Non-photochemical fluorescence quenching and the diadinoxanthin cycle in a marine diatom

The diadinoxanthin cycle (DD-cycle) in chromophyte algae involves the interconversion of two carotenoids, diadinoxanthin (DD) and diatoxanthin (DT). We investigated the kinetics of light-induced DD-cycling in the marine diatom Phaeodactylum tricornutum and its role in dissipating excess excitation energy in PS II. Within 15 min following an increase in irradiance, DT increased and was accompanied by a stoichiometric decrease in DD. This reaction was completely blocked by dithiothreitol (DTT). A second, time-dependent, increase in DT was detected 20 min after the light shift without a concomitant decrease in DD. DT accumulation from both processes was correlated with increases in non-photochemical quenching of chlorophyll fluorescence. Stern-Volmer analyses suggests that changes in non-photochemical quenching resulted from changes in thermal dissipation in the PS II antenna and in the reaction center. The increase in non-photochemical quenching was correlated with a small decrease in the effective absorption cross section of PS II. Model calculations suggest however that the changes in cross section are not sufficiently large to significantly reduce multiple excitation of the reaction center within the turnover time of steady-state photosynthetic electron transport at light saturation. In DTT poisoned cells, the change in non-photochemical quenching appears to result from energy dissipation in the reaction center and was associated with decreased photochemical efficiency. D1 protein degradation was slightly higher in samples poisoned with DTT than in control samples. These results suggest that while DD-cycling may dynamically alter the photosynthesis-irradiance response curve, it offers limited protection against photodamage of PS II reaction centers at irradiance levels sufficient to saturate steady-state photosynthesis.Abbreviations CAP chloramphenicol - D1 PS II reaction center protein - DD diadinoxanthin - DD cycle-diadinoxanthin cycle - DT diatoxanthin - DTT dithiothreitol - FCP fucoxanthin chlorophyll a-c protein - F_m maximum fluorescence yield in the dark-adapted state - F_o minimum fluorescence yield in the dark-adapted state - F_m and F_o maximum and minimum fluorescence yields respectively in some light adapted state - F_v maximum variable fluorescence yield in the dark-adapted state - I_k Irradiance at the intercept of the initial slope of the photosynthesis-irradiance curve and the maximum photosynthetic rate - k_D first order rate constant for nonradiative de-excitation of excitions in the PS II antenna - k_d first order rate constant for non-radiative de-excitation of excitons in the PS II reaction center - k_F first order rate constant for fluorescence - k_T first order rate constant for exciton transfer to the reaction center - k_t first order rate constant for exciton transfer from the reaction center to the antenna - Rubisco ribulose bisphosphate carboxylase - SV_m Stern-Volmer quenching coefficient of the maximum fluorescence yield - SV_o Stern-Volmer quenching coefficient of the miniximum fluorescence yield - _{PS II} apparent absorption cross-section of PS II - _arr average interval between exciton arrival to the PS II reaction center (ms) - _rem average interval between electron turnover during photosynthesis in the PS II reaction center (ms) - _d the probability that an exciton is non-radiatively dissipated in the reaction center - _T the probability that an exciton in the antenna is transferred to the reaction center - _t the probability that an exciton is transferred back from the reaction center to the antenna     

Effects of amphipathic peptides,including presequences,on the functional integrity of rat liver mitochondrial membranes

A number of amphipathic peptides were tested for their effects on structural and functional properties of isolated rat liver mitochondria. The peptides included the matrix targeting sequence of subunit IV of (yeast) cytochromec oxidase. Titration experiments in which the mitochondria were incubated with increasing concentrations of the peptides revealed two major stages in the interaction. First, at low peptide/mitochondria ratios, peptide binding to the outer membrane occurred which was accompanied by gradual lysis of the outer membrane at higher ratios. The latter was deduced from the release of adenylate kinase, the classical marker enzyme of the intermembrane space. Secondly, at still higher peptide/mitochondria ratios, the permeability of the inner membrane progressively increased, as evidenced by measurements of respiratory control and of the membrane potential. Complete uncoupling of respiration seemed to precede dissipation of the membrane potential.     

Composition and function of cytochrome b559 in reaction centers of photosystem II of green plants

A review of a recent study of the spectral and thermodynamic properties of cytochrome b559 as well as of the electron transfer between b559 and photosystem II reaction center cofactors in isolated D1/D2/cytochrome b559 complex RC-2 is presented. Attention is paid to the existence of intermediary-potential (IP, +150 mV) and extra-low-potential (XLP, –45 mV) hemes located close to the acceptor (quinone) and donor (P680) sides of the reaction center cofactors, respectively. These hemes found in isolated RC-2 probably correspond to the high-potential and low-potential hemes in chloroplasts, respectively. The above location of the hemes is believed to allow the photoreduction of the XLP heme and photooxidation of the IP heme. The electron transfer between the two hemes is discussed in terms of the cyclic electron flow and possible involvement in water splitting.     

Accumulation of cesium and radiocesium in forest litter in selected regions of poland and its influence on litter-to-mushroom transfer factor

The aim of this work was to check whether the stable cesium content in forest litter affects the value of radiocesium from litter-to-mushroom transfer factorTf or not. Total cesium in litter, measured by AAS, varied from 0.1–2.7 μg/g. These data, combined with earlier results for mushrooms, showed no simple correlation forTf. More complex relationships provided very high correlation coefficients, but their validity needs further investigation.     

Structure and function of the photosynthetic reaction center fromRhodobacter sphaeroides

The three-dimensional structure of the photosynthetic reaction center fromRhodobacter sphaeroides is described. The reaction center is a transmembrane protein that converts light into chemical energy. The protein has three subunits: L, M, and H. The mostly helical L and M subunits provide the scaffolding and the finely tuned environment in which the chromophores carry out electron transfer. The details of the protein-chromophore interactions are from studies of a trigonal crystal form that diffracted to 2.65-Å resolution. Functional studies of the multi-subunit complex by site-specific replacement of key amino acid residues are summarized in the context of the molecular structure.This work was supported in part by the U.S. Department of Energy, Office of Health and Environmental Research, under Contract No. W-31-109-ENG-38 and by Public Health Service Grant GM36598.     

Singlet oxygen production in thylakoid membranes during photoinhibition as detected by EPR spectroscopy

Exposure of isolated spinach thylakoids to high intensity illumination (photoinhibition) results in the well-characterized impairment of Photosystem II electron transport, followed by degradation of the D1 reaction centre protein. In the present study we demonstrate that this process is accompanied by singlet oxygen production. Singlet oxygen was detected by EPR spectroscopy, following the formation of stable nitroxide radicals from the trapping of singlet oxygen with a sterically hindered amine TEMP (2,2,6,6-tetramethylpiperidine). There was no detectable singlet oxygen production during anaerob photoinhibition or in the presence of sodium-azide. Comparing the kinetics of the loss of PS II function and D1 protein with that of singlet oxygen trapping suggests that singlet oxygen itself or its radical product initiates the degradation of D1.Abbreviations HEPES 4-(2-hydroxyethyl)-1-piperazine ethanesulphonle acid - PS Photosystem - TEMP 2,2,6,6-tetramethylpiperidine - TEMPO 2,2,6,6-tetramethylpiperidine-1-oxyl     

Photosystem II in a mutant of Chlamydomonas reinhardtii lacking the 23 kDa psbP protein shows increased sensitivity to photoinhibition in the absence of chloride

The psbP gene product, the so called 23 kDa extrinsic protein, is involved in water oxidation carried out by Photosystem II. However, the protein is not absolutely required for water oxidation. Here we have studied Photosystem II mediated electron transfer in a mutant of Chlamydomonas reinhardtii, the FUD 39 mutant, that lacks the psbP protein. When grown in dim light the Photosystem II content in thylakoid membranes of FUD 39 is approximately similar to that in the wild-type. The oxygen evolution is dependent on the presence of chloride as a cofactor, which activates the water oxidation with a dissociation constant of about 4 mM. In the mutant, the oxygen evolution is very sensitive to photoinhibition when assayed at low chloride concentrations while chloride protects against photoinhibition with a dissociation constant of about 5 mM. The photoinhibition is irreversible as oxygen evolution cannot be restored by the addition of chloride to inhibited samples. In addition the inhibition seems to be targeted primarily to the Mn-cluster in Photosystem II as the electron transfer through the remaining part of Photosystem II is photoinhibited with slower kinetics. Thus, this mutant provides an experimental system in which effects of photoinhibition induced by lesions at the donor side of Photosystem II can be studied in vivo.Abbreviations Chl chlorophyll - DCIP 2,6-dichlorophenolindophenol - DPC 2,2-diphenylcarbonic dihydrazide - HEPES 4-(2-hydroxyethyl)-1-piperazinethanesulfonic acid - P₆₈₀ the primary electron donor to PS II - PpBQ phenyl-p-benzoquinone - PS II Photosystem II - Q_A the first quinone acceptor of PS II - Q_B the second quinone acceptor of PS II - SDS sodium dodecyl sulfate - Tris tris(hydroxymethyl)aminomethane - Tyr_D accessory electron donor on the D₂-protein - Tyr_Z tyrosine residue, acting as electron carrier between P₆₈₀ and the water oxidizing system     

A proper transmembrane Ca2+ gradient is essential for the stimulation of adenylate cyclase activity by stimulatory GTP-binding protein

Stimulatory GTP-binding Protein (Gs) and adenylate cyclase prepared from bovine brain cortices were co-reconstituted into asolectin vesicles with or without 1000-fold transmembrane Ca²⁺ gradient. The results showed that both basal activity and Gs-stimulated activity of adenylate cyclase were highest in proteoliposomes with a transmembrane Ca²⁺ gradient similar to physiological condition (1 M Ca²⁺ outside and 1 mM Ca²⁺ inside) and lowest when the transmembrane Ca²⁺ gradient was in the inverse direction. Such a difference could be diminished following dissipation of the transmembrane Ca²⁺ gradient by A23187. Comparable conformational changes of Gs in proteoliposomes were also observed when Gs was labeled with the fluorescence probe, acrylodan. These results may indicate that a proper transmembrane Ca²⁺ gradient is essential not only for higher adenylate cyclase activity but also for its stimulation by Gs.     

Leaf chemistry and food selection by common langurs (Presbytis entellus) in Rajaji National Park,Uttar Pradesh,India

Food selection in folivorous primates has been hypothesized to be correlated with leaf chemistry. To test this hypothesis, we conducted a 5-month study on Presbytis entellusin a moist deciduous forest in the Rajaji National Park,Uttar Pradesh, India. The study period covered two seasons, winter and spring. We used the percentage time spent feeding on each food item as an index of food selection, which we estimated from group scan data collected from one study group for 6 days each month. We estimated the selection ratio for each item as the ratio of time spent feeding to availability. We estimated food availability from vegetation sampling and phenological data in the home range of the study group. We estimated crude protein (CP), acid detergent fiber (ADF) and tannins in mature and young leaves of 12 major food species in the laboratory. Food selection is positively correlated with CP in winter and negatively correlated with ADF in both seasons. It also is correlated with CPJADF ratio, but to a lesser extent than the best predictor in winter and spring. Selection ratio is not correlated with CP and ADF in both seasons. It is probable that the inclusion of other factors, such as micronutrients,condensed tannins, and digestibility, might give a better prediction of food selection.     

Protein synthesis is not required for the inhibitory effect of selenite on cell colony formation and RNA synthesis

Selenite has been shown to undergo intracellular metabolism that results in its conversion to other low molecular weight Secontaining species and also to its incorporation into a selenocysteine residue in selenoprotein. In order to investigate whether the incorporation into protein is required for the cytotoxic effects of selenite, we have examined whether inhibition of protein synthesis prevents the inhibitory effect of selenite on the ability of cells to form colonies or to synthesize RNA. We have found that treatment of HeLa cells with cycloheximide inhibited protein synthesis by >90% but had no effect on the inhibitory effect of selenite on cell colony formation or RNA synthesis. Since protein synthesis is not necessary for these cytotoxic effects of selenite they are unlikely to result from an increase in the synthesis of selenoproteins.