Functional size of Photosystem II determined by radiation inactivation

The functional size of Photosystem II (PS II) was investigated by radiation inactivation. The technique provides an estimate of the functional mass required for a specific reaction and depends on irradiating samples with high energy -rays and assaying the remaining activity. The analysis is based on target theory that has been modified to take into account the temperature dependence of radiation inactivation of proteins. Using PS II enriched membranes isolated from spinach we determined the functional size of primary charge separation coupled to water oxidation and quinone reduction at the Q_B site: H₂O (Mn)₄ Y_z P680 Pheophytin Q phenyl-p-benzoquinone. Radiation inactivation analysis indicates a functional mass of 88 ± 12 kDa for electron transfer from water to phenyl-p-benzoquinone. It is likely that the reaction center heterodimer polypeptides, D1 and D2, contribute approximately 70 kDa to the functional mass, in which case polypeptides adding up to approximately 20 kDa remain to be identified. Likely candidates are the and subunits of cytochrome b ₅₅₉and the 4.5 kDa psbI gene product.Abbreviations Cyt cytochrome - PS Photosystem - P680 primary electron donor of Photosystem II - Q_A primary quinone acceptor of Photosystem II - Q_B secondary quinone acceptor of Photosystem II - Y_z tyrosine donor to P680     

Photosynthetic oxygen evolution: H/D isotope effects and the coupling between electron and proton transfer during the redox reactions at the oxidizing side of Photosystem II

The oxygen evolving complex (OEC) of Photosystem II (PS II) incorporates a Mn-cluster and probably a further redox cofactor, X. Four quanta of light drive the OEC through the increasingly oxidized states S0 S1 S2 S3 S4 to yield O2 during the transition S4 S0. It has been speculated that the oxidation of water might be kinetically facilitated by the abstraction of hydrogen. This implied that the respective electron acceptor is deprotonated upon oxidation. Whether YZ and X fulfill this expectation is under debate. We have previously inferred a 'chemical' deprotonation of X based on the kinetics of proton release (Haumann M, Drerenstedt W, Hundelt M and Junge W (1996) Biochim Biophys Acta 1273: 237–250. Here, we investigated the rates of electron transfer and proton release as function of the D2O/H2O ratio, the pH, and the temperature both in thylakoids and PS II core particles. The largest kinetic isotope effect on the rate of electron transfer (factor of 2.1–2.4) and the largest pH-dependence (factor of about 2 between pH 5 and 8) was found on S2 S3 where X is oxidized. During the other transitions both factors were much smaller ( 1.4). Electron transfer is probably kinetically steered by proton transfer only during S2 S3. These results corroborate the notion that X serves as a hydrogen acceptor for bound water during S4 S0. We propose a consistent scheme for the final reaction with water to yield dioxygen: two two-electron (hydrogen) transfers in series with a peroxide intermediate.     

9-Aminoacridine and dibucaine exhibit competitive interactions and complicated inhibitory effects that interfere with measurements of ΔpH and xanthophyll cycle-dependent Photosystem II energy dissipation

This study concerns measurements and interpretations of the trans-thylakoid membrane pH gradient, pH, and xanthophyll cycle-dependent energy dissipation in Photosystem II (PS II). Compared and contrasted are the concentration-dependent inhibitory effects and interactions between two lipophilic tertiary amines, namely, 9-aminoacridine the pH indicator and dibucaine a local anesthetic reported to inhibit both the pH and xanthophyll cycle deepoxidation. Chlorophyll a fluorescence monitored both electron transport efficiency and xanthophyll cycle-dependent energy dissipation, high-performance liquid chromatography monitored deepoxidase and chloroplast ATPase activities and steady-state fluorescence monitored various activities of the amines in solution. Low concentrations (up to 2 M) of both 9-aminoacridine and dibucaine showed similar fluorescence properties and pH-dependent uptake into thylakoids. Importantly both amines exhibited mutually competitive inhibitory effects with respect to this pH-dependent uptake and fluorescence quenching. The fluorescence yields of both compounds in aqueous solution were strongly quenched by sodium ascorbate, a necessary cofactor for in vitro deepoxidation. Both compounds similarly inhibited several light induced activities including deepoxidation, photosynthetic electron transport and PS II energy dissipation. However, for all these activities 9-aminoacridine was 2 to 5 times more potent. Importantly, 9-aminoacridine inhibited deepoxidation with an I₅₀1 M, a concentration far below that which inhibits the pH, ATP synthesis/hydrolysis or electron transport. The inhibitory effects of both compounds on PS II energy dissipation were exerted at 3 to 5 times lower concentration if added before as opposed to after a saturating level of deepoxidation. This result confirms the important role for deepoxidation in mediating PS II energy dissipation. Compared to 9-aminoacridine and in contrast to similar effects on the light-induced activities, dibucaine exhibited significantly different inhibitory effects on ATPase activity and ATPase mediated PS II energy dissipation. However, we conclude from the more potent inhibition by 9-aminoacridine and the similar inhibitory patterns of all the light-induced activities that neither 9-aminoacridine nor dibucaine possess unique capacities to neutralize the light-mediated pH. DCMU–3-(3,4-dichlorophenyl)-1,1-dimethylurea; DTT–dithiothreitol; f_x–fractional intensity of fluorescence lifetime component x; F()_m–maximal PS II Chl a fluorescence intensity with all Q_A reduced in the absence (presence) of thylakoid membrane energization; F_o–minimal PS II Chl a fluorescence intensity with all Q_A oxi dized; F_s–steady state PS II Chl a fluorescence; HPLC–high performance liquid chromatography; I_(o)–intensity of fluorescence in the presence (absence) of quencher; K_a–association constant between Z (and A) and protonated PS II units; LA–local anesthetic; NaAsc–sodium ascorbate; NR–neutral red; PAM–pulse-amplitude modulation fluorometer; PFD–photon-flux density, mols photons m^-2 s^-1; PS I–Photosystem I; PS II–Photosystem II; [PS II^-+]–concentration of PS II units with inactive/deprotonated (active/protonated ) xanthophyll binding sites; [PS II_tot]–total concentration of PS II units; [PS II⁺-Z]–concentration of PS II units with Z or A bound; Q–fraction of fluorescence intensity that is quenched; Q_max–fraction of fluorescence intensity that is quenched under control conditions; Q_A–primary quinone electron acceptor of PS II; V–violaxanthin; Z–zeaxanthin; 9AA–9-aminoacridine; pH–trans-thylakoid membrane proton gradient; _f–lifetime of Chl a fluorescence     

Cyclic electron flow around Photosystem II in vivo

The oxygen flash yield (Y_O2) and photochemical yield of PS II (_{PS II}) were simultaneously detected in intact Chlorella cells on a bare platinum oxygen rate electrode. The two yields were measured as a function of background irradiance in the steady-state and following a transition from light to darkness. During steady-state illumination at moderate irradiance levels, Y_O2 and _{PS II} followed each other, suggesting a close coupling between the oxidation of water and Q_A reduction (Falkowski et al. (1988) Biochim. Biophys. Acta 933: 432–443). Following a light-to-dark transition, however, the relationship between Q_A reduction and the fraction of PS II reaction centers capable of evolving O₂ became temporarily uncoupled. _{PS II} recovered to the preillumination levels within 5–10 s, while the Y_O2 required up to 60 s to recover under aerobic conditions. The recovery of Y_O2 was independent of the redox state of Q_A, but was accompanied by a 30% increase in the functional absorption cross-section of PS II (_{PS II}). The hysteresis between Y_O2 and the reduction of Q_A during the light-to-dark transition was dependent upon the reduction level of the plastoquinone pool and does not appear to be due to a direct radiative charge back-reaction, but rather is a consequence of a transient cyclic electron flow around PS II. The cycle is engaged in vivo only when the plastoquinone pool is reduced. Hence, the plastoquinone pool can act as a clutch that disconnects the oxygen evolution from photochemical charge separation in PS II.Abbreviations ADRY acceleration of the deactivation reactions of the water-splitting enzyme (agents) - Chl chlorophyll - cyt cytochrome - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F_O minimum fluorescence yield in the dark-adapted state - F_I minimum fluorescence yield under ambient irradiance or during transition from the light-adapted state - F_M maximum fluorescence yield in the dark-adapted state - F_M maximum fluorescence yield under ambient irradiance or during transition from light-adapted state - F_V, F_V variable fluorescence (F_V=F_M–F_O ; F_V=F_M–F_I) - FRR fast repetition rate (fluorometer) - _{PS II} quantum yield of Q_A reduction (_{PS II}=(F_M – F_O)/F_M or _{PS II})=(F_M= – F_I=)/F_M=) - LHCII Chl a/b light harvesting complexes of Photosystem II - OEC oxygen evolving complex of PS II - P680 reaction center chlorophyll of PS II - PQ plastoquinone - POH₂ plastoquinol - PS I Photosystem I - PS II Photosystem II - RC II reaction centers of Photosystem II - PS II the effective absorption cross-section of PHotosystem II - TL thermoluminescence - Y_O2 oxygen flash yield The US Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Spectral hole burning of the primary electron donor state of Photosystem I

Persistent photochemical hole burned profiles are reported for the primary electron donor state P700 of the reaction center of PS I. The hole profiles at 1.6 K for a wide range of burn wavelengths (_B) are broad (FWHM310 cm^-1) and for the 45:1 enriched particles studied exhibit no sharp zero-phonon hole feature coincident with _B. The _B hole profiles are analyzed using the theory of Hayes et al. [J Phys Chem 1986, 90: 4928] for hole burning in the presence of arbitrarily strong linear electron-phonon coupling. A Huang-Rhys factor S in the range 4–6 and a corresponding mean phonon frequency in the range 35–50 cm^-1 together with an inhomogeneous line broadening of100 cm^-1 are found to provide good agreement with experiment. The zero-point level of P700^* is predicted to lie at710 nm at 1.6K with an absorption maximum at702 nm. The hole spectra are discussed in the context of the hole spectra for the primary electron donor states of PS II and purple bacteria.Abbreviations NPHB nonphotochemical hole burning - O.D. optical density - PSBH phonon sideband hole - PS I Photosystem I P680 - P700, P870, P960 the primary electron donors of Photosystem II, Photosystem I, Rhodobacter sphaeroides, Rhodopseudomonas viridis - PED primary electron donor - RC reaction center - ZPH zero-phonon holes     

Evidence for the existence of trimeric and monomeric Photosystem I complexes in thylakoid membranes from cyanobacteria

In cyanobacteria, solubilization of thylakoid membranes by detergents yields both monomeric and trimeric Photosystem I (PS I) complexes in variable amounts. We present evidence for the existence of both monomeric and trimeric PS I in cyanobacterial thylakoid membranes with the oligomeric state depending in vitro on the ion concentration. At low salt concentrations (i.e.10 mM MgSO₄) PS I is mainly extracted as a trimer from these membranes and at high salt concentrations (i.e.150 mM MgSO₄) nearly exclusively as a monomer, irrespective of the type of salt used (i.e. mono- or bivalent ions) and the temperature (i.e. 4°C or 20°C). Once solubilized, the PS I trimer is stable over a wide range of ion concentrations (i.e. beyond 0.5 M). A model is presented which suggests a monomer-oligomer equilibrium of PS I, but also of PS II and the cyt. b6/f-complex in the cyanobacterial thylakoid membrane. The possible physiological role of this equilibrium in the regulation of state transitions is discussed.Abbreviations -DM dodecyl--D-maltoside - Chl chlorophyll - cyt. b6f cytochrome b6f complex - EM electron microscopy - HPLC high performance liquid chromatography - LDAO N, N-dimethyl-N-dodecyl amine oxide - MES 4-morpholino ethane sulfonic acid - PAGE polyacrylamide gel electrophoresis - PBS phycobilisome - PS photosystem - SDS sodium dodecyl sulfate - 2D two dimensional - 3D three dimensional     

Electroluminescence

An overview is presented of research based on the observation by Arnold and Azzi (1971) (Photochem Photobiol 14: 233–240), that an electric field induces charge-recombination luminescence in a suspension of photosynthetic membrane vesicles. The electroluminescence signals from Photosystems I and II are discussed in relation to the shape of the vesicles and the membrane potentials generated by the externally applied electric field. The use of the electroluminescence amplitude as a probe to study the kinetics and energetics of charge separation, and of its kinetics to monitor the electric-field induced charge recombination process are reviewed. Currently unresolved issues regarding the emission yield of electroluminescence are briefly discussed and the properties are summarized of the unexplained Photosystem II luminescence which is not sensitive to the membrane potential.Abbreviations DCMU 3(3,4-dichlorophenyl)-1,1-dimethylurea - EL electroluminescence - PS I, II Photosystem I, II - TPB tetraphenylboron, an artificial electron donor for PS II - P primary electron donor - S_i Y_z P680 Pheo Q_A Q_B sequence of electron transfer components in PS II - plastocyanin P700 A₀ A₁ F_x F_A (or F_B) sequence of electron transfer components in PS I     

The mechanisms contributing to photosynthetic control of electron transport by carbon assimilation in leaves

Photosynthetic control describes the processes that serve to modify chloroplast membrane reactions in order to co-ordinate the synthesis of ATP and NADPH with the rate at which these metabolites can be used in carbon metabolism. At low irradiance, optimisation of the use of excitation energy is required, while at high irradiance photosynthetic control serves to dissipate excess excitation energy when the potential rate of ATP and NADPH synthesis exceed demand. The balance between pH, ATP synthesis and redox state adjusts supply to demand such that the [ATP]/[ADP] and [NADPH]/[NADP⁺] ratios are remarkably constant in steady-state conditions and modulation of electron transport occurs without extreme fluctuations in these pools.Abbreviations FBPase Fructose-1,6-bisphosphatase - PS I Photosystem I - PS II Photosystem II - Pi inorganic phosphate - PGA glycerate 3-phosphate - PQ plastoquinone - Q_A the bound quinone electron acceptor of PS II - q_P Photochemical quenching of chlorophyll fluorescence associated with the oxidation of Q_A - q_N non-photochemical quenching of chlorophyll fluorescence - q_E non-photochemical quenching associated with the high energy state of the membrane - RuBP ribulose-1,5-bisphosphate - TP triose phosphate - intrinsic quantum yield of PS II - quantum yield of electron transport - quantum yield of CO₂ assimilation     

Supramolecular architecture of cyanobacterial thylakoid membranes: How is the phycobilisome connected with the photosystems?

Cyanobacteria, as the most simple organisms to perform oxygenic photosynthesis differ from higher plants especially with respect to the thylakoid membrane structure and the antenna system used to capture light energy. Cyanobacterial antenna systems, the phycobilisomes (PBS), have been shown to be associated with Photosystem 2 (PS 2) at the cytoplasmic side, forming a PS 2-PBS-supercomplex, the structure of which is not well understood. Based on structural data of PBS and PS 2, a model for such a supercomplex is presented. Its key features are the PS 2 dimer as prerequisite for formation of the supercomplex and the antiparallel orientation of PBS-cores and the two PS 2 monomers which form the contact area within the supercomplex. Possible consequences for the formation of superstructures (PS 2-PBS rows) within the thylakoid membrane under so-called state 1 conditions are discussed. As there are also indications for specific functional connections of PBS with Photosystem 1 (PS 1) under so-called state 2 conditions, we show a model which reconciles the need for a structural interaction between PBS and PS 1 with the difference in structural symmetry (2-fold rotational symmetry of PBS-cores, 3-fold rotational symmetry of trimeric PS 1). Finally, the process of dynamic coupling and uncoupling of PBS to PS 1 and PS 2, based on the presented models, shows analogies to mechanisms for the regulation of photosynthetic electron flow in higher plants-despite the very different organization of their thylakoid membranes in comparison to cyanobacteria.Abbreviations APC allophycocyanin - b ₆ f cytochrome b ₆ f complex - CP chlorophyll protein - FNR ferredoxin-NADP⁺-oxidoreductase - LD linkerprotein-domain - LHC light-harvesting complex - Pc plastocyanin - PC phycocyanin - PD phycobiliprotein-domain - PS 1 Photosystem 1 - PS 2 Photosystem 2 - PBS phycobilisome Dedicated to Prof. Dr. Horst Senger on the occasion of his 65th birthday.     

Heat-induced reversible changes in Photosystem 1 absorption cross-section of pea chloroplasts and sub-chloroplast preparations

Reversible changes in the room temperature fluorescence quenching at 685 nm and light scattering level at 577 nm, indicating about 15% of granal unstacking, induced by high temperature treatment (40°C, for 5 min) of pea chloroplasts were shown. Analysis of the low temperature excitation fluorescence spectra of the 735 nm Photosystem 1 (PS 1) band (F735), in the 635–725 nm region, has revealed the involvement of light-harvesting (LHC 2, maxima at 650 and 676 nm) and the proximal Photosystem 2 antenna (maxima 668, 687 nm) in heat-induced enhancement of the PS 1 long wavelength antenna absorption cross-section. It was found that the two PS 1 sub-chloroplast preparations, achieved by the digitonin method, possessed different characteristics of this enhancement. For the heavier fraction (100 000 g) the additional absorption cross-section was formed mostly at the expense of PS 2 antennas (apparently spillover), but for the lighter PS 1 fraction (145 000 g) the changes have indicated an -transfer mechanism, i.e., participation of only LHC 2 in the energy transfer towards PS 1. This may indicate the heterogeneous character of the temperature-induced energy redistribution across the PS 1-containing chloroplast membrane compartments. The model of heat-induced changes in the pigment-protein complex arrangement is discussed in terms of domain organisation of the thylakoid membrane.Abbreviations Chl a/b ratio between chlorophyll a and chlorophyll b concentrations - CP43 and CP47 proximal Photosystem 2 antenna complexes - D1/D2 complex Photosystem 2 reaction centre complex - EDTA ethylenediaminetetraacetic acid - F685 and F696 Photosystem 2 low temperature fluorescence bands - F735 Photosystem 1 low temperature fluorescence band - Fp free pigment band in green gel electrophoresis - LHC 2 light-harvesting chlorophyll a/b complex - LHCP I, II and III light-harvesting bands in green gel electrophoresis - Cp1 and Cpa bands in green gel electrophoresis which are associated with Photosystem 1 and 2 reaction centre complexes with internal antennas - P700 Photosystem 1 reaction centre - PPC pigment-protein complex - PS 1 and Photosystem 1 alpha and Photosystem 1 beta - PS 2 and Photosystem 2 alpha and Photosystem 2 beta - RC reaction centre - SDS-PAGE sodiumdodecylsulphate-polyacrylamide gel electrophoresis - St1-St2 state-1-state-2 transitions     

Fluorescence studies on interaction between phospho-LHC II and subchloroplast Photosystem 1 preparations

Chloroplast proteins were phosphorylated under two test conditions: white light irradiance alone and white light irradiance with the addition of glucose and glucose oxidase, used to produce an anaerobic medium. The interaction of phospho-LHC II with Photosystem 1 (PS 1) was studied for two types of PS I preparation. Changes in the chlorophyll a/b ratio and the ratio of 650 and 680 nm band intensities (E650/E680) in fluorescence excitation spectra were used in calculating the phospho-LHC II portion which became associated with PS 1. It is shown that the associated portion of phospho-LHC II varies for each of the PS 1 preparations and phosphorylation procedures. Possible conclusions as regards the transfer of various sets of LHC II subpopulations under different phosphorylation procedures and the differences of interaction with PS 1 are discussed.Abbreviations PS 1 Photosystem 1 - PS 2 Photosystem 2 - LHC II light-harvesting chlorophyll a/b protein complex II - Chl chlorophyll - fluorescence quantum yield - _f life time of fluorescence at =685 nm - F735 fluorescence band with a maximum at 735 nm - F685 fluorescence band with a maximum at 685 nm - E650/E680 ratio of amplitudes in excitation fluorescence spectrum at 650 and 680 nm     

Multiple action sites for photosystem II herbicides as revealed by delayed fluorescence

DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) at concentrations higher than 10 M suppresses the second time range delayed fluorescence (DF) of pea chloroplasts, due to inhibition of the oxidizing side of photosystem II (PS II). The inhibition of the reducing side of PS II resulting in the suppression of millisecond DF takes place at much lower (0.01 M) DCMU concentrations. The variation in the herbicide-affinities of the reducing and oxidizing sides of PS II is not the same for DCMU and phenol-type herbicides. The DCMU-affinity of the oxidizing side considerably increases and approximates that of the reducing side upon mild treatment of chloroplasts with oleic acid. Probably this is a result of some changes in the environment of the binding site at the oxidizing side. At DCMU concentrations higher than 1 mM, the chaotropic action of DCMU leads to the generation of millisecond luminescence which is not related to the functioning of the reaction centres.Abbreviations D-1 The 32 kDa herbicide-binding intrinsic polypeptide of PS II, the apoprotein of Q_B - D-2 The 32–34 kDa intrinsic polypeptide of PS II, probably the apoprotein of Z - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DF Delayed fluorescence - Dinoseb 2,4-dinitro-6-sec-butylphenol - DNOC 4,6-dinitro-o-cresol - F_m Maximal fluorescence yield (when all traps are closed) - F_o Constant fluorescence yield (when all traps are open) - PS Photosystem - Q_A and Q_B The primary and secondary plastoquinone acceptors of PS II, correspondingly - Z A plastoquinol electron donor, presumably associated with the D-2 protein     

The role of light in formation of modified S2-state upon low pH-treatment of oxygen-evolving Photosystem II

An abnormal, structurally modified, kinetically stable S₂-state has been reported to be induced when Photosystem II was treated with NaCl-EGTA (or EDTA) in the light or with pH in darkness, both are assumed to release functional Ca²⁺. In order to compare the mechanism of induction of modified S₂-state between the two treatments, effects of illumination during or before low pH-treatment on formation of the abnormal S₂-state were investigated by means of thermoluminescence measurements and low temperature EPR spectroscopy. Following results have been obtained: Flash illumination during low pH-treatment did not practically induce the abnormal S₂-state, whereas preflash illumination given immediately before low pH-treatment efficiently induced the abnormal S₂-state, and its amplitude showed a period-four oscillation on varying the preflash number with maxima at the second and sixth flashes. The abnormal S₂-state thus induced by preflashes was identical with the modified S₂-state that could be induced in dark-low pH-treated Photosystem II by excitation at 0°C after neutralization to pH 6.5. It was inferred that in low pH-treatment, modified S₂-state can be formed from both S₂- and S₃-states, but its yield from the latter is much higher than from the former, consistent with the early results by Boussac et al. obtained for NaCl-EGTA-light or NaCl-citrate-light treatment.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - EDTA ethylenediaminetetraactate - EGTA ethylene glycol bis(-aminoethylether)-N,N,N,N-tetraacetic acid - Mes 2-(N-morpholino)ethanesulfonic acid - PS II Photosystem II     

Time-resolved chlorophyll fluorescence studies on photosynthetic mutants of Chlamydomonas reinhardtii: origin of the kinetic decay components

The room temperature chlorophyll fluorescence decay kinetics of photosynthetic mutants of Chlamydomonas reinhardtii have been measured as a function of Photosystem 2 (PS2) trap closure, DNB-induced quenching at F_M, and time-resolved emission spectra. The overall decays have been analyzed in terms of three or four kinetic components where necessary. A comparison of the characteristics of the decay components exhibited by the mutants with the wild-type has been carried out to elucidate the precise origins of the different emissions in relation to the observed pigment-protein complexes. It is shown that a) charge recombination in PS2 is not necessary for the presence of long-lived decay components, b) there are two rapid PS1-associated emissions (=30 and 150–200 ps), c) a slow PS1 decay is observed (=1.73 ns) in the absence of PS1 reaction centres, d) the two variable components (=0.25–1.2 and 0.5–2.2 ns) observed in the wild-type arise from LHC2 and e) a rapid (=50–250 ps) decay is associated with the PS2 core antenna (CP3 and CP4). These results show that the intact thylakoid membrane system is too complex to distinguish all of the individual kinetic components.Abbreviations Aexp preexponential factor (Amplitude) - chl chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - DNB m, dinitrobenzene - F_M maximum chl fluorescence level - F₀ initial chl fluorescence level - F_v variable chl fluorescence (F_M–F₀) - LHC light harvesting chl a/b protein complex - PS photosystem - Q_A primary stable electron acceptor of PS2     

Formation of nucleoside 5′-phosphoramidates under potentially prebiological conditions

Summary Adenosine 5-phosphoramidates form when solutions containing adenosine 5-polyphosphates p_nA (n 3) or P₁, P₂-diadenosine 5-diphosphate and amines are allowed to dry out. Mg ions catalyze these reactions. We have studied systems containing ammonia, imidazole, glycine, ethylenediamine and histamine. The yields of adenosine 5-phosphoramidates range from 10–50 % based on the nucleotide. The prebiotic significance of the reactions is discussed.Abbreviations Im imidazole - hist histamine - gly glycine - en ethylenediamine - CDI 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride - EDTA ethylenediaminetetraacetic acid - A adenosine - P_n (n = 1, 2 ) linear polyphosphate containing n phosphate residues - p_nA adenosine 5-polyphosphate containing n phosphate residues - ADP adenosine 5-diphosphate - ATP adenosine 5-triphosphate - AppA P₁, P₂-diadenosine 5-diphosphate - gly-pA adenylyl-(5N)-glycine - ImpA adenosine 5-phosphorimidazolide - NH₂-pA adenosine 5-phosphoramidate - en-pA adenylyl-(5N)-ethylenediamine - hist (NH) - pA adenosine 5-phospho-[2-(4-imidazolyl)-ethylamide] - hist(Im)-pA adenosine 5-phospho-[4-(2-aminoethyl)-imidazolide] - enP_1,2 phosphoramidates of ethylenediamine derived from H₃PO₄ and H₄P₂O₇     

In situ evidence that chilling in the light does not cause uncoupling of photophosphorylation or detachment of coupling factor in chilling-sensitive plants

The potential involvement of impaired photophosphorylation in the chilling sensitivity of photosynthesis in warm climate plant species has been a topic of investigation for more than two decades. With recent advances in the analysis of photosynthetic energy transduction in intact leaves, experiments are now possible that either address or avoid important uncertainties in the significance and interpretation of earlier in vitro work. Nevertheless, different laboratories using different techniques to analyze the effects of chilling in the light on photophosphorylation in intact cucumber (Cucumis sativus) leaves have come to very different conclusions regarding the role of impaired ATP formation capacity in the inhibition of net photosynthesis. In order to evaluate these discrepancies and bring this issue to a final resolution, in this investigation, we have made a detailed analysis of the decay of the flash-induced electrochromic shift and changes in chlorophyll fluorescence yield in cucumber leaves before, during and after a 5 h light-chill at chill temperatures of between 4 and 10°C. We feel that our findings address the major discrepancies in both data and interpretation as well as provide convincing evidence that photophosphorylation is not disrupted in cucumber leaves during or after light and chilling exposure. It follows that impaired photophosphorylation is not a contributing element to the inhibition of net photosynthesis that is widely observed in warm climate plants as a result of chilling in the light.Abbreviations CF chloroplast coupling factor or CF₁CF₀-ATP synthase - A₅₁₈ flash-induced electrochromic absorbance change measured at 518 nm - DCCD N,N'-dicyclohexylcarbodiimide - _H ⁺ transmembrane electrochemical potential of hydrogen ions - the electrical charge component of _H ⁺ - pH the hydrogen ion concentration component of _H ⁺ - F₀ and F_m the yields of chlorophyll fluorescence from dark-adapted material when all Photosystem II centers are open (F₀) or closed (F_m) - F₀' and F_m' F₀ and F_m measured in light-adapted material - F_s steady-state chlorophyll fluorescence yield in light-adapted material - Q_A primary quinone electron acceptor of Photosystem II - PPFD photosynthetic photon flux density     

A hydrogen-atom abstraction model for the function of YZ in photosynthetic oxygen evolution

Recent magnetic-resonance work on Y suggests that this species exhibits considerable motional flexibility in its functional site and that its phenol oxygen is not involved in a well-ordered hydrogen-bond interaction (Tang et al., submitted; Tommos et al., in press). Both of these observations are inconsistent with a simple electron-transfer function for this radical in photosynthetic water oxidation. By considering the roles of catalytically active amino acid radicals in other enzymes and recent data on the water-oxidation process in Photosystem II, we rationalize these observations by suggesting that Y functions to abstract hydrogen atoms from aquo- and hydroxy-bound managanese ions in the (Mn)₄ cluster on each S-state transition. The hydrogen-atom abstraction process may occur either by sequential or concerted kinetic pathways. Within this model, the (Mn)₄/Y_Z center forms a single catalytic center that comprises the Oxygen Evolving Complex in Photosystem II.     

Evaluation of the role of State transitions in determining the efficiency of light utilisation for CO2 assimilation in leaves

Wheat leaves were exposed to light treatments that excite preferentially Photosystem I (PS I) or Photosystem II (PS II) and induce State 1 or State 2, respectively. Simultaneous measurements of CO₂ assimilation, chlorophyll fluorescence and absorbance at 820 nm were used to estimate the quantum efficiencies of CO₂ assimilation and PS II and PS I photochemistry during State transitions. State transitions were found to be associated with changes in the efficiency with which an absorbed photon is transferred to an open PS II reaction centre, but did not correlate with changes in the quantum efficiencies of PS II photochemistry or CO₂ assimilation. Studies of the phosphorylation status of the light harvesting chlorophyll protein complex associated with PS II (LHC II) in wheat leaves and using chlorina mutants of barley which are deficient in this complex demonstrate that the changes in the effective antennae size of Photosystem II occurring during State transitions require LHC II and correlate with the phosphorylation status of LHC II. However, such correlations were not found in maize leaves. It is concluded that State transitions in C₃ leaves are associated with phosphorylation-induced modifications of the PS II antennae, but these changes do not serve to optimise the use of light absorbed by the leaf for CO₂ assimilation.Abbreviations Fm, Fo, Fv maximal, minimal and variable fluorescence yields - Fm, Fv maximal and variable fluorescence yields in a light adapted state - LHC II light harvesting chlorophyll a/b protein complex associated with PS II - q_P photochemical quenching - A₈₂₀ light-induced absorbance change at 820 nm - _{PS I}, _{PS II} relative quantum efficiencies of PS I and PS II photochemistry - _{CO 2} quantum yield of CO₂ assimilation     

Ultrastructure of lycopene containing chromoplasts in fruits ofAglaonema commutatum schott (Araceae)

Summary The ripening process in fruits ofA. commutatum is characterized by clearly distinguishable developmental stages of their pericarp plastids. With respect to predominant pigments and ultrastructural features the following scheme is proposed: 1. The green stage with a tendency to thylakoid degeneration and plastoglobule formation leads to 2. the yellow stage. An increasing number of globules, mostly being membrane associated, are converted to tubules. In this stage, the main pigments are -carotene and cryptoxanthine. The development of membraneous invaginations from the inner plastid envelope leads to 3. the red stage. Concomitantly with lycopene synthesis and incorporation, these envelope-derived membranes expand and become electron dense (after KMnO₄ treatment), but maintain their triple-layered structure. Chromoplasts of deep red coloured fruits (1,400 g lycopene g^–1 dry wt) contain lycopene crystals within the lumina of membraneous sacs which are also derived from the inner envelope membrane. The molar ratio between the three main pigments -carotene, cryptoxanthine, and lycopene is about 1110 in this final state. GA/OsO₄ fixation is unable to stabilize the lycopenic structures (membranes as well as crystals).     

Mechanisms for controlling balance between light input and utilisation in the salt tolerant alga Dunaliella C9AA

The yield of photosynthetic O₂ evolution was measured in cultures of Dunaliella C9AA over a range of light intensities, and a range of low temperatures at constant light intensity. Changes in the rate of charge separation at Photosystem I (PS I) and Photosystem II (PS II) were estimated by the parameters _{PS I} and _{PS II} . _{PS I} is calculated on the basis of the proportion of centres in the correct redox state for charge separation to occur, as measured spectrophotometrically. _{PS II} is calculated using chlorophyll fluorescence to estimate the proportion of centres in the correct redox state, and also to estimate limitations in excitation delivery to reaction centres. With both increasing light intensity and decreasing temperature it was found that O₂ evolution decreased more than predicted by either _{PS I} or _{PS II}. The results are interpreted as evidence of non-assimilatory electron flow; either linear whole chain, or cyclic around each photosystem.Abbreviations F₀ dark level of chlorophyll fluorescence yield (PS II centres open) - F_m maximum level of chlorophyll fluorescence yield (PS II centres closed) - F_v variable fluorescence (F_m-F₀) - PS I Photosystem I - PS II Photosystem II - P₇₀₀ reaction centre chlorophyll(s) of PS I - q_N coefficient of non-photochemical quenching of chlorophyll fluorescence - q_P coefficient of photochemical quenching of fluorescence yield - q_E high-energy-state quenching coefficient - _{PS I} yield of PS I - _{PS II} yield of PS II - _S yield of photosynthetic O₂ evolution - _P intrinsic yield of open PS II centres