Difference spectra and extinction coefficients of P 700

Photochemical activities and organization of photosynthetic apparatus of C3 and C 4 plants grown under different light intensities

Changes in the photochemical activities, influenced by variation in the growth light intensity, were followed in typical C3 (Phaseolus, Ipomoea) and C4 (Amaranthus, Sorghum) plants. Progressive decrease in the growth light intensity accelerated the O-P fluorescence induction in whole leaves. Such acceleration of the fluorescence kinetics was found to be not due to enhanced photosystem II activity but possibly a result of reduced rate of electron flow between the two photosystems. This is supported by 4 lines of evidence: (1) by the Hill activity determined in the presence of electron acceptors functioning before and after plastoquinone; (2) the photosynthetic unit size determined after flash excitation showing variations that were apparently too small to account for the changes observed fluorescence induction; (3) modification of the kinetics of secondrange light-induced absorbance changes at 520 nm; and (4) absence of significant changes in the ratio of P700/total chlorophyll ratio.The P700/cytochrome f ratio, however, increased from the usual 1-1.5 to 3-4 in plants grown under 9% sunlight. Increase in the P700/cytochrome f ratio was found to be due to a decrease in the cytochrome f/chlorophyll ratio, and this was due to perhaps to a simultaneous increase in chlorophyll and decrease in cytochrome content.

Partial characterization of six chlorophyll a-protein complexes isolated from a blue-green alga by a nondetergent method

The chlorophyll a-protein complexes of a blue-green alga, Phormidium luridum, are resolved in the absence of detergent, by the combination of a Sepharose 4B column and sucrose density gradient centrifugation, into six chlorophyll a-containing zones. These six complexes are termed here as F15, F25, F35, F40, F60, and F65, according to their appearance in the sucrose density gradient after centrifugation. The absorption spectra of these six isolated complexes are reported, as well as the fluorescence emission spectra at room temperature and liquid-nitrogen temperature. The F60 complex was enriched in Photosystem I, while the F35 and F40 complexes contained both Photosystem I and II. The F60 complex is the predominant band and accounts for about 49% of the total chlorophyll a of the cells. The extinction coefficient of this complex is determined to be 68.7 mM-1 cm-1 at 680 nm in 50 mM tris(hydroxymethyl)amino methane buffer at pH 8.0. In addition, the effect of the detergent, sodium dodecyl sulfate, on these spectra are also reported for comparison. The chemically induced difference spectra of F35, F40, and F60 complexes also indicate the presence of the reaction center, P700, of Photosystem I. These three complexes have been shown to contain P700 in a ratio of approximately one reaction center molecule per 100 light-harvesting chlorophyll molecules. The simple exposure of the F60 fraction to sodium dodecyl sulfate results in an "apparent" enhancement of the P700 to chlorophyll ratio to one P700 per 51 light harvesting chlorophyll. Room temperature electron spin resonance measurements of photooxidized F60 are consistent with the presence of P700 and with the chlorophyll/P700 ratio observed by chemical assay. The amino acid compositions of F60 and F65 complexes are studied. Gel electrophoresis patterns of these six isolated complexes are presented and are significantly different from those reported for detergent-treated chlorophyll-protein complexes.

Quantum efficiency and antenna size of photosystems II alpha, II beta and I in tobacco chloroplasts

Reaction center concentrations were determined in chloroplasts of tobacco, cv John William's Broadleaf, and its mutants Su/su and Su/su var. Aurea. Quantum yields of the primary reactions of Photosystems I, II alpha and II beta (Melis, A. and Homann, P.H. (1975) Photochem. Photobiol. 21, 431--437) were obtained by measurement of their rate constants and the absorbed energy, under conditions where all three photosystems operated simultaneously and produced almost irreversibly a single charge separation. The concentrations and reaction rates of the photosystems were different in chloroplasts from the wild type and the mutants, but in chloroplasts of each type of plant used essentially all quanta absorbed by chlorophyll caused a charge separation in PS I, PS II alpha or PS II beta. since the quantum efficiency of each photosystem was close to one, kinetic differences between the photosystems and between different kinds of chloroplasts were only due to differences in antenna size. From the rate constants the number of chlorophyll molecules in the antenna of each photosystem could be calculated. It is argued that PS II alpha and PS II beta must be different, independent structures.

Relative activities of linear and cyclic electron flows during chloroplast CO2-fixation

Evolution of oxygen and turnover of cytochromes b-563 and f were measured upon illumination of isolated intact spinach chloroplasts with a series of flashes. The flash yield of cytochrome f oxidation approximated the sum of the yields of cytochrome b-563 reduction and electron transfer through Photosystem II, regardless of whether HCO(-3), 3-phosphoglycerate or O2 served as the terminal electron acceptor. No absorbance contribution form cytochrome b-559 was discerned within the time range studied. Some pseudocyclic electron flow occurred when both HCO(-3) and 3-phosphoglycerate were omitted, and possibly also during induction of photosynthesis; however, the flash yield data suggest tht O2 is not reduced at a significant rate during steady state photosynthesis. The maximum rate of cytochrome f turnover (1000 microequiv./mg chlorophyll per h) was adequate to support the highest rates of photosynthesis observed in isolated chloroplasts. These results agree with the concept that cytochrome f is a component both of the linear and cyclic pathways whereas cytochrome b-563 functions only in the cyclic pathway. NH4Cl decreased the half time of cytochrome b-563 oxidation fro 11.6 to 8.2 ms and decreased the half time of cytochrome f reduction from 7.2 to 2.8 ms. The cyclic and linear pathways thus seem to be jointly regulated by a transthylakoid H+ gradient through a common control point on the reducing side of cytochrome f. Cyclic turnover also increased during the induction phase of photosynthesis, when linear throughput is limited by the rate of utilization of NADPH. The slow rise in the P-518 transient correlated with increased cyclic activity under the above conditions. It is proposed that flexibility in the utilization of linear and cyclic pathways allows the chloroplast to generate ATP and NADPH in ratios appropriate to varying needs.

Components and activity of the photosynthetic electron transport system of intact heterocysts isolated from the blue-green alga Nostoc muscorum

Heterocysts of the blue-green alga Nostoc muscorum have been isolated by prolonged treatment with lysozyme. Quantitative data are presented which show the occurrence of cytochromes c-553, f-557 and b-563 in heterocysts in amounts comparable to vegetative cells. Particularly the content of the water-soluble cytochrome c-553 can be used to evaluate the intactness of a heterocyst preparation. Cytochrome f-557 has been partially purified and found to be a c-type cytochrome corresponding to cytochrome f of higher plants and other algae. Cytochrome b-559 is present in vegetative cells but not in heterocysts. The content of plastoquinone in heterocysts is reduced to 42% of the amount present in vegetative cells. These data suggest a degradation of Photosystem II during heterocyst differentiation. Measurements of photosynthetic electron transport in heterocysts proved the inability of the photosynthetic apparatus to carry out electron transport with electrons donated by water or diphenylcarbazide. In Tris-washed thylakoids from vegetative cells, however, diphenylcarbazide can act as an electron donor to Photosystem II.

Effect of detergents on the reliability of a chemical assay for P-700

A chemical assay for P-700 was developed using 0.36 mM potassium ferricyanide as oxidant and 1.6 mM sodium ascorbate as reductant. The major difference from other chemical assays for P-700 is procedural. The method is designed to take advantage of the availability of microprocessor-linked spectrophotometers to obtain greater accuracy by minimizing the spectral changes due to irreversible oxidized antenna chlorophyll molecules. The value measured for the P-700 concentration in a sample of chloroplasts was not changed by the presence of EDTA, Mg2+ or sucrose in the assayed solution. Similarly, half of the detergents tested (Triton X-100, Nonidet P-40, digitonin, Deriphat 160, Miranol S2M-SF and Miranol M2M) did not alter the value when added to the chloroplasts. The remainder of the detergents examined caused a significant decrease or increase in the value for P-700 content. Sodium dodecyl sufate, of particular interest due to its widespread use, caused a doubling in the amount of apparent P-700. This effect may be due to this detergent and some others enabling an additional long wavelength form of chlorophyll, possibly an intermediary electron acceptor in Photosystem I, to be chemically oxidized and reduced under the assay conditions.

Absorption and fluorescence spectra of chlorophyll-proteins isolated from Euglena gracilis

A spectroscopic study of chlorophyll-protein complexes isolated from Euglena gracilis membranes was carried out to gain information about the state of chlorophyll in vivo and energy transfer in photosynthesis. The membranes were dissociated by Triton X-100 and separated into fractions by sucrose gradient centrifugation and hydroxyapatite chromatography. Four different types of chlorophyll-protein complexes were distinguished from each other and from detergent-solubilized chlorophyll in these fractions by examination of their absorption, fluorescence excitation (400--500 nm) and emission spectra at low temperature. These types were: (1). A mixture of antenna chlorophyll a- and chlorophyll ab-proteins with an absorption maximum at 669 and emission at 682 nm; (2) a P-700-chlorophyll a-protein (chlorophyll : P-700 = 30 : 1), termed CPI with an absorption maximum at 676 nm and emission maxima at 698 and 718 nm; (3) a second chlorophyll a-protein (CPI-2) less enriched in P-700, with an absorption maximum at 676 nm and emission maxima at 680, 722 and 731 nm; (4) a third chlorophyll a-protein (CPa1) with no P-700, absorption maxima at 670 and 683 nm, and an unusually sharp emission maximum at 687 nm. Treatment of CPa1 with sodium dodecyl sulfate drastically altered its spectroscopic properties indicating that at least some chlorophyll-proteins isolated with this detergent are partially denatured. The results suggest that the complex absorption spectra of chlorophyll in vivo are caused by varying proportions of different chlorphyll-protein complexes, each with different groups of chlorophyll molecules bound to it and making up a unique entity in terms of electronic transitions.

P-700 content and polypeptide profile of chlorophyll-protein complexes of spinach and barley thylakoids

Of the six chlorophyll-protein complexes of spinach and barley resolved by mild gel electrophoresis, two were chlorophyll a-protein complexes of PS I, namely CP1a and CP1, which accounted for up to 30% of the total chlorophyll. Both of these complexes had one P-700 per 120 chlorophyll a molecules. Since spinach and barley thylakoids have some 400 chlorophyll molecules per P-700, these complexes may not have lost any of the chlorophyll associated with them in vivo. This may account for CP1a and CP1 having the characteristic low-temperature fluorescence normally associated with PS I in vivo, which is not found in complexes with low chlorophyll/P-700 ratios. Two-dimensional electrophoresis showed that all of the chlorophyll a and P-700 of CP1 was bound to 70 kilodalton polypeptides. The PS I reaction centre complex of lowest mobility, CP1a, contained CP1 and four additional low molecular weight polypeptides. The three light-harvesting complexes resolved had major 25 and 23 kilodalton polypeptides. The presumed reaction centre complex of PS II contained major 50 and 47 kilodalton polypeptides.

The P-700-chlorophyl alpha-protein complex and two major light-harvesting complexes of Acrocarpia paniculata and other brown seaweeds

Acrocarpia paniculata thylakoids were fragmented with Triton X-100 and the pigment-protein complexes so released were isolated by sucrose density gradient centrifugation. Three main chlorophyll-carotenoid-protein complexes with distinct pigment compositions were isolated. (1) A P-700-chlorophyll a-protein complex, with a ratio of 1 P-700: 38 chlorophyll a: 4 beta-carotene molecules, had similar absorption and fluorescence characteristics to the chlorophyll-protein complex 1 isolated with Triton X-100 from higher plants, green algae and Ecklonia radiata. (2) an orange-brown complex had a chlorophyll a : c2 : fucoxanthin molar ratio of 2 : 1 : 2. this complex had no chlorophyll c1 and contained most of the fucoxanthin present in the chloroplasts. This pigment complex is postulated to be the main light-harvesting complex of brown seaweeds. (3) A green complex had a chlorophyll a : c1 : c2 : violaxanthin molar ratio of 8 : 1 : 1. This also is a light-harvesting complex. the absorption and fluorescence spectral characteristics and other physical properties were consistent with the pigments of these three major complexes being bound to protein. Differential extraction of brown algal thylakoids with Triton X-100 showed that a chlorophyll c2-fucoxanthin-protein complex was a minor pigment complex of these thylakoids.

The fractionation of plant photoactive pigment-protein complexes I and II

The pigment-protein complexes enriched with Photosystem I (PPC-I) and Photosystem II (PPC-II) were obtained using sievorptive chromatography on DEAE-Sephadex column. Both types of complexes contain Chlorophyll a, beta-carotene and minor quantities of Chl b. Red absorbance maxima are located at 676 nm and 673 nm for PPC-I and PPC-II, respectively. The degrees of reaction centre enrichment were measured by the method of differential spectrophotometry: PPC-I has one P-700 per 35 bulk Chl a molecules, PPC-II contains one P-680 per 18 bulk Chl a molecules. The yield of PPC-II is 7--10 times lower than that of PPC-I. After one chromatographic procedure the amount of P-680 in PPC-I preparation does not exceed 7% of that of P-700, the amount of P-700 in PPC-II preparation 2% of that of P-680. The product of PPC-II degradation was studied.

Flash-induced charge separation and dark recombination in a photosystem-II subchloroplast particle

The decay time of flash-induced absorption changes in a Photosystem-II subchloroplast fragment is very temperature sensitive down to 210 K, below which it remains constant at 1.25 +/- 0.05 ms. The difference spectrum from the near-infra-red to the ultraviolet regions indicates that the monophasic decay represents charge recombination between P-680+ and the reduced primary acceptor. The charge recombination proceeds by electron tunneling. The P-680 concentration in the TSF-IIa fragment was estimated to be one in 30 +/- 5 total chlorophyll molecules.

Electron and proton transfers from P-430 to ferredoxin-NADP-reductase in Chlorella cells

After blocking Photosystem II on whole Chlorella cells, we measured the absorption changes between 0 degrees C and -10 degrees C. The absorption changes measured 2 mus after the beginning of a Xenon Flash are the sum of changes due to P(+)-700 and changes due to P(-)-430 (after the subtraction of the carotenoid triplet change and of the electrochromic effect). The reduction of P(-)-430 is not resolved by our technique. Its reoxidation presents a half-time around 1 mus at 0 degrees C and around 2 mus at -10 degrees C. The reduction and protonation of ferredoxin-NADP-reductase to its neutral semi-quinoid form FNRH present a half-time of about 3 mus at 0 degrees C and 6 mus at -10 degrees C. The presence of only one photoreducible ferredoxin-NADP-reductase per Photosystem I center is confirmed, but is an acceptor X' the differential extinction coefficients of which are weak or null from 420 nm to 480 nm. Tentative explanations which would reconcile these results with what was already known about ferredoxin are proposed.

Contribution to the structural characterization of eucaryotic PSI reaction centre - II. Characterization of a highly purified photoactive SDS-CP1 complex

Under precise conditions, SDS PAGE† allows purification of a photoactive P700-chla-protein complex from eucaryotic cells. The yield of P700 recovery is close to 100%. A total protein content equivalent to about 140 kD for one mole of P700 has been estimated by chemical analysis, and electrophoresis revealed the presence of two peptidic chains with MWs close to 65 kD. Photochemical and structural properties of this complex are given and compared with those of other complexes previously isolated.

The intensification of absorbance changes in leaves by light-dispersion : Differences between high-light and low-light leaves

In dispersive samples, like leaves, the absorbance of pigments is intensified. The intensification is due to a longer optical path through the dispersive sample. However, in chloroplast suspensions the optical path is not much longer than in clear solutions. The factor of intensification β (=the lengthening of the optical path) is calculated by comparing the absorbance of leaves and the absorbance of chloroplast suspensions with equal pigment-content. This method also includes the influence of possible sieve effects which could decrease absorbance. The measurements are carried out with high- and low-light leaves of different thickness and pigment content. The intensification of absorbance was 2-2.5 fold. In highlight leaves it was somewhat less than in low-light leaves. The factor β is better correlated to the pigment content than to the thickness of the leaves. The plot of absorbance versus the pigment content of the leaves shows that β decreases with increasing pigment content. In contrast, chloroplast suspensions show a linear dependence as expected from Lambert-Beer's law. Thus, in leaves with very low pigment content the absorbance is intensified up to 6 fold while the intensification decreases with increasing absorbance. These results are in good agreement with measurements of Tsel'niker (1975) and with the theoretical predictions of Butler's formula (1960). Absorbance changes due to photooxidation of P-700 and cytochrome f in intact leaves are measured, and β is used to calculate the amount of the oxidized components. Without correction for β the values would be much greater than the amount actually present. The corrected data show that between 70 and 90% of the present P-700 and cytochrome f can be photooxidized in the intact leaf.

Measurements of cytochrome f and P-700 in intact leaves of Sinapis alba grown under high-light and low-light conditions

The oxidation and reduction of cytochrome f and P-700 is measured spectrophotometrically in leaves of low-light and high-light plants. After illumination with red light, an induction phenomenon for cytochrome f oxidation is observed which indicates a regulation of photosystem I activity through energy distribution between the pigment systems by the energy state of the membrane. After far-red excitation the reduction of cytochrome f in the dark is much slower in low-light leaves. This shows that cyclic electron transport is not improved in low-light plants under these conditions. P-700 is oxidized on excitation with far-red light. However, with high intensities of far-red light, P-700 is partially reduced again which is due to a low extent of photosystem II excitation with the far-red used in the experiments. The low-light leaves show greater sensitivity of photosystem II to this excitation. The initial rate of the cytochrome f oxidation-rate is the same in low-light and high-light leaves. This shows that several P-700 are connected with only one electron transport chain. The consequences of these results concerning the tripartite concept and the photosynthetic unit are discussed. In the high-light plants the experimental data can be well explained by the tripartite organization of the photosynthetic unit. In low-light plants, however, a multipartite organization has to be postulated. In the partition regions of the grana, several antennae systems I, antennae systems II, and light-harvesting complexes can communicate with one electron transport chain.

The electrophoretic isolation and partial characterization of three chlorophyll-protein complexes from blue-green algae

Three chlorophyll-protein complexes have been resolved from blue-green algae using an improved procedure for membrane solubilization and electrophoretic fractionation. One complex has a red absorbance maximum of 676 nm and a molecular weight equivalency of 255 000 +/- 15 000. A second complex has an absorbance maximum of 676 nm, a molecular weight equivalency of 118 000 +/- 8000, and resembles the previously described P-700-chlorophyll a-protein (CPI) of higher plants and algae. The third chlorophyll-protein has a red absorbance maximum of 671 nm and a molecular weight equivalency of 58 000 +/- 5000. Blue-green algal membrane fractions enriched in Photosystem I and heterocyst cells do not contain this third chlorophyll-protein, whereas Photosystem II-enriched membrane fractions and vegetative cells do. A component of the same spectral characteristics and molecular weight equivalency was also observed in chlorophyll b-deficient mutants of barley and maize. It is hypothesized that this third complex is involved in some manner with Photosystem II.

Cytochrome function in the cyclic electron transport pathway of chloroplasts

Flash excitation of isolated intact chloroplasts promoted absorbance transients corresponding to the electrochromic effect (P-518) and the alpha-bands of cytochrome b6 and cytochrome f. Under conditions supporting coupled cyclic electron flow, the oxidation of cytochrome b6 and the reduction of cytochrome f had relaxation half-times of 15 and 17 ms, respectively. Optimal poising of cyclic electron flow, achieved by addition of 0.1 microM 3-(3,4-dichlorophenyl)-1,1-dimethylurea, increased phosphorylation of endogenous ADP and prolonged these relaxation times. The presence of NH4Cl, or monensin plus NaCl, decreased the half-times for cytochrome relaxation to approximately 2 ms. Uncouplers also revealed the presence of a slow rise component in the electrochromic absorption shift with formation half-time of about 2 ms. Ths inhibitors of cyclic phosphorylation antimycin and 2,5-dibromo-3-methyl-6-isoprophy-p-benzoquinone abolished the slow rise in the electrochromic shift and prolonged the uncoupled relaxation times of cytochromes b6 and f by factors of ten or more. These observations indicate that cytochrome b6, plastoquinone and cytochrome f participated in a coupled electron transport process responsible for cyclic phosphorylation in intact chloroplasts. Estimations of cyclic phosphorylation rates from 40 to 120 mumol ATP/mg chlorophyll per h suggest that this process can provide a substantial fraction of the ATP needed for CO2 fixation.