Secondary Electron Acceptor Of Photosystem Research Articles

The secondary electron acceptor of photosystem I in Gloeobacter violaceus PCC 7421 is menaquinone-4 that is synthesized by a unique but unknown pathway

The secondary electron acceptor of photosystem (PS) I in the cyanobacterium Gloeobacter violaceus PCC 7421 was identified as menaquinone-4 (MQ-4) by comparing high performance liquid chromatograms and absorption spectra with an authentic compound. The MQ-4 content was estimated to be two molecules per one molecule of chlorophyll (Chl) a′, a constituent of P700. Comparative genomic analyses showed that six of eight men genes, encoding phylloquinone/MQ biosynthetic enzymes, are missing from the G. violaceus genome. Since G. violaceus clearly synthesizes MQ-4, the combined results indicate that this cyanobacterium must have a novel pathway for the synthesis of 1,4-dihydroxy-2-naphthoic acid.

Reversed-phase HPLC determination of chlorophyll a' and naphthoquinones in photosystem I of red algae: existence of two menaquinone-4 molecules in photosystem I of Cyanidium caldarium.

Chlorophyll (Chl) a', the C13(2)-epimer of Chl a, is one of the two Chl molecules constituting the primary electron donor (P700) of photosystem (PS) I of a thermophilic cyanobacterium Synechococcus elongatus. To examine whether PS I of other oxygenic photosynthetic organisms in general contain one Chl a' molecule in P700, the pigment composition of thylakoid membranes and PS I preparations isolated from red algae Porphyridium purpureum and Cyanidium caldarium was examined by reversed-phase HPLC with particular attention to Chl a' and phylloquinone (PhQ), the secondary electron acceptor of PS I. The two red algae contained one Chl a' molecule at the core part of PS I. In PS I of C. caldarium, two menaquinone-4 (MQ-4) molecules were detected in place of PhQ used by higher plants and cyanobacteria. The 1:2:1 stoichiometry among Chl a', PhQ (MQ-4) and P700 in PS I of the red algae indicates that one Chl a' molecule universally exists in PS I of oxygenic photosynthetic organisms, and two MQ-4 molecules are associated with PS I of C. caldarium.

Reversed-phase HPLC determination of chlorophyll a' and phylloquinone in Photosystem I of oxygenic photosynthetic organisms. Universal existence of one chlorophyll a' molecule in Photosystem I.

Chlorophyll (Chl) a', the C132-epimer of Chl a, is a constituent of the primary electron donor (P700) of Photosystem (PS) I of a thermophilic cyanobacterium Synechococcus (Thermosynechococcus) elongatus, as was recently demonstrated by X-ray crystallography. To determine whether PS I of oxygenic photosynthetic organisms universally contains one molecule of Chl a', pigment compositions of thylakoid membranes and PS I complexes isolated from the cyanobacteria T. elongatus and Synechocystis sp. PCC 6803, the green alga Chlamydomonas reinhardtii, and the green plant spinach, were examined by simultaneous detection of phylloquinone (the secondary electron acceptor of PS I) and Chl a' by reversed-phase HPLC. The results were compared with the Chl a/P700 ratio determined spectrophotometrically. The Chl a'/PS I ratios of thylakoid membranes and PS I were about 1 for all the organisms examined, and one Chl a' molecule was found in PS I even after most of the peripheral subunits were removed. Chl a' showed a characteristic extraction behaviour significantly different from the bulk Chl a in acetone/methanol extraction upon varying the mixing ratio. These findings confirm that a single Chl a' molecule in P700 is the universal feature of PS I of the Chl a-based oxygenic photosynthetic organisms.

Separation and determination of minor photosynthetic pigments by reversed-phase HPLC with minimal alteration of chlorophylls.

Reversed-phase HPLC conditions for separation of chlorophyll (Chl) a, Chl a' (the C132-epimer of Chl a), pheophytin (Pheo) a (the primary electron acceptor of photosystem (PS) II), and phylloquinone (PhQ) (the secondary electron acceptor of PS 1), have been developed. Pigment extraction conditions were optimized in terms of pigment alteration and extraction efficiency. Pigment composition analysis of light-harvesting complex II, which would not contain Chl a' nor Pheo a, showed the Chl a'/Chl a ratio of 3-4 x 10(-4) and the Pheo a/Chl a ratio of 4-5 x 10(-4), showing that the conditions developed here were sufficiently inert for Chl analysis. Preliminary analysis of thylakoid membranes with this analytical system gave the PhQ/Chl a' ratio of 0.58 +/- 0.03 (n = 4), in line with the stoichiometry of one molecule of Chl a' per PS I.

HPLC determination of chlorophyll a' and phylloquinone in photosystem I of higher plants and cyanobacteria

Stoichiometries of Chlorophyll (Chl) a?, C132-epimer of Chl a, within Photosystem (PS) I of higher plants and cyanobacteria were determined by simultaneous detection of Chl a? and phylloquinone, the secondary electron acceptor of PS I (two molecules per PS I), with reversed-phase HPLC analyses, and spectrophotometric determination of P700, the primary donor of PS I. For thermophilic cyanobacterium Synechococcus elongatus, the Chl a?/PhQ ratio was about 0.5 for cell, thylakoid membranes, and PS I trimer. The Chl a?/P700 ratio and the PhQ/P700 ratio were about 1 and 2, respectively, for thylakoid membranes and PS I trimer. Theses results showed that a 1:1:2 stoichiometry is established among Chl a?, P700, and PhQ within PS I of S. elongatus. PS I trimer isolated from Synechocystis PCC 6803 also showed the same stoichiometry as that found for S. elongatus. Native PS I complexes of spinach and barley also showed the 1:1:2 stoichiometry among Chl a?, P700, and PhQ. Treatments of PS I with water-saturated/dry diethyl ether mixtures enriched Chl a? up to the Chl a/Chl a? ratio of 9-10 with keeping the Chl a?/P700 ratio at about 1. These findings show that higher plant PS I contains one molecule of Chl a? in close association with the primary donor, P700. Pigment composition analyses with reversed-phase HPLC indicate that one molecule of Chl a? is universally present in PS I of oxygenic photo-autotroph as the indispensable ingredient, possibly as the constituent of the primary donor, P700.

Recruitment of a Foreign Quinone into the A1 Site of Photosystem I: I. GENETIC AND PHYSIOLOGICAL CHARACTERIZATION OF PHYLLOQUINONE BIOSYNTHETIC PATHWAY MUTANTS IN SYNECHOCYSTIS SP. PCC 6803

Genes encoding enzymes of the biosynthetic pathway leading to phylloquinone, the secondary electron acceptor of photosystem (PS) I, were identified in Synechocystis sp. PCC 6803 by comparison with genes encoding enzymes of the menaquinone biosynthetic pathway in Escherichia coli. Targeted inactivation of the menA and menB genes, which code for phytyl transferase and 1,4-dihydroxy-2-naphthoate synthase, respectively, prevented the synthesis of phylloquinone, thereby confirming the participation of these two gene products in the biosynthetic pathway. The menA and menB mutants grow photoautotrophically under low light conditions (20 microE m(-2) s(-1)), with doubling times twice that of the wild type, but they are unable to grow under high light conditions (120 microE m(-2) s(-1)). The menA and menB mutants grow photoheterotrophically on media supplemented with glucose under low light conditions, with doubling times similar to that of the wild type, but they are unable to grow under high light conditions unless atrazine is present to inhibit PS II activity. The level of active PS II per cell in the menA and menB mutant strains is identical to that of the wild type, but the level of active PS I is about 50-60% that of the wild type as assayed by low temperature fluorescence, P700 photoactivity, and electron transfer rates. PS I complexes isolated from the menA and menB mutant strains contain the full complement of polypeptides, show photoreduction of F(A) and F(B) at 15 K, and support 82-84% of the wild type rate of electron transfer from cytochrome c(6) to flavodoxin. HPLC analyses show high levels of plastoquinone-9 in PS I complexes from the menA and menB mutants but not from the wild type. We propose that in the absence of phylloquinone, PS I recruits plastoquinone-9 into the A(1) site, where it functions as an efficient cofactor in electron transfer from A(0) to the iron-sulfur clusters.

The molecular basis of triazine herbicide resistance in Senecio vulguris L.

The widespread use of triazine herbicides for weed control has resulted in the appearance of triazine-resistant populations of plants of several species throughout the world ( LeBaron & Gressel, 1982). Triazine herbicides act by inhibiting photosynthetic electron transfer on the reducing side of Photosystem 11, and, by the use of the photoaffinity label azido-atrazine, i t has been shown that the herbicide binds to the 32 kDa polypeptide which is the binding site for QR. the secondary electron acceptor of Photosystem I 1 (Steinback et ul.. 1981), and analysis of the genes from atrazine-resistant and atrazine-susceptible biotypes of Atnurunthus li>dwidu.s (Hirschberg & Mclntosh, 1983) and Solunum nigrum (Hirschberg et d., 1984; Goloubinoff ct u/., 1984) by nucleotide sequencing has indicated that a single base change resulting in a single amino acid change is responsible for the sensitivity of the plants to atrazine. A population of groundsel (Scwecio vulguris L.) plants resistant to triazines were discovered in a tree nursery in Stone, Staffordshire. U.K., in 1981 (Putwain, cited in LeBaron & Gressel, 1982). The molecular basis of atrazine resistance in these plants is being investigated. The results of [I4C]atrazine binding to isolated chloroplast membranes (Pfister et ul., 1979) indicate a significant difference in the binding of this herbicide between resistant and suceptible groundsel plants. Chloroplast membranes from the susceptible plants had a binding constant (K) of4.0 x 10 M-atrazine whereas no detectable binding of atrazine was observed with chloroplast membranes from the atrazine-resistant plants.

Interference by DDT and cyclodiene types of insecticides with chloroplast-associated reactions

The effects of DDT, some of its analogs, and selected cyclodiene insecticides on isolated spinach ( Spinacea oleracea L.) thylakoids were identified, characterized, and compared to responses induced by selected herbicides. Except for endrin, the insecticides inhibited light-induced electron transport, altered chlorophyll fluorescence transients, and competitively displaced [ 14C]atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)- s-triazine], a known photosystem II inhibitor, from the membranes. The insecticides appeared to act at, or near B, the secondary electron acceptor of photosystem II. Binding of DDT and dieldrin was estimated at 900 and 2200 molecules, respectively, per photosynthetic unit (490 chlorophyll molecules). The insecticides also inhibited valinomycin-induced swelling of the thylakoid membrane. Whereas inhibition of electron transport can be attributed to interaction by the insecticides with a proteinaceous component of the thylakoid membrane, interference with the action of valinomycin may involve interaction with lipoidal constituents of the membrane.

Localization of electron transport inhibition in plastoquinone reactions.

Reduction kinetics of P700 following a short flash are measured in spinach chloroplasts after oxidation of the electron carriers between the two photoreactions by far-red light. Three features of the kinetics allow us to localize simultaneously inhibition at different sites between photoreaction II and the reducing site of plastoquinol. These are the initial lag, the halftime and the area under the transient of the P700 absorbance change, which indicate the electron transfer time from photoreaction II to the reducing site of plastoquinol, the rate of plastoquinol oxidation, and the number of electrons transferred to the special plastoquinone B functioning as secondary electron acceptor of photosystem II, respectively. As an additional diagnostic parameter for inhibition before and after the plastoquinone pool, the area under the transient of the P700 absorbance change is used after long flashes. This area is proportional to the amount of reduced plastoquinone as shown by the absorbance change at 265 nm. The effects of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) are compared with those of 2-bromo-4-nitrothymol, 2,4-dinitrophenyl ether of 2-iodo-4-nitrothymol, and Illoxan as representatives for new classes of inhibitors. While 2-halogeno-4-nitrothymols inhibit the reduction of plastoquinone similarly to DCMU, their diphenyl ether derivatives inhibit selectively the oxidation of plastoquinol.

Effect of High Cation Concentrations on Photosystem II Activities

The effects of wide concentration ranges of NaCl, KCl, and MgCl(2) on ferricyanide reduction and the fluorescence induction curve of isolated spinach (Spinacia oleracea) chloroplasts were investigated. Concentrations of the monovalent salts above 100 mm and MgCl(2) above 25 mm produced a decrease in the rate of ferricyanide reduction by thylakoids uncoupled with 2.5 mm NH(4)Cl which cannot be attributed to changes in the primary photochemical capacity of photosystem II. Salt-induced decreases in the effective concentration of the secondary electron acceptor of photosystem II, plastoquinone, reduce the capacity for secondary photochemistry of photosystem II and this could contribute to the reduction in ferricyanide reduction by uncoupled thylakoids at high salinities. The rate of ferricyanide reduction by coupled thylakoids is little affected by salinity changes, indicating that the rate-limiting phosphorylation mechanism in electron flow from water to ferricyanide in coupled thylakoids is salt-tolerant, whereas the rate-limiting reaction in uncoupled ferricyanide reduction is considerably affected by salinity changes. Salt-induced changes in the fluorescence induction curve are interpreted in terms of changes in the rate constants for excitation decay by radiationless transitions, exciton transfer from photosystem II chlorophylls to other associated chlorophyll species, and photochemistry.

Inhibition of the reoxidation of the secondary electron acceptor of Photosystem II by bicarbonate depletion

In bicarbonate-depleted chloroplasts, the chlorophyll a fluorescence decayed with a halftime of about 150 ms after the third flash, and appreciably faster after the first and second flash of a series of flashes given after a dark period. After the fourth to twentieth flashes, the decay was also slow. After addition of bicarbonate, the decay was fast after all the flashes of the sequence. This indicates that the bicarbonate depletion inhibits the reoxidation of the secondary acceptor R 2− by the plastoquinone pool; R is the secondary electron acceptor of pigment system II, as it accepts electrons from the reduced form of the primary electron acceptor (Q −). This conclusion is consistent with the measurements of the DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea)-induced chlorophyll a fluorescence after a series of flashes in the presence and the absence of bicarbonate, if it is assumed that DCMU not only causes reduction of Q if added in the state QR −, but also if added in the state QR 2−.

Wirkungsmechanismus und Struktur-Aktivitätsbeziehungen des Aminotriazinon-Herbizids Metribuzin. Hemmung des photosynthetischen Elektronentransports von Chloroplasten durch Metribuzin / Mode of Action and Structure-Acitivity-Relationships of the Aminotriazinone Herbicide Metribuzin. Inhibition of Photosynthetic Electron Transport in 'Chloroplasts by Metribuzin

The influence of the aminotriazinone herbicide Metribuzin on photosynthetic reactions of isolated chloroplasts is investigated. Metribuzin inhibits all Hill-reactions when water is the electron donor, but not photoreductions by photosystem I at the expense of an artifical electron donor. The PI59-value is 6.7. Cyclic photophosphorylation is not affected by Metribuzin. Measurements of the prompt and delayed fluorescence of the photosynthetically active chlorophyll support the notion, that Metribuzin inhibits photosynthetic electron flow between the primary and seondary electron acceptor of photosystem II (Q and plastoquinone). The relationship of inhibitory potency to chemical structure is investigated by comparing a number of related aminotriazinones. The effect of various substituents is discussed.