Flash-induced absorption changes of the primary donor of photosystem II at 820 nm in chloroplasts inhibited by low pH or tris-treatment

A comparative study is made, at 15 °C, of flash-induced absorption changes around 820 nm (attributed to the primary donors of Photosystems I and II) and 705 nm (Photosystem I only), in normal chloroplasts and in chloroplasts where O₂ evolution was inhibited by low pH or by Tris-treatment.At pH 7.5, with untreated chloroplasts, the absorption changes around 820 nm are shown to be due to P-700 alone. Any contribution of the primary donor of Photosystem II should be in times shorter than 60 μs.When chloroplasts are inhibited at the donor side of Photosystem II by low pH, an additional absorption change at 820 nm appears with an amplitude which, at pH 4.0, is slightly higher than the signal due to oxidized P-700. This additional signal is attributed to the primary donor of Photosystem II. It decays ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ about 180 μs) mainly by back reaction with the primary acceptor and partly by reduction by another electron donor. Acid-washed chloroplasts resuspended at pH 7.5 still present the signal due to Photosystem II ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ about 120 μs). This shows that the acid inhibition of the first secondary donor of Photosystem II is irreversible.In Tris-treated chloroplasts, absorption changes at 820 nm due to the primary donor of Photosystem II are also observed, but to a lesser extent and only after some charge accumulation at the donor side. They decay with a half-time of 120 μs.     

A flash photolysis ESR study of Photosystem II Signal IIvf, the physiological donor to P-680+

In flash-illuminated, oxygen-evolving spinach chloroplasts and green algae, a free radical transient has been observed with spectral parameters similar to those of Signal II ($\text{g ≈ 2.0045}$, $\text{ΔH}{}_{\mathrm{<mtext>pp</mtext>}}\text{≈ 19}\text{G}$). However, in contrast with ESR Signal II, the transient radical does not readily saturate even at microwave power levels of 200 mW. This species is formed most efficiently with “red” illumination ($\text{λ < 680}\text{nm}$ and occurs stoichiometrically in a 1 : 1 ratio with $\text{P-700}{}^{\mathrm{+}}$. The Photosystem II transient is formed in less than 100 μs and decays via first-order kinetics with a halftime of 400–900 μs. Additionally, the $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ for radical decay is temperature independent between 20 and 4 °C; however, below 4 °C the transient signal exhibits Arrhenius behavior with an activation energy of approx. 10 kcal · mol^?1. Inhibition of electron transport through Photosystem II by $\text{o-}\text{phenanthroline}$, 3-(3,4-dichlorophenyl)-1,1-dimethylurea or reduced $\text{2,5-}\text{dibromo}\text{-3-}\text{methyl}\text{-6-}\text{isopropyl}\text{-p-}\text{benzoquinone}$ suppresses the formation of the light-induced transient. At low concentrations (0.2 mM), $\text{2,5-}\text{dibromo}\text{-3-}\text{methyl}\text{-6-}\text{isopropyl}\text{-p-}\text{benzoquinone}$ partially inhibits the free radical formation, however, the decay kinetics are unaltered. High concentrations of $\text{2,5-}\text{dibromo}\text{-3-}\text{methyl}\text{-6-}\text{isopropyl}\text{-p-}\text{benzoquinone}$ (1–5 mM) restore both the transient signal and electron flow through Photosystem II. These findings suggest that this “quinoidal” type ESR transient functions as the physiological donor to the oxidized reaction center chlorophyll, $\text{P-680}{}^{\mathrm{+}}$.     

Isolation and characterization of Photosystem I and II membrane particles from the blue-green alga, Synechococcus cedrorum

Fractions enriched in either Photosystem I or Photosystem II activity have been isolated from the blue-green alga, Synechococcus cedrorum after digitonin treatment. Sedimentation of this homogenate on a 10–30% sucrose gradient yielded three green bands: the upper band was enriched in Photosystem II, the lowest band was enriched in Photosystem I, while the middle band contained both activities. Large quantities of both particles were isolated by zonal centrifugation, and the material was then further purified by chromatography on DEAE-cellulose.The resulting Photosystem II particles carried out light-induced electron transport from semicarbizide to ferricyanide of over 2000 μmol/mg Chlorophyll per h (which was sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea), and was nearly devoid of Photosystem I activity. This particle contains β-carotene, very little phycocyanin, has a chlorophyll absorption maximum at 675 nm, and a liquid N₂ fluorescence maximum at 685 nm. The purest Photosystem II particles have a chlorophyll to cytochrome $\text{b-559}$ ratio of 50 : 1. The Photosystem I particle is highly enriched in $\text{P-700}$, with a chlorophyll to $\text{P-700}$ ratio of 40 : 1. The physical structure of the two Photosystem particles has also been studied by gel electrophoresis and electron microscopy. These results indicate that the size and protein composition of the two particles are distinctly different.     

Changes in the redox state of the secondary acceptor of Photosystem II associated with light-induced thylakoid protein phosphorylation

Thylakoid membrane protein phosphorylation affects photochemical reactions of Photosystem II. Incubation of thylakoids in the light with ATP leads to: (1) an increase in the amplitude of three components (4–6, 25–45 and 280–300 μs) of delayed light emission after a single flash without any change in their kinetics; (2) a reduction of the flash-dependent binary oscillations of chlorophyll a fluorescence yield associated with electron transfer from the primary quinone acceptor, Q, to the secondary quinone acceptor, B; (3) an increase in the $\text{B}{}^{\mathrm{?}}\text{B}$ ratio resulting from an increase in stability of the semiquinone anion during dark adaptation; and (4) no change in the redox state of the plastoquinone pool as determined by flash-induced photooxidation of the Photosystem I reaction center, P-700. All the above observations are reversible upon dephosphorylation of the thylakoid membranes. These data are explained by a protein phosphorylation-induced stabilization of the bound semiquinone anion, B^?. It is proposed that this increased stability may be due to an alteration in the accessibility of an endogenous reductant to B, or to an increase in dissipative cycling of charge around Photosystem II.     

Kinetics of reduction of the oxidized primary electron donor of Photosystem II in spinach chloroplasts and in Chlorella cells in the microsecond and nanosecond time ranges following flash excitation

Absorption changes (ΔA) at 820 nm, following laser flash excitation of spinach chloroplasts and Chlorella cells, were studied in order to obtain information on the reduction time of the photooxidized primary donor of Photosystem II at physiological temperatures.In the microsecond time range the difference spectrum of ΔA between 750 and 900 nm represents a peak at 820 nm, attributable to a radical-cation of chlorophyll a. In untreated dark-adapted material the signal can be attributed solely to P⁺?700; it decays in a polyphasic manner with half-times of 17 μs, 210 μs and over 1 ms. The oxidized primary donor of Photosystem II (P⁺_II) is not detected with a time resolution of 3 μs. After treatment with 3–10 mM hydroxylamine, which inhibits the donor side of Photosystem II, P⁺_II is observed and decays biphasically (a major phase with $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 20–40 μ}\text{s}$, and a minor phase with $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{? 200 μ}\text{s}$), probably by reduction by an accessory electron donor.In the nanosecond range, which was made accessible by a new fast-response flash photometer operating at 820 nm, it was found the P⁺_II is reduced with a half-time of 25–45 ns in untreated dark-adapted chloroplasts. It is assumed that the normal secondary electron donor is responsible for this fast reduction.     

Photosynthetic electron transport and electrochromic effects at sub-zero temperatures

Spinach chloroplasts, suspended in a liquid medium containing ethyleneglycol, showed reversible absorbance changes near 700 and 518 nm due to P-700 and “P-518” in the region from ?35 to ?50 °C upon illumination. The kinetics were the same at both wavelengths, provided absorbance changes due to Photosystem II were suppressed. At both wavelengths, the decay was slowed down considerably, not only by the System I electron acceptor methyl viologen, but also by silicomolybdate. The effect of the latter compound is probably not due to the oxidation of the reduced acceptor of Photosystem I by silicomolybdate, but to the enhanced accessibility of the acceptor to some other oxidant.In the presence of both an electron donor and acceptor for System I, a strong stimulation of the extent of the light-induced absorbance increase at 518 nm was observed. The most effective donor tested was reduced N-methylphenazonium methosulphate (PMS). The light-induced difference spectrum was similar to spectra obtained earlier at room temperature, and indicated electrochromic band shifts of chlorophylls a and b and carotenoid, due to a large potential over the thylakoid membrane, caused by sustained electron transport. It was estimated that steady-state potentials of up to nearly 500 mV were obtained in this way; the potentials reversed only slowly in the dark, indicating a low conductance of the membrane. This decay was accelerated by gramicidin D. The absorbance changes were linearly proportional to the membrane potential.     

On the functional organization of electron transport from plastoquinone to Photosystem I

Signal I, the EPR signal of P-700, induced by long flashes as well as the rate of linear electron transport are investigated at partial inhibition of electron transport in chloroplasts. Inhibition of plastoquinol oxidation by dibromothymoquinone and bathophenanthroline, inhibition of plastocyanin by KCN and HgCl₂, and inhibition by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide are used to study a possible electron exchange between electron-transport chains after plastoquinone. (1) At partial inhibition of plastocyanin the reduction kinetics of P-700⁺ show a fast component comparable to that in control chloroplasts and a new slow component. The slow component indicates P-700⁺ which is not accessible to residual active plastocyanin under these conditions. We conclude that P-700 is reduced via complexed plastocyanin. (2) The rate of linear electron transport at continuous illumination decreases immediately when increasing amounts of plastocyanin are inhibited by KCN incubation. This is not consistent with an oxidation of cytochrome f by a mobile pool of plastocyanin with respect to the reaction rates of plastocyanin being more than an order of magnitude faster than the rate-limiting step of linear electron transport. It is evidence for a complex between the cytochrome b₆ - f complex and plastocyanin. The number of these complexes with active plastocyanin is concluded to control the rate-limiting plastoquinol oxidation. (3) Partial inhibition of the electron transfer between plastoquinone and cytochrome f by dibromothymoquinone and bathophenanthroline causes decelerated monophasic reduction of total P-700⁺. The P-700 kinetics indicate an electron transfer from the cytochrome b₆ - f complex to more than ten Photosystem I reaction center complexes. This cooperation is concluded to occur by lateral diffusion of both complexes in the membrane. (4) The proposed functional organization of electron transport from plastoquinone to P-700 in situ is supported by further kinetic details and is discussed in terms of the spatial distribution of the electron carriers in the thylakoid membrane.     

Contrasting effects of plastocyanin on the photoreduction and photooxidation of cytochrome f in chloroplasts

Removal of plastocyanin from Photosystem I subchloroplast particles had no effect on the Photosystem I photooxidation of cytochrome f. Chloroplasts depleted of plastocyanin by sonication lost the ability to reduce cytochrome f in Photosystem II light. Addition of plastocyanin restored the photoreduction of cytochrome f. These results are consistent with a plastocyanin site on the reducing side of cytochrome f.     

Photoisomerization of lycopene during carotenogenesis in mutant C-6D of Scenedesmus obliquus

Mutant C-6D of the unicellular green alga Scenedesmus obliquus has lost the ability to form cyclic carotenoids during heterotrophic growth in the dark. In the dark it accumulates acyclic intermediates, i.e. lycopene, neurosporene and ζ-carotene. The lycopene and two neurosporene forms were identified to be cis-isomers. Upon transfer to light, intermediates decrease and a normal set of carotenoids is synthesized. Inhibition of the cyclization reaction by nicotine reveals a lightinduced isomerization of cis-lycopene to trans-lycopene. Since the spectral characteristics of these two isomers differ drastically the isomerization can be followed in vivo by measuring a light-induced absorbance change. This absorbance change has its maximum at 520 nm and shows fast kinetics under high light intensities reaching a saturation level after about 2 min. Fluence-response curves for this absorbance change were performed for different wavelengths of actinic light. From the linear parts of these curves an action spectrum was caculated showing maxima at 670, 630 and 440 nm originating from chlorophyll and a maximum at shorter wavelengths (400–510 nm) which is interpreted to derive from ζ-carotene. A model for the light regulation of carotenogenesis in mutant C-6D is presented and the relation to the so-called 520-change observed in many plants is discussed.     

The site of inhibition of photosystem II by 3-(3,4-dichlorophenyl)-N-N'-dimethylurea in thylakoids of the cyanobacterium Anabaena cylindrica.

Thylakoids isolated from the cyanobacterium $\text{Anabaena}\text{cylindrica}$ exhibit Photosystem II activity. Photosynthetic electron transfer from water to ferricyanide and to 2,6-dichlorophenolindophenol is inhibited by 3-(3,4-dichlorophenyl)-N-N′-dimethyl urea. Diphenylcarbazide stimulates ferricyanide and 2,6-dichlorphenolindophenol photoreduction, whilst inhibiting oxygen evolution. Diphenylcarbazide-supported Photosystem II activity is completely insensitive to 3-(3,4-dichlorophenyl)-N-N′-dimethyl urea, indicating that the site of action of this inhibitor lies on the donor side of Photosystem II in $\text{A}\text{.}\text{cylindrica}$, before the site of electron donation by diphenylcarbazide.     

Picosecond time-resolved energy transfer in Porphyridium cruentum. Part I. In the intact alga

The wavelength-resolved fluorescence emission kinetics of the accessory pigments and chlorophyll a in Porphyridium cruentum have been studied by picosecond laser spectroscopy. Direct excitation of the pigment B-phycoerythrin with a 530 nm, 6 ps pulse produced fluorescence emission from all of the pigments as a result of energy transfer between the pigments to the reaction centre of Photosystem II. The emission from B-phycoerythrin at 576 nm follows a nonexponential decay law with a mean fluorescence lifetime of 70 ps, whereas the fluorescence from R-phycocyanin (640 nm), allophycocyanin (660 nm) and chlorophyll a (685 nm) all appeared to follow an exponential decay law with lifetimes of 90 ps, 118 ps and 175 ps respectively. Upon closure of the Photosystem II reaction centres with 3-(3,4-dichlorophenyl)-1,1-dimethylurea and preillumination the chlorophyll a decay became non-exponential, having a long component with an apparent lifetime of 840 ps. The fluorescence from the latter three pigments all showed finite risetimes to the maximum emission intensity of 12 ps for R-phycocyanin, 24 ps for allophycocyanin and 50 ps for chlorophyll a.A kinetic analysis of these results indicates that energy transfer between the pigments is at least 99% efficient and is governed by an exp $\text{?At}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ transfer function. The apparent exponential behaviour of the fluorescence decay functions of the latter three pigments is shown to be a direct result of the energy transfer kinetics, as are the observed risetimes in the fluorescence emissions.     

Lateral mobility of the light-harvesting complex in chloroplast membranes controls excitation energy distribution in higher plants

Chloroplast thylakoid protein phosphorylation produces changes in light-harvesting properties and in membrane structure as revealed by freeze-fracture electron microscopy. Protein phosphorylation resulted in an increase in the 77 °K fluorescence signal at 735 nm relative to that at 685 nm. In addition, a decrease in connectivity between Photosystem II centers (PS II) and a dynamic quenching of the room temperature variable fluorescence was observed upon phosphorylation. Accompanying these fluorescence changes was a 23% decrease in the amount of stacked membranes. Microscopic analyses indicated that 8.0-nm particles fracturing on the P-face moved from the stacked into the unstacked regions upon phosphorylation. The movement of the 8.0-nm particles was accompanied by the appearance of chlorophyll b and 25 to 29 kD polypeptides in isolated stroma lamellae fractions. We conclude that phosphorylation of a population of the light-harvesting chlorophyll $\text{a}\text{b}$ protein complexes (LHC) in grana partitions causes the migration of these pigment proteins from the PS II-rich appressed membranes into the Photosystem I (PS I) enriched unstacked regions. This increases the absorptive cross section of PS I. In addition, we suggest that the mobile population of LHC functions to interconnect PS II centers in grana partitions; removal of this population of LHC upon phosphorylation limits PS II → PS II energy transfer and thereby favors spillover of energy from PS II to PS I.     

Electron transfer between the two photosystems. I. Flash excitation under oxidizing conditions

The redox changes of cytochrome b-563 (cytochrome b), cytochrome f, plastocyanin and P-700 were measured on dark-adapted chloroplasts after illumination by a series of flashes in oxidizing conditions (0.1 mM ferricyanide). In these conditions, the plastoquinone pool is fully oxidized and the only available plastoquinol are those formed by Photosystem (PS) II reaction. According to the two-electron gate mechanism proposed by Bouges-Bocquet (Bouges-Bocquet, B. (1973) Biochim. Biophys. Acta 314, 250–256), plastoquinol is mainly formed after the second and the fourth flashes. After the second flash, the reoxidation of plastoquinol occurs by a concerted reaction which reduces most of the cytochrome b present and a fraction of PS I donors. Most of these electrons are stored on P-700, which implies a large equilibrium constant between the secondary PS I donors and P-700. One electron is stored on cytochrome b during a time ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{≈ 1}\text{s}$) much longer than the dark interval between flashes. After the fourth flash, a new plastoquinol molecule is formed, which induces the reduction of PS I donors with no corresponding further reduction of cytochrome b. The number of electrons transferred after the fourth flash is larger than that transferred after the second flash although the rate of transfer is lower. To interpret these data, we assume that the plastoquinol formed after the fourth flash is reoxidized by a second concerted reaction: one electron is directly transferred to PS I donors while the other cooperates with the electron stored on cytochrome b to reduce a plastoquinone molecule localized on a site close to the outer face of the membrane. This newly formed plastoquinol crosses the membrane and transfers a second electron to PS I donors. This interpretation resembles a model proposed by Velthuys (Velthuys, B.R. (1979) Proc. Natl. Acad. Sci. U.S.A. 76, 2765?2769) and which belongs to the modified Q-cycle class of models.     

EPR characteristics of oxygen-evoloving Photosytem II particles from Phormidium laminosum

The EPR characteristics of oxygen evolving particles prepared from Phormidium laminosum are described. These particles are enriched in Photosystem II allowing EPR investigation of signals which were previously small or masked by those from Photosystem I in other preparations. EPR signals from a Signal II species and high potential cytochrome b-559 appear as they are photooxidised at cryogenic temperatures by Photosystem II. The Signal II species is a donor close to the Photosystem II reaction centre and may represent part of the charge accumulation system of water oxidation. An EPR signal from an iron-sulphur centre which may represent an unidentified component of photosynthetic electron transport is also described.The properties of the oxygen evolving particles show that the preparation is superior to chloroplasts or unfractionated algal membranes for the study of Photosystem II with a functional water oxidation system.     

The role of the membrane-bound iron-sulphur centres A and B in the Photosystem I reaction centre of spinach chloroplasts

Photosystem I particles prepared from spinach chloroplast using Triton X-100 were frozen in the dark with the bound iron-sulphur Centre A reduced. Illumination at cryogenic temperatures of such samples demonstrated the photoreduction of the second bound iron-sulphur Centre B. Due to electron spin-electron spin interaction between these two bound iron-sulphur centres, it was not possible to quantify amounts of Centre B relative to the other components of the Photosystem I reaction centre by simulating the line-shape of its EPR spectrum. However, by deleting the free radical signal I from the EPR spectra of reduced Centre A alone or both Centres A plus B reduced, it was possible to double integrate these spectra to demonstrate that Centre B is present in the Photosystem I reaction centre in amounts comparable to those of Centre A and thus also signal I ($\text{P-700}$) and X.Oxidation-reduction potential titrations confirmed that Centre A had $\text{E}{}_{\mathrm{<mtext>m</mtext>}}\text{? ?550}\text{mV}$, Centre B had $\text{E}{}_{\mathrm{<mtext>m</mtext>}}\text{? ?585}\text{mV}$. These results, and those presented for the photoreduction of Centre B, place Centre B before Centre A in the sequence of electron transport in Photosystem I particles at cryogenic temperatures. When both A and B are reduced, $\text{P-700}$ photooxidation is reversible at low temperature and coupled to the reduction of the component X. The change from irreversible to reversible $\text{P-700}$ photooxidation and the photoreduction of X showed the same potential dependence as the reduction of Centre B with $\text{E}{}_{\mathrm{<mtext>m</mtext>}}\text{? ?585}$ mV, substantiating the identification of X as the primary electron acceptor of Photosystem I.     

Determination and modification of the redox state of the secondary acceptor of Photosystem II in the dark

The redox state of the secondary electron acceptor B of Photosystem II was studied using fluorescence measurements. Preillumination of algae or chloroplasts with a variable number of short saturating flashes followed rapidly by the addition of 3-(3,4-dichlorophenyl)-1, 1-dimethylurea induces oscillations of the initial level of fluorescence. The phase of these oscillations is characteristic of a given $\text{B}\text{B}{}^{\mathrm{?}}$ ratio in the dark-adapted samples.We conclude from our results that about 50% of the secondary electron acceptors are singly reduced in the dark in Chlorella cells, but that more than 70% are fully oxidized in the dark adapted chloroplasts.Benzoquinone treatment modifies this distribution in Chlorella leading to the same situation as in chloroplasts, i.e. more than 70% of the secondary acceptors are oxidized in the dark.The same ratio is observed if these algae are illuminated and then dark-adapted, unless an artificial donor (hydroxylamine) is added before this illumination. In that case about 50% B^? is generated and stabilized in the dark.     

Phosphorylation of the light-harvesting chlorophyll-protein regulates excitation energy distribution between photosystem II and photosystem I

The kinetics of thylakoid membrane protein phosphorylation in the presence of light and adenosine triphosphate is correlated to an incease in the 77 °K fluorescence emission at 735 nm (F735) relative to that at 685 nm (F685). Analysis of detergent-derived submembrane fractions indicate phosphorylation only of the polypeptides of Photosystem II, and the light-harvesting chlorophyll-protein complex serving Photosystem II (LHC-II). Although several polypeptides are phosphorylated, only the dephosphorylation kinetics of LHC-II follow the kinetics of the decrease of the $\text{F735}\text{F685}$ fluorescence emission ratios. The relative quantum yield of Photosystem II was significantly lower in phosphorylated membranes compared to dephosphorylated membranes. Reversible LHC-II phosphorylation thus provides the physiological mechanism for the control of the distribution of absorbed excitation energy between the two photosystems.