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     

Light-dependent modification of Photosystem II in spinach leaves

In dark-adapted spinach leaves approximately one third of the Photosystem II (PS II) reaction centers are impaired in their ability to transfer electrons to Photosystem I. Although these inactive PS II centers are capable of reducing the primary quinone acceptor, Q_A, oxidation of Q_A ^– occurs approximately 1000 times more slowly than at active centers. Previous studies based on dark-adapted leaves show that minimal energy transfer occurs from inactive centers to active centers, indicating that the quantum yield of photosynthesis could be significantly impaired by the presence of inactive centers. The objective of the work described here was to determine the performance of inactive PS II centers in light-adapted leaves. Measurements of PS II activity within leaves did not indicate any increase in the concentration of active PS II centers during light treatments between 10 s and 5 min, showing that inactive centers are not converted to active centers during light treatment. Light-induced modification of inactive PS II centers did occur, however, such that 75% of these centers were unable to sustain stable charge separation. In addition, the maximum yield of chlorophyll fluorescence associated with inactive PS II centers decreased substantially, despite the lack of any overall quenching of the maximum fluorescence yield. The effect of light treatment on inactive centers was reversed in the dark within 10–20 mins. These results indicate that illumination changes inactive PS II centers into a form that quenches fluorescence, but does not allow stable charge separation across the photosynthetic membrane. One possibility is that inactive centers are converted into centers that quench fluorescence by formation of a radical, such as reduced pheophytin or oxidized P680. Alternatively, it is possible that inactive PS II centers are modified such that absorbed excitation energy is dissipated thermally, through electron cycling at the reaction center.Abbreviations A518 absorbance change at 518 nm, reflecting the formation of an electric field across the thylakoid membrane - A_FL1 amplitude of the fast (<100 ms) phase of A518 induced by the first of two saturating, single-turnover flashes spaced 30 ms apart - A_FL2 amplitude of the fast (<100 ms) phase of A518 induced by the second of two saturating, single-turnover flashes spaced 50 ms apart - DCBQ 2,6-dichloro-p-benzoquinone - Fo yield of chlorophyll fluorescence when Q_A is fully oxidized - Fm yield of chlorophyll fluorescence when Q_A is fully reduced - Fx yield of chlorophyll fluorescence when Q_A is fully reduced at inactive PS II centers, but fully oxidized at active PS II centers - Pheo pheophytin - P680 the primary donor of Photosystem II - PPFD photosynthetic photon flux density - Q_A Primary quinone acceptor of PS II - Q_B secondary quinone acceptor of PS II     

Absorbance difference spectra of the S-state transitions in Photosystem II core particles

Redox changes of the oxygen evolving complex in PS II core particles were investigated by absorbance difference spectroscopy in the UV-region. The oscillation of the absorbance changes induced by a series of saturating flashes could not be explained by the minimal Kok model (Kok et al. 1970) consisting of a 4-step redox cycle, S₀ S₁ S₂ S₃ S₀, although the values of most of the relevant parameters had been determined experimentally. Additional assumptions which allow a consistent fit of all data are a slow equilibration of the S₃ state with an inactive state, perhaps related to Ca²⁺-release, and a low quantum efficiency for the first turnover after dark-adaptation. Difference spectra of the successive S-state transitions were determined. At wavelengths above 370 nm, they were very different due to the different contribution of a Chl bandshift in each spectrum. At shorter wavelengths, the S₁ S₂ transition showed a difference spectrum similar to that reported by Dekker et al. 1984b and attributed to an Mn(III) to Mn(IV) oxidation. The spectrum of absorbance changes associated with the S₂ S₃ transition was similar to that reported by Lavergne 1991 for PS II membranes. The S₀ S₁ transition was associated with a smaller but still substantial absorbance increase in the UV. Differences with the spectra reported by Lavergne 1991 are attributed to electrostatic effects on electron transfer at the acceptor side associated with the S-state dependence of proton release in PS II membranes.Abbreviations Bis-Tris (bis[2-hydroxyethyl]imino-tris[hydroxymethyl]methane) - DCBQ 2,5-dichloro-p-benzoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - PS II Photosystem II - Q_A secondary electron acceptor of PS II - S₀ to S₄ redox state of the oxygen evolving complex - Z secondary electron donor of PS II     

Development of photosynthetic electron transport reactions under the influence of phytohormones and nitrate nutrition in greening cucumber cotyledons

Development of the photosynthetic electron transport system, under the influence of hormones and nitrate-nutrition, in greening cucumber cotyledon was investigated. Both photosystems, PS I measured as DCPIP MV, and PS II as H₂O pBQ, were significantly promoted by GA and kinetin with kinetin being more effective. PS II/PS I ratio, though increased in control, did not change significantly with GA or kinetin treatment. Other partial reactions (H₂O MV/K₃Fe(CN)₆/NADP) were also promoted. Addition of KNO₃ showed concentration-dependent effects on growth and photosynthetic electron transport reactions (H₂O MV/K₃Fe(CN)₆/NADP). It is concluded that both hormones and nutritional status influence development of the photosynthetic electron transport system in greening cucumber cotyledons.Abbreviations PS I Photosystem I - PS II Photosystem II - BSA Bovine Serum Albumin - DCMU 3-(3,4-Dichlorophenyl)-1,1-Dimethyl Urea - DCPIP 2,6-Dichlorophenol Indophenol - EDTA Ethylene Diamine Tetra-acetic Acid - GA Gibberellic acid (GA₃) - HEPES (N-2-Hydroxyethyl Piperazine-N-2-Ethanesulphonic Acid) - IAA Indole-3-acetic acid - MV Methyl Viologen - NADP Nicotinamide Adenine Dinucleotide Phosphate - pBQ p-benzoquinone     

The relationship between Photosystem II intrinsic quantum yield and millisecond luminescence in thylakoids

The relationship between charge recombination at Photosystem II (PS II), as indicated by millisecond luminescence, and PS II quantum yield was studied in spinach thylakoids during electron flow to methylviologen. Under the low magnesium conditions used, a decrease in quantum yield was observed in the absence of non-photochemical excitation quenching, and therefore cannot be due to a restriction in excitation delivery to the reaction centre. It was found that the decrease of the parameter _p, which is a measure of the intrinsic quantum yield of open PS II centers, correlates with an increase in luminescence per open center. The relationship between these two parameters was the same whether _p was manipulated by dissipation of the transthylakoid pH gradient or of the electrical potential. This indicates that the mechanism by which _p decreases depends in the same way on the two components of the protonmotive force as does the charge recombination at PS II. Calculation of the yield of luminescence with respect to the back reaction will be necessary to determine whether the charge recombination occurs at a sufficiently high rate to be directly responsible for the _p decrease.     

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     

Site inhomogeneity and exciton delocalization in the photosynthetic antenna

The influence of energy disorder on exciton states of molecular aggregates (the dimer and the circular aggregate) was analyzed. The dipole strength and inhomogeneous line shapes of exciton states were calculated by means of numerical diagonalization of Hamiltonian with diagonal energy disorder without intersite correlation. The disorder degree corresponding to destruction of coherent exciton states was estimated. The circular aggregates were treated as a model of light-harvesting antenna structures of photosynthetic bacteria. It was concluded that the site inhomogeneity typical for LH1 and LH2 complexes of purple bacteria cannot significantly influence the exciton delocalization over the whole antenna.Abbreviations BChl- bacteriochlorophyll - LH1 and LH2- core and peripheral light-harvesting complexes from purple bacteria - RC- reaction center     

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.     

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     

Effect of Photosystem II inhibitor K-15 on photochemical reactions of the isolated D1/D2 cytochrome b559 complex

Effect of a highly efficient inhibitor of Photosystem II (PS II), K-15 (4-[methoxy-bis-(trifluoromethyl)methyl)-2,6-dinitrophenyl hydrazone methyl ketone), was investigated using the D1/D2/cytochrome b559 reaction centre (RC) complex. A novel approach for photoaccumulating reduced pheophytin (Pheo^–) in the absence of the strong reducing agent, sodium dithionite, was demonstrated which involved illumination in the presence of TMPD (from 5 to 100 M) under anaerobic conditions. The addition of K-15 at concentrations of 0.5 M and 2 M resulted in approx. 50% and near 100%, respectively, inhibition of this photoreaction, while subsequent additions of dithionite eliminated the inhibitory effect of K-15. Methyl viologen induced similar inhibition at much higher concentrations (>1 mM). Moreover, K-15 efficiently quenched the variable part of chlorophyll fluorescence (which is the recombination luminescence of the pair P₆₈₀ ⁺ Pheo^–). A 50% inhibition was induced by 5 M K-15 and the effect was maximal in the range 20 to 200 M. Photooxidation of P₆₈₀ in the presence of 0.1 mM silicomolybdate was also efficiently inhibited by K-15 (50% inhibition at 15 M). The data are consistent with the idea put forward earlier (Klimov et al. 1992) that the inhibitory effect of K-15 is based on facilitating a rapid recombination between Pheo^– and P₆₈₀ ⁺ (or Z⁺) via its redox properties. The inhibitor can be useful for suppressing PS II reactions in isolated RCs of PS II which are resistant to all traditional inhibitors, like diuron, and probably functions by substituting for Q_A missing in the preparation.At a concentration of 0.5–50 M K-15 considerably increased both the rate and extent of cytochrome b559 photoreduction in the presence, as well as in the absence, of 5 mM MnCl₂. Consequently it is suggested that K-15 also serves as a mediator for electron transfer from Pheo^– to cytochrome b559.Abbreviations K-15 4-[methoxy-bis-(trifluoromethyl)methyl]-2,6-dinitrophenyl hydrazone methyl ketone - P₆₈₀ the primary electron donor of PS II - Pheo pheophytin - PS II Photosystem II - Q_A and Q_B the primary and the secondary electron acceptor of PS II - RC reaction centre - SiMo silicomolybdate - TMPD N,N,N,,N,-tetramethyl-p-phenylenediamine - Z secondary electron donor of PS II     

The catalysis of nucleotide polymerization by compounds of divalent lead

Summary Soluble lead salts and a number of lead-containing minerals catalyze the formation of oligonucleotides from nucleoside 5-phosphorimidazolides. The effectiveness of lead compounds correlates strongly with their solubility. Under optimal conditions we were able to obtain 18% of pentamer and higher oligomers from ImpA. Reactions involving ImpU gave smaller yields.Abbreviations A adenosine - U uridine - Im imidazole - MeIm 1-methyl-imidazole - EDTA ethylenediaminetetraacetic acid - pA adenosine 5-phosphate - pU uridine 5-phosphate - Ap adenosine cyclic 2:3-phosphate - ATP adenosine 5-triphosphate - AppA P₁,P₂-diadenosine 5-diphosphate - pNp (N = A,U) nucleotide 2(3), 5-diphosphate - ImpA adenosine 5-phosphoreimidazolide - ImpU uridine 5-phosphorimidazolide - A ²pA adenylyl-[25]-adenosine - A ³pA adenylyl-[35]-adenosine - pA ²pA 5-phospho-adenylyl-[25]-adenosine - pA ³pA 5-phospho-adenylyl-[35]-adenosine - pUpU 5-phospho-uridylyl-uridine - pApU 5-phospho-adenylyl-uridine - pUpA 5-phospho-uridylyladenine - (pA)n (n, 2,3,4,) oligoadenylates with 5 terminal phosphate - ImpApA 5-phosphorimidazolide of adenylyl adenosine - (pA) ₅₊ pentamer and higher oligoadenylates with 5 terminal phosphate - (Ap)nA (n = 2,3,4) oligoadenylates without terminal phosphates In the following we do not specify the nature of the internucleotide linkageIn the following we do not specify the nature of the internucleotide linkage     

Characterization of the photosystem II centers inactive in plastoquinone reduction by fluorescence induction

In order to characterize the photosystem II (PS II) centers which are inactive in plastoquinone reduction, the initial variable fluorescence rise from the non-variable fluorescence level F_o to an intermediate plateau level F_i has been studied. We find that the initial fluorescence rise is a monophasic exponential function of time. Its rate constant is similar to the initial rate of the fastest phase (-phase) of the fluorescence induction curve from DCMU-poisoned chloroplasts. In addition, the initial fluorescence rise and the -phase have the following common properties: their rate constants vary linearly with excitation light intensity and their fluorescence yields are lowered by removal of Mg⁺⁺ from the suspension medium. We suggest that the inactive PS II centers, which give rise to the fluorescence rise from F_o to F_i, belong to the -type PS II centers. However, since these inactive centers do not display sigmoidicity in fluorescence, they thus do not allow energy transfer between PS II units like PS II.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - DMQ 2,5-dimethyl-p-benzoquinone - F_o initial non-variable fluorescence yield - F_m maximum fluorescence yield - F_i intermediate fluorescence yield - PS II photosystem II - Q_A primary quinone acceptor of PS II - Q_B secondary quinone acceptor of PS II     

Formation of nucleoside 5′-polyphosphates under potentially prebiological conditions

Summary Aqueous solutions of linear inorganic polyphosphates incubated in presence of Mg ions depolymerize to give trimetaphosphate. The presence of a nucleoside 5-phosphate has little influence upon the reaction. Drying the products obtained by incubating a linear polyphosphate with Mg ions in the presence of a nucleoside 5-phosphate yields nucleoside 5-polyphos-phates. The prebiological relevance of the reactions is discussed.Abbreviations P_n(n=1,2,3,) linear polyphosphate containing n phosphate residues - P₃! trimetaphosphate - A adenosine - p_nN nucleoside 5-polyphosphate containing n phosphate residues, e.g. with N = A, n = 4 - p₄N adenosine 5-tetraphosphate - _P ^* _lp_mA p_nA, (n = 1 + m); adenosine 5-polyphosphate containing n phosphate units with³³p-label on terminal 1 phosphate groups     

Evolution of PS IIα and PS IIβ centers during the greening of Euglena gracilis Z: Correlations with changes in lipid content

The building up of the two types of reaction centers, PS II and PS II, was investigated during the greening of Euglena gracilis Z cells in resting medium. The maximal values in the proportion of PS II centers (55%) and in the oxygen evolved per chlorophyll were reached at the outbreak of greening, when accumulation of galactolipids (MGDG and DGDG) rich in unsaturated fatty acids occurred, and when anionic lipids (SQDG and PG) emerged. As the greening progressed, the chlorophyll accumulation corresponded to a secondary enrichment in PS II centers, which built up more rapidly than PS II centers; correlatively, a general saturation of the fatty acids constitutive of all lipid classes took place.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DGDG digalactosyldiacylglycerol - FAME Tatty acid methyl esters - HEPES acide (N-[2-hydroxyethyl]piperazine-N-[2-ethane sulfonic] - MGDG monogalactosyldiacylglycerol - PC phosphatidylcholine - PE phosphatidylethanolamine - PG phosphatidylglycerol - PQ plastoquinone - PS I Photosystem I - PS II Photosystem II - Q_A primary quinone electron acceptor of PS II - Q_B secondary quinone electron acceptor of PS II - SQDG sulfoquinovosyldiacylglycerol     

Thermodynamics of Mn2+-binding to goat α-lactalbumin

By means of reaction calorimetry we measured the apparent enthalpy change, H^app, of the binding of Mn²⁺-ions to goat -lactalbumin as a function of temperature. The observed H^app can be written as the sum of contributions resulting from a conformational and a binding process. In combination with the thermal unfolding curve of goat -lactalbumin, we succeeded in separating the complete set of thermodynamic parameters (H, G, S, C_p) into the binding and conformational contributions. By circular dichroism we showed that NH ₄ ⁺ -ions, upon binding to bovine a-lactalbumin, induce the same conformational change as do Na⁺ and K⁺: the binding constant equals 98 ± 9 M^–1.Abbreviations BLA bovine -lactalbumin - GLA goat -lactalbumin - HLA human -lactalbumin - CD circular dichroism Offprint requests to: H. Van DaelDeceased     

The inheritance of host plant resistance and its effect on the relative infection efficiency of Magnaporthe grisea in rice cultivars

The inheritance of host plant resistance and its effect on the relative infection efficiency for leaf blast was studied in the crosses IR36/CO39 (partially resistant × highly susceptible) and IR36/IR64 (both partially resistant). On the natural scale, gene action appeared multiplicative. After log transformation, additive effects described most of the genetic variation in the cross IR36/CO39, while additive and dominance effects were about equal in magnitude in the cross IR36/IR64. Dominance was towards increased resistance. No transgressive segregation occurred in the cross IR36/CO39. The number of genes that reduce lesion number was estimated to be zero in CO39 and five or more in IR36. The cross IR36/IR64 showed transgressive segregation in both directions, and IR36 and IR64 each contain at least one gene that is not present in the other cultivar. The heritabilities (narrow sense) in the F₂ were low (range 0.06–0.16), while narrow sense heritabilities based on F₃ lines were much higher (range 0.41–0.68). Lesion numbers in F₃ lines were reasonably correlated with those in F₅ progenies derived from the same F₂ plant (r was±0.6 in both crosses). Partial resistance can be effectively improved by selecting the most resistant plants from the most resistant F₃ lines.     

Geometry of the dry-state oligomerization of 2′,3′-cyclic phosphates

Summary Evaporation of a solution of thymidine plus either theexo or theendo diastereomer of uridine cyclic 2,3-O, O-phosphorothioate (U > p(S) in 1,2-diaminoethane hydrochloride buffer gave the 2,5 and 3,5 isomers of (P-thio) uridylylthymidine (Up(S)dT) in a ratio of 1:2 with a combined yield of about 20%. These isomers were re-converted to U > p(S) and dT by a reaction that is known to proceed by an in-line mechanism. Both the 2,5 and 3,5 isomers gave as product the same diastereomer of U > p(S) that had been used originally in their formation. These dry-state prebiotic reactions (Verlander, Lohrmann, and Orgel 1973) are thus shown to be stereospecific, and both the 2,5 and 3,5 internucleotide bonds are formed by an in-line mechanism.Abbreviations DAE 1,2-diaminoethane - HPLC high pressure liquid chromatography - RNase bovine pancreatic ribonuclease A, EC 3.1.4.22 - TEAB triethylammonium bicarbonate - tris tris(hydroxymethyl)aminomethane - UMP(S) uridine monophosphorothioate - U > p uridine cyclic 2,3-phosphate - U > p(S) uridine cyclic 2,3-O, O-phosphorothioate - Up(S)dT (P-thio)uridylylthymidine - U2p(R_p-S)5dT (P-thio)uridylylthymidine with theR configuration at phosphorous, and a 2,5 internucleotide linkage     

Light dependence of quantum yields of Photosystem II and CO2 fixation in C3 and C4 plants

The light dependence of quantum yields of Photosystem II (_II) and of CO₂ fixation were determined in C₃ and C₄ plants under atmospheric conditions where photorespiration was minimal. Calculations were made of the apparent quantum yield for CO₂ fixation by dividing the measured rate of photosynthesis by the absorbed light [A/I=_CO2 and of the true quantum yield by dividing the estimated true rate of photosynthesis by absorbed light [(A+R_l)/I_a=_CO2·], where R_L is the rate of respiration in the light. The dependence of the _II/_CO2 and _II/_CO2 ^* ratios on light intensity was then evaluated. In both C₃ and C₄ plants there was little change in the ratio of _II/_CO2 at light intensities equivalent to 10–100% of full sunlight, whereas there was a dramatic increase in the ratio at lower light intensities. Changes in the ratio of _II/_CO2 can occur because respiratory losses are not accounted for, due to changes in the partitioning of energy between photosystems or changes in the relationship between PS II activity and CO₂ fixation. The apparent decrease in efficiency of utilization of energy derived from PS II for CO₂ fixation under low light intensity may be due to respiratory loss of CO₂. Using dark respiration as an estimate of R_L, the calculated _II/_CO2 ^* ratio was nearly constant from full sunlight down to approx 5% of full sunlight, which suggests a strong linkage between the true rate of CO₂ fixation and PS II activity under varying light intensity. Measurements of photosynthesis rates and _II were made by illuminating upper versus lower leaf surfaces of representative C₃ and C₄ monocots and dicots. With the monocots, the rate of photosynthesis and the ratio of _II/_CO2 exhibited a very similar patterns with leaves illuminated from the adaxial versus the abaxial surface, which may be due to uniformity in anatomy and lack of differences in light acclimation between the two surfaces. With dicots, the abaxial surface had both lower rates of photosynthesis and lower _II values than the adaxial surface which may be due to differences in anatomy (spongy versus palisade mesophyll cells) and/or light acclimation between the two surfaces. However, in each species the response of _II/_CO2 to varying light intensity was similar between the two surfaces, indicating a comparable linkage between PS II activity and CO₂ fixation.Abbreviations A measured rate of CO₂ assimilation - A+R_L true rate of CO₂ assimilation; e - CO₂ estimate of electrons transported through PSII per CO₂ fixed by RuBP carboxylase - f fraction of light absorbed by Photosystem II - F'_m yield of PSII chlorophyll fluorescence due to a saturating flash of white light under steady-state photosynthesis - F_s variable yield of fluorescence under steady-state photosynthesis; PPFD-photosynthetic photon flux density - I_a absorbed PPFD - PS II Photosystem II - R_d rate of respiration in the dark - R_I rate of respiration in the light estimated from measurement of R_d or from analysis of quantum yields - apparent quantum yield of CO₂ assimilation under a given condition (A/absorbed PPFD) - true quantum yield of CO₂ assimilation under a given condition [(A+R_L)/(absorbed PPFD)] - quantum yield for photosynthetic O₂ evolution - electrons transported via PS II per quantum absorbed by PS II Supported by USDA Competitive Grant 90-37280-5706.     

High misses after odd flashes in oxygen evolution in thoroughly dark-adapted thylakoids from pea and Chenopodium album

In Photosystem II (PS II), water is oxidized to molecular oxygen and plastoquinone is reduced to plastoquinol. The oxidation of water requires the accumulation of four oxidizing equivalents, through the so-called S-states of the oxygen evolving complex; the production of plastoquinol requires the accumulation of two reducing equivalents on a bound plastoquinone, Q_B. It has been generally believed that during the flash-induced transition of each of the S-states (S_n S_n+1, where n=0, 1, 2 and 3), a certain small but equal fraction of the PS II reaction centers are unable to function and, thus, miss being turned over. We used thoroughly dark-adapted thylakoids from peas (Pisum sativum) and Chenopodium album (susceptible and resistant to atrazine) starting with 100% of the oxygen evolving complex in the S₁ state. Thylakoids were illuminated with saturating flashes, providing a double hit parameter of about 0.07. Our experimental data on flashnumber dependent oscillations in the amount of oxygen per flash fit very well with a binary pattern of misses: 0, 0.2, 0, 0.4 during S₀ S₁, S₁ S₂, S₂ S₃ and S₃ S₀ transitions. Addition of 2 mM ferricyanide appears to shift this pattern by one flash. These results are consistent with the bicycle model recently proposed by V. P. Shinkarev and C. A. Wraight (Oxygen evolution in photosynthesis: From unicycle to bicycle, 1993, Proc Natl Acad Sci USA 90: 1834–1838), where misses are due to the presence of P⁺ or Q_A ^- among the various equilibrium states of PS II centers.Abbreviations miss parameter - double hit parameter - PS II Photosystem II - Q_A primary one-electron acceptor of PS II, a plastoquinone molecule - Q_B secondary plastoquinone two-electron acceptor of PS II - S-states (S_n, where n=0, 1, 2, 3 or 4) redox states of the oxygen evolving complex     

The formation of peptides from the 2′(3′)-glycyl ester of a nucleotide

Summary We have synthesized 2(3)-O-(glycyl)-adenosine-5-(O-methylphos-phate), an analogue of the 3-terminus of aminoacylated tRNA. A 0.4M solution of this compound maintained at pH 8.2, yields 5.5% of diglycine and 11.5% of diketopiperazine, in addition to the hydrolysis products glycine and adenosine-5-(O-methylphosphate). Under the same conditions, glycine ethyl ester reacts much more slowly, but ultimately gives similar yields of diglycine and diketopiperazine.The aminolysis of 2(3)-O-(glycyl)-adenosine-5-(O-methylphosphate) by free glycine is relatively inefficient, but serine reacts 20 times more rapidly and yields up to 50% of N-glycylserine. The prebiotic significance of these reactions is discussed.Abbreviations MepA adenosine-5-(O-methylphosphate) - MepA-gly 2(3)-O-(glycyl)-adenosine-5-(O-methylphosphate) - MepA-bis-gly 2,3-O-(bis-glycyl)-adenosine-5-(O-methylphosphate) - DKP diketopiperazine - gly Et glycine ethyl ester - gly-ser N-glycylserine - O-gly-ser O-glycylserine - O-(gly)-gly-ser O-(glycyl)-glycylserine - Boc-gly N-tert-butyloxycarbonylglycine - MepA-Boc-gly 2(3)-O-(Boc-glycyl)-adenosine-5-(O-methylphosphate) - MepA-bis-Boc-gly 2,3-O-(bis-Boc-glycyl)-adenosine-5(O-methylphosphate) - (gly)₂ diglycine - (gly)₃ triglycine