 and 6 were accumulating at similar level, regardless of the light irradiance experienced. Another group consisting of the subcomplexes 3 and 4 (the most basic ones) showed a progressive increase in relative abundance with increasing an irradiance intensity whereas the subcomplexes 7-9 (the most acidic ones) exhibited a progressive decline in their relative abundance during an acclimation of spinach plants to elevated irradiances thus they may collectively represent an elevated irradiance-responsive subunit of LHCII.

Acclimatization↗
PubMed2000

Degradation pathway(s) of chlorophyll: what has gene cloning revealed?

The mechanism responsible for the degreening of plants and the degradation of chlorophyll was unclear for many years. However, recent studies have identified the colorless intermediates and helped to construct a basic pathway for degradation. After the successive removal of phytol and Mg21 from the chlorophyll molecule by chlorophyllase and 'Mg dechelatase', pheophorbide a is cleaved and reduced to yield a colorless, open tetrapyrrole intermediate. After further modifications, this is finally transported to the vacuole. Cloning the genes for chlorophyllase isozymes and the reductase should help to elucidate the physiological roles of each enzyme at a molecular level.

Amino Acid Sequence↗
PubMed2002

Photo-electrochemical control of photosystem II chlorophyll fluorescence in vivo.

The effect of electric field on chlorophyll fluorescence is considered on the basis of the reversible radical pair model. The hypothesis is presented that the electric fields generated by photosynthetic charge separation in reaction centers and propagated laterally through the thylakoid lumen are associated with changes in chlorophyll fluorescence yield.

Chlorophyll↗
PubMed2001

Characterization of carotenoid and chlorophyll photooxidation in photosystem II.

Photosystem II (PSII) contains two accessory chlorophylls (Chl(Z), ligated to D1-His118, and Chl(D), ligated to D2-His117), carotenoid (Car), and heme (cytochrome b(559)) cofactors that function as alternate electron donors under conditions in which the primary electron-donation pathway from the O(2)-evolving complex to P680(+) is inhibited. The photooxidation of the redox-active accessory chlorophylls and Car has been characterized by near-infrared (near-IR) absorbance, shifted-excitation Raman difference spectroscopy (SERDS), and electron paramagnetic resonance (EPR) spectroscopy over a range of cryogenic temperatures from 6 to 120 K in both Synechocystis PSII core complexes and spinach PSII membranes. The following key observations were made: (1) only one Chl(+) near-IR band is observed at 814 nm in Synechocystis PSII core complexes, which is assigned to Chl(Z)(+) based on previous spectroscopic studies of the D1-H118Q and D2-H117Q mutants [Stewart, D. H., Cua, A., Chisholm, D. A., Diner, B. A., Bocian, D. F., and Brudvig, G. W. (1998) Biochemistry 37, 10040-10046]; (2) two Chl(+) near-IR bands are observed at 817 and 850 nm in spinach PSII membranes which are formed with variable relative yields depending on the illumination temperature and are assigned to Chl(Z)(+), and Chl(D)(+), respectively; (3) the Chl and Car cation radicals have significantly different stabilities at reduced temperatures with Car(+) decaying much faster; (4) in Synechocystis PSII core complexes, Car(+) decays by recombination with Q(A)(-) and not by Chl(Z)/Chl(D) oxidation, with multiphasic kinetics that are attributed to an ensemble of protein conformers that are trapped as the protein is frozen; and (5) in spinach PSII membranes, Car(+) decays mainly by recombination with Q(A)(-), but also partly by formation of the 850 nm Chl cation radical. The greater stability of Chl(Z)(+) at low temperatures enabled us to confirm that resonance Raman bands previously assigned to Chl(Z)(+) are correctly assigned. In addition, the formation and decay of these cations provide insight into the alternate electron-donation pathways to P680(+).

Carotenoids↗
PubMed2001

Kinetic analysis of nonphotochemical quenching of chlorophyll fluorescence. 2. Isolated light-harvesting complexes.

The chlorophyll fluorescence yield of purified photosystem II light-harvesting complexes can be lowered by manipulation of experimental conditions. In several important respects, this quenching resembles the nonphotochemical quenching observed in isolated chloroplasts and leaves, therefore providing a model system for investigating the underlying mechanism. A methodology based on the principles of enzyme kinetic analysis has already been applied to isolated chloroplasts, and this same experimental approach was used here with purified LHCIIb, CP26, and CP29. It was found that the kinetics of the decrease in fluorescence yield robustly fitted a second-order kinetic model with respect to time after induction of quenching. The second-order rate constant was dependent upon the complex that was analyzed, the detergent concentration, the solution pH, and the presence of exogenous xanthophyll cycle carotenoids. In contrast, the formation of an absorbance change at 683 nm that accompanies quenching displayed first-order kinetics. The reversal of quenching also displayed second-order kinetics. These data show that quenching results from a binary reaction, possibly arising between two chlorophyll molecules. On the basis of these data, a model for the regulation of nonphotochemical quenching based upon the allosteric control of the conformation of light-harvesting complexes by protonation and xanthophyll binding is presented.

Chlorophyll↗
PubMed2002

The mechanism of UV-A radiation-induced inhibition of photosystem II electron transport studied by EPR and chlorophyll fluorescence.

The UV-A (320-400 nm) component of sunlight is a significant damaging factor of plant photosynthesis, which targets the photosystem II complex. Here we performed a detailed characterization of UV-A-induced damage in photosystem II membrane particles using EPR spectroscopy and chlorophyll fluorescence measurements. UV-A irradiation results in the rapid inhibition of oxygen evolution accompanied by the loss of the multiline EPR signal from the S(2) state of the water-oxidizing complex. Gradual decrease of EPR signals arising from the Q(A)(-)Fe(2+) acceptor complex, Tyr-D degrees, and the ferricyanide-induced oxidation of the non-heme Fe(2+) to Fe(3+) is also observed, but at a significantly slower rate than the inhibition of oxygen evolution and of the multiline signal. The amplitude of Signal II(fast), arising from Tyr-Z degrees in the absence of fast electron donation from the Mn cluster, was gradually increased during the course of UV-A treatment. However, the amount of functional Tyr-Z decreased to a similar extent as Tyr-D as shown by the loss of amplitude of Signal II(fast) that could be measured in the UV-A-treated particles after Tris washing. UV-A irradiation also affects the relaxation of flash-induced variable chlorophyll fluorescence. The amplitudes of the fast (600 micros) and slow (2 s) decaying components, assigned to reoxidation of Q(A)(-) by Q(B) and by recombination of (Q(A)Q(B))(-) with donor side components, respectively, decrease in favor of the 15-20 ms component, which reflects PQ binding to the Q(B) site. In the presence of DCMU, the fluorescence relaxation is dominated by a 1 s component due to recombination of Q(A)(-) with the S(2) state. After UV-A radiation, this is partially replaced by a much faster component (30-70 ms) arising from recombination of Q(A)(-) with a stabilized intermediate PSII donor, most likely Tyr-Z degrees. It is concluded that the primary damage site of UV-A irradiation is the catalytic manganese cluster of the water-oxidizing complex, where electron transfer to Tyr-Z degrees and P(680)(+) becomes inhibited. Modification and/or inactivation of the redox-active tyrosines and the Q(A)Fe(2+) acceptor complex are subsequent events. This damaging mechanism is very similar to that induced by the shorter wavelength UV-B (280-320) radiation, but different from that induced by the longer wavelength photosynthetically active light (400-700 nm).

Chlorophyll↗
PubMed2004

Supramolecular organization and dual function of the IsiA chlorophyll-binding protein in cyanobacteria.

A significant part of global primary productivity is provided by cyanobacteria, which are abundant in most marine and freshwater habitats. In many oceanographic regions, however, the concentration of iron can be so low that it limits growth. Cyanobacteria respond to this condition by expressing a number of iron stress inducible genes, of which the isiA gene encodes a chlorophyll-binding protein known as IsiA or CP43'. It was recently shown that 18 IsiA proteins encircle trimeric photosystem I (PSI) under iron-deficient growth conditions. We report here that after prolonged growth of Synechocystis PCC 6803 in an iron-deficient medium, the number of bound IsiA proteins can be much higher than previously known. The largest complexes bind 12-14 units in an inner ring and 19-21 units in an outer ring around a PSI monomer. Fluorescence excitation spectra indicate an efficient light harvesting function for all PSI-bound chlorophylls. We also find that IsiA accumulates in cyanobacteria in excess of what is needed for functional light harvesting by PSI, and that a significant part of IsiA builds supercomplexes without PSI. Because the further decline of PSI makes photosystem II (PSII) increasingly vulnerable to photooxidation, we postulate that the surplus synthesis of IsiA shields PSII from excess light. We suggest that IsiA plays a surprisingly versatile role in cyanobacteria, by significantly enhancing the light harvesting ability of PSI and providing photoprotection for PSII.

Bacterial Proteins↗
PubMed2004

FTIR spectroscopy of synechocystis 6803 mutants affected on the hydrogen bonds to the carbonyl groups of the PsaA chlorophyll of P700 supports an extensive delocalization of the charge in P700+.

P700, the primary electron donor of photosystem I is an asymmetric dimer made of one molecule of chlorophyll a' (P(A)) and one of chlorophyll a (P(B)). While the carbonyl groups of P(A) are involved in hydrogen-bonding interactions with several surrounding amino acid side chains and a water molecule, P(B) does not engage in hydrogen bonding with the protein. Light-induced FTIR difference spectroscopy of the photooxidation of P700 has been combined with site-directed mutagenesis in Synechocystis sp. PCC 6803 to investigate the influence of these hydrogen bonds on the structure of P700 and P700(+). When the residue Thr A739, which donates a hydrogen bond to the 9-keto C=O group of P(A), is changed to Phe, a differential signal at 1653(+)/1638(-) cm(-1) in the P700(+)/P700 FTIR difference spectrum upshifts by approximately 30-40 cm(-1), as expected for the rupture of the hydrogen bond or, at least, a strong decrease of its strength. The same upshift is also observed in the FTIR spectrum of a triple mutant in which the residues involved in the three main hydrogen bonds to the 9-keto and 10a-carbomethoxy groups of P(A) have been changed to the symmetry-related side chains present around P(B). In addition, the spectrum of the triple mutant shows a decrease of a differential signal around 1735 cm(-1) and the appearance of a new signal around 1760 cm(-1). This is consistent with the perturbation of a bound 10a-ester C=O group that becomes free in the triple mutant. All of these observations support the assignment scheme proposed previously for the carbonyls of P700 and P700(+) [Breton, J., Nabedryk, E., and Leibl, W. (1999) Biochemistry 38, 11585-11592] and therefore reinforce our previous conclusions that the positive charge in P700(+) is largely delocalized over P(A) and P(B).

Adhesins, Bacterial↗