Further characterization of a photosystem-II particle isolated from spinach chloroplasts by triton treatment: the reaction-center components

Interaction between the intermediary electron acceptor (pheophytin) and a possible plastoquinone-iron complex in photosystem II reaction centers

Photoreduction of the intermediary electron acceptor, pheophytin (Pheo), in photosystem II reaction centers of spinach chloroplasts or subchloroplast particles (TSF-II and TSF-IIa) at 220 K and redox potential E(h) = -450 mV produces an EPR doublet centered at g = 2.00 with a splitting of 52 G at 7 K in addition to a narrow signal attributed to Pheo([unk]) (g = 2.0033, DeltaH approximately 13 G). The doublet is eliminated after extraction of lyophilized TSF-II with hexane containing 0.13-0.16% methanol but is restored by reconstitution with plastoquinone A (alone or with beta-carotene) although not with vitamin K(1). TSF-II and TSF-IIa are found to contain approximately 2 nonheme Fe atoms per reaction center. Incubation with 0.55 M LiClO(4) plus 2.5 mM o-phenanthroline (but not with 0.55 M LiClO(4) alone) decreases this value to approximately 0.6 and completely eliminates the EPR doublet, but photoreduction of Pheo is not significantly affected. Partial restoration of the doublet (about 25%) was achieved by subsequent incubation with 0.2 mM Fe(2+), but not with either Mn(2+) or Mg(2+). The Fe removal results in the development of a photoinduced EPR signal (g = 2.0044 +/- 0.0003, DeltaH = 9.2 +/- 0.5 G) at E(h) = 50 mV, which is not observed after extraction with 0.16% methanol in hexane. It is ascribed to plastosemiquinone no longer coupled to Fe in photosystem II reaction centers. The results show that a complex of plastoquinone and Fe can act as the stable "primary" electron acceptor in photosystem II reaction centers and that the interaction of its singly reduced form with the reduced intermediary acceptor, Pheo([unk]), is responsible for the EPR doublet.

Photosynthetic electron transport from water to NADP driven by photosystem II in inside-out chloroplast vesicles

It is now widely held that the light-induced noncyclic (linear) electron transport from water to NADP(+) requires the collaboration in series of the two photosystems that operate in oxygen-evolving cells: photosystem II (PSII) photooxidizes water and transfers electrons to photosystem I (PSI); PSI photoreduces ferredoxin, which in turn reduces NADP(+) (the Z scheme). However, a recently described alternative scheme envisions that PSII drives the noncyclic electron transport from water to ferredoxin and NADP(+) without the collaboration of PSI, whose role is limited to cyclic electron transport [Arnon, D. I., Tsujimoto, H. Y. & Tang, G. M.-S. (1981) Proc. Natl. Acad. Sci. USA 78, 2942-2946]. Reported here are findings at variance with the Z scheme and consistent with the alternative scheme. Thylakoid membrane vesicles were isolated from spinach chloroplasts by the two-phase aqueous polymer partition method. Vesicles, originating mainly from appressed chloroplast membranes that are greatly enriched in PSII, were turned inside-out with respect to the original sidedness of the membrane. With added plastocyanin, ferredoxin, and ferredoxin-NADP(+) reductase, the inside-out vesicles enriched in PSII gave a significant photoreduction of NADP(+) with water as electron donor, under experimental conditions that appear to exclude the participation of PSI.

Redox titration of fluorescence yield of photosystem II

The variable fluorescence yield of photosystem II is dependent on the redox state of the fluorescence quencher molecule or the primary electron acceptor of the system. We have carried out redox titrations of fluorescence yield of a photochemically active photosystem-II reaction-center particle and have measured the redox potential of the photosystem-II primary acceptor.During reductive titrations using dithionite as the reductant, only a single quenching transition was observed. For instance, at pH 7.0, the midpoint potential of the fluorescence transition is -325 mV, and those at a pH between 6.0 and 7.5 are consistent with a pH dependence of about 60 mV/pH unit. At a given pH, the midpoint potential of the transition closely corresponds to that of the most negative transition previously measured in unfractionated chloroplasts (both by chemical reductive titration). Oxidative titrations using ferricyanide as the oxidant yielded hysteresis in the titration curves.Similar changes in fluorescence yield were observed in redox titrations by electrochemical reduction or oxidation. Electrochemical reductive and oxidative titrations yielded reversible transitions, contrary to the hysteresis observed during chemical oxidative titration. From coulometric-titration data, we have estimated that most likely one electron is involved in the redox transition of the fluorescence-quencher or primary-electron-acceptor molecule of photosystem II. These findings are consistent with the current proposal that a membrane-bound plastoquinone functions as the primary acceptor of photosystem II.

Spectral and kinetic evidence for two early electron acceptors in photosystem I

Triton-fractionated photosystem-I particles poised at -625 mV, where the two bound iron-sulfur proteins are reduced, have been studied by optical and electron paramagnetic resonance spectroscopies from 293 to 5 K. At 5-9 K, these particles exhibit two decay components with lifetimes of 1.3 and 130 msec in the laser pulse-induced absorption and electron paramagnetic resonance signal changes. Spectral properties of the 130-msec decay component reflect the charge separation between P-700 and some iron-sulfur center having a broad optical absorbance in the 400- to 550-nm region and a previously reported electron paramagnetic resonance signal with g = 1.78, 1.88, and 2.08. Spectral properties of the 1-msec decay component indicate photoinduced charge separation between P-700 and a chlorophyll a dimer having absorption bands at 420, 450, and 700 nm. It is assumed that these two acceptors participate in the electron transfer from P-700(*) to the bound iron-sulfur proteins.

The chlorophyll-protein complexes of higher plant photosynthetic membranes or Just what green band is that?

Higher plant thylakoid membranes can be fractionated into a bewildering array of macrocomplexes, chlorophyll-protein complexes and chlorophyll-proteins with various deteregents and separations techniques. The chemical nature of each of these entities depends on the particular methods used to obtain them. This review summarizes the current status of the biochemical identification and characterization of individual chlorophyll-proteins and chlorophyll-protein complexes, and attempts to clarify the relationships among them.

Preparation and properties of the water-soluble chlorophyll-bovine serum albumin complexes

By mixing chlorophyll (Chl) a or b with a dense bovine serum albumin solution, the water-soluble Chl-bovine serum albumin complexes were prepared. These complexes, eluted near the void volume on a gel filtration, were separated well from unreacted bovine serum albumin, indicating an aggregation of such molecules in the complexes. Preparation of chlorophyllide (Chlide) a- or Chlide b-bovine serum albumin complex was unsuccessful, while the phytol-, and beta-carotene-bovine serum albumin complexes could be obtained. Chls in the Chl-bovine serum albumin complexes had the following characteristics. Main absorption peak of Chl a or b in the red region occurred at 675 nm or 652 nm, respectively. The Chl a-bovine serum albumin complex having absorption peak at 740 nm was also prepared. As compared with the stabilities of Chl a and b in Triton X-100. Both Chls in the bovine serum albumin-complexes were stable against oxidative stresses, such as photobleaching, Fenton reagent, peroxidase-H2O2 system. But they were easily hydrolyzed by chlorophyllase. These properties of Chls in the bovine serum albumin-complexes were similar to those of Chls in the isolated light-harvesting Chl a/b protein complex. A possible localization of Chls within the bovine serum albumin complexes was suggested that the porphyrin moiety of Chl was buried in bovine serum albumin; however, the hydrophilic edge of porphyrin ring, adjacent to the phytol group, occurred in the hydrophilic region of a bovine serum albumin molecule.

Membrane-surface electric properties of triton-fractionated spinach subchloroplast fragments

Surface charge density of subchloroplast fragments fractionated from spinach by Triton X-100 treatment was estimated from cation-induced quenching of chlorophyll fluorescence, with the premise that the fluorescence yield is dependent on the surface electric potential of the preparations. Application of the Gouy-Chapman theory of diffuse double layer to the subchloroplast preparations, or treating the surface of the preparations under electric charge regulation conditions yielded a result suggesting the Photosystem II reaction-center preparation (TSF-IIa) to be more negatively charged than the Photosystem I reaction-center preparation (TSF-I). Isoelectric points of the subchloroplast fragments were determined by measuring 90 degrees light scattering and more directly by gel isoelectric focusing. Isoelectric points of TSF-I and -IIa were estimated to be 4.8 and 4.0 from light-scattering experiments, and 4.5 and 4.1 from gel electrophoresis, respectively. The TSF-II preparation that contains both a light-harvesting complex and the reaction-center (core) complex showed a small cation-induced quenching of chlorophyll fluorescence. This fluorescence quenching may be ascribed mostly to the regulation of energy transfer in the preparation (Yamamoto, Y. and Ke, B. (1980) Biochim, Biophys. Acta 592, 296-302). Furthermore, the TSF-II preparation showed a broad and indefinite peak in light scattering in the pH range 3-8, suggesting that the complex probably carries a small amount of charge in this pH range. The physiological role of the membrane surface charge of the subchloroplast preparations in membrane structure and cation regulated processes in chloroplast is discussed.

Regulation of electron transport in photosystem-II fragments by magnesium ions

In Photosystem-II reaction-center particles (TSF-IIa) fractionated from spinach chloroplasts by Triton X-100 treatment, divalent cations appear to regulate electron-transport reactions. Oxidation of cytochrome b-559 after illumination of the particles was accelerated by the presence of Mg2+, whereas photoreduction of 2,6-dichlorophenolindophenol (DCIP) by diphenyl carbazide was inhibited, both at a half-effective concentration of Mg2+ of approx. 0.1 mM. The site of regulation was shown to be on the oxidizing side of Photosystem II, near P-680, based on the effects of actinin-light intensity and nature of the electron donors on DCIP photoreduction. Mg2+ was effective in quenching chlorophyll fluorescence in TSF-IIa particles, but the quenching was sensitive to the presence of 3(3,4-dichlorophenyl)-1,1-dimethylurea. In the reaction-center (core) complex of Photosystem II, where the light-harvesting chlorophyll-protein complex is absent, there seems to be no regulation by Mg2+ on excitation-energy distribution.

Regulation of excitation energy distribution in photosystem-II fragments by magnesium ions

Fluorescence characteristics of Photosystem-II subchloroplasts (TSF-II and TSF-IIa) fractionated by Triton X-100 treatment were studied in relation to cation-induced regulation of excitation-energy distribution within subchloroplast fragments. Absorption spectra and fluorescence-emission spectra at 77 K showed that TSF-II contains the light-harvesting chlorophyll-protein complex in addition to the reaction-center complex, which is present alone in TSF-IIa. Mg2+ increased the ratio of F695nm to F685nm in the fluorescence-emission spectrum of TSF-II particles at 77 K, but had no effect on TSF-IIa particles. Mg2+ also induced a quenching of chlorophyll fluorescence at room temperature in TSF-II, an effect that was insensitive to the presence of DCMU. The DCMU-insensitive fluorescence quenching was not observed in the TSF-IIa preparation. These results suggest an existence of cation-induced regulation of excitation-energy transfer in TSF-II preparations. Presence of antenna chlorophyll molecules alone does not seem to be sufficient for observing energy-transfer regulation by cations in Photosystem-II preparations.

Isolation of highly active photosystem II particles from a mutant of Chlamydomonas reinhardtii

Highly active photosystem-II particles were rapidly isolated using detergents and obtained in good yield from a mutant of the green alga Chlamydomonas reinhardtii. The particles are completely devoid of reaction centers of photosystem I, and of the secondary electron acceptor to photosystem II. They show: (a) a specific activity (delta A of C550/unit chlorophyll) 4--7 times that of the starting material and of spinach chloroplasts: (b) an antenna size of 40 to 50 chlorophyll molecules containing little light-harvesting chlorophyll a/b complex (chlorophyll a/chlorophyll b = 4--6.4); (c) a ratio of variable to dark-adapted fluorescence yield of up to 3. Further treatment of these particles by ion-exchange chromatography largely removes five proteins and further decreases the antenna size with little loss in primary photoactivity.

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 spin echo studies on chloroplasts. Spectral characteristics of electron transport components and light-induced transients

Electron-spin resonance echoes are used to study the complex overlapping ESR spectra of whole chloroplasts. By varying the repetition rate of the microwave pulse sequence, delay time, and pulse width, signals with different longitudinal and transverse relaxation times were extracted. We have identified the echo signals due to plastocyanin and ferredoxins. In addition, we have found a strong signal at g = 4.3, that possibly arises from distorted cytochrome, and weak signals in the region g = 6-9. The strong echo signal at g = 2.0047 (Signal II), is made up of at least three "dark" components that differ in their relaxation times. Upon illumination at 1.2 K several of the echo signals including Signal II show reversible light-induced components. The kinetics of these transients depend on the addition of 3(3,4-dichlorophenyl)-1,1-dimethyl urea. Part of the transients are believed to arise from cyclic electron flow around Photosystem I.

Redox titration of electron acceptor Q and the plastoquinone pool in photosystem II

The primary photochemical quencher Q and the secondary electron acceptor pool in Photosystem II have been titrated. We used particles of Scenedesmus mutant No. 8 that lack System I and allowed the system to equilibrate with external redox mediators in darkness prior to measurement of the fluorescence rise curve. The titration of Q, as indicated by the dark level of Fi, occurs in two discrete steps. The high-potential component (Qh) has a midpoint potential of +68 mV (pH 7.2) and accounts for approximately 67% of Q. The pH sensitivity of the midpoint potential is -60 mV, indicating the involvement of 1 H+/e. The low-potential component (Q1) accounts for the remaining 33% of Q and shows a midpoint potential near--300 mV (pH 7.2). The plastoquinone pool, assayed as the half-time of the fluorescence rise curve, titrates as a single component with a midpoint potential 30--40 mV more oxidizing than that of Qh, i.e., at 106 mV (pH 7.2). The Em shows a pH sensitivity of -60 mV/pH unit, indicating the involvement of 1 H+/e. The observation that all 12--14 electron equivalents in the pool titrate as a single component indicates that the heterogeneity otherwise observed in the secondary acceptor system is a kinetic rather than a thermodynamic property. Illumination causes peculiar, and as yet unclarified, changes of both Q and the secondary pool under anaerobic conditions that are reversed by oxygen.

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 mumol/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 beta-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 b-559 ratio of 50 : 1. The Photosystem I particle is highly enriched in P-700, with a chlorophyll to 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.

Photochemical activities, pigment distribution and photosynthetic unit size of subchloroplast particles isolated from synchronized cells of Scenedesmus obliquus

Photosystem II (PS II) reactions of chloroplast particles show the same variations during the synchronous life cycle of Scenedesmus obliquus, strain D3 (Gaffron Biol. Zbl. 59, 302 1939), as the whole cells they derived from. Photosystem I (PS I) reactions of whole cells and of subchloroplast particles show little or no variation in their activity, whereas PS I reactions of chloroplast particles vary like PS II reactions during the life cycle. The variation in chloroplast particles could be attributed to the change in the reoxidation capacity of plastoquinone still attached to PS I. Digitonin-treatment of chloroplast particles from Scenedesmus and subsequent sucrose density gradient separation yielded 3 distinct fractions: Fraction I contained pure PS I particles with the most efficient PS I-mediated methylviologen (MV) reduction with subsequent oxygen uptake (3 mmol O2/mg Chl·h); no Hill reaction; and a high chlorophyll a/b ratio, and a vast amount of unbound protein xanthophyll complexes. Fraction II is enriched in PS II particles, with little PS I activity (less than 10% of the PS I particles) and a low chlorophyll a/b ratio. The activity of the water-splitting system was completely lost. This fraction must also contain most of the light-harvesting pigment system. Fraction III is also enriched in PS II with even less PS I activity, but the ratio of chlorophyll a/b is slightly higher than in whole cells and the water-splitting system is intact. β-carotene was part of all fractions whereas functional xanthophylls seemed to be restricted to the PS II particles. From the constant chlorophyll P/700 ratio we had to conclude that size of the photosynthetic unit does not change during the life cycle of a synchronized Scenedesmus obliquus culture.

Electrochemical and spectro-kinetic evidence for an intermediate electron acceptor in photosystem I

Absorption changes accompanying light-induced P-700 formation and its decay in the dark at 15 K in Photosystem-I particles poised at various redox potentials have been examined. In unpoised samples, the light-induced absorption change is practically irreversible. At increasingly negative potentials, an increasing fraction of the absorption change, proportional to the fraction of bound iron-sulfur protein chemically reduced, becomes reversible, and the titration curve has a midpoint potential of --530 mV (vs. normal hydrogen electrode). At --66 mV, the P-700 absorption change is 97% reversible. The total P-700-signal amplitude decreases over the same potential span and levels off at about 43% (to slightly over 50% at a substantially higher excitation intensity). These results provide additional support to previous suggestions of an existence of an intermediate electron acceptor located between the primary donor, P-700, and the more stable primary electron acceptor (P-430 or bound iron-sulfur protein).

Photooxidation of chlorophyll in spinach chloroplasts between 10 and 180 K

Electron paramagnetic resonance (EPR) and optical absorbance difference spectra and kinetics upon illumination by saturating flashes and continuous light of spinach chloroplasts frozen under various conditions were measured between 10 and 180 K. 1. At 100 K illumination with continuous light caused an EPR signal which decayed during the light in about 30 ms. This change is probably due to the reduction of P+-680, the oxidized primary electron donor of Photosystem II, by a secondary electron donor, cytochrome b-559. Flash illumination yielded the previously observed rapid (2 ms) transient. This transient has been ascribed to a back-reaction of the two primary reagents of Photosystem II (Malkin, R. and Bearden, A.J. (1975) Biochim. Biophys. Acta 396, 250-259; Visser, J.W.M. (1975) Thesis, Leiden). 2. Between 10 and 40 K, illumination with continuous light showed a transient which decayed in about 500 ms. The extent decreased with increasing temperature. However, the half time appeared to be temperature independent. This signal was also attributed to P+-680. 3. At 180 K it appeared to be impossible to observe the 2 and 30 ms components in dark frozen chloroplasts. However, they could be observed again if two short saturating flashes were given shortly before freezing. These changes seem to be dependent on the S-state of the reaction center. 4. After oxidizing the sample with ferricyanide (Eh = 540 mV), the light induced absorbance difference spectrum showed a bleaching near 676 nm. This change is ascribed to the irreversible oxidation of a dimeric chlorophyll molecule which acts as electron donor to P+-680 under these conditions. 5. Titration curves of the irreversible light-induced absorbance change at 676 nm and the irreversible light-induced EPR change near g = 2.00 provide strong evidence that these two changes reflect the same compound. Finally, a model is given to explain the observed reactions of Photosystem II at 10-180 K. The model involves three different ultimate and one intermediate electron donor to P+-680 at these temperatures.

The role of plastoquinone and beta-carotene in the primary reaction of plant photosystem II

Extraction of Triton Photosystem II chloroplast fragments with 0.2% methanol in hexane for 3 h results in the removal of 90 to 95% of the plastoquinone in the original preparation. The extracted fragments (chlorophyll:plastoquinone ratio, 900: 1) showed no P-680 photooxidation at 15 K after a single laser flash. The extracted fragments also showed no light-induced C-550 absorbance change at 77 K. Reconstitution of the primary reaction of Photosystem II, as evidenced by restoration of low-temperature photooxidation of P-680, could be obtained by the addition of plastoquinone A but not by the addition of beta-carotene. The addition of beta-carotene plus plastoquinone A restored the C-550 absorbance change. These results indicate that plastoquinone functions as the primary electron acceptor of Photosystem II and that beta-carotene does not play a direct role in the primary photochemistry but is required for the C-550 absorbance change.

Light-induced absorption changes in photosystem I at low temperatures

Light-induced absorption changes associated with the primary photochemical reaction and dark relaxation in Photosystem I were measured at various low temperatures. A possible temperature-dependent long-range electron tunneling process was suggested to account for the unique temperature dependence of the dark decay process. The kinetics of the light-induced absorption changes are in good agreement with the light-induced EPR changes reported earlier (Ke, B., Sugahara, K., Shaw, E.R., Hansen, R. E., Hamilton, W. D. and Beinert, H. (1974) Biochim, Biophys. Acta 368, 401--408) for the same Photosystem I subchloroplast fragment at comparable temperatures. All absorption changes between 400 and 725 nm at 86degreesK have identical kinetics. The light-minus-dark difference spectrum is very similar to that of P-700 at room temperature, with an additional prominent positive change at 690 nm. Possible contributions by P-430 to the blue and red spectral changes were discussed. It was demonstrated that the intensity of the measuring beam has a drastic effect on the light-induced absorption changes of Photosystem I at low temperatures. Various pretreatments of the Photosystem I fragments such as those that photochemically (or chemically) oxidize the primary donor or photoreduce the primary acceptor abolish the subsequent photochemical reaction. Continuous illumination of the Photosystem I fragments before and during freezing has the same effect. In the temperature range of --20 to --60degreesC, an unusual counter absorption change as well as a counter EPR change were observed.

Light-induced changes of absorbance and electron spin resonance in small photosystem II particles

Photosystem II reaction center components have been studied in small system II particles prepared with digitonin. Upon illumination the reduction of the primary acceptor was indicated by absorbance changes due to the reduction of a plastoquinone to the semiquinone anion and by a small blue shifts of absorption bands near 545 nm (C550) and 685 nm. The semiquinone to chlorophyll ratio was between 1/20 and 1/70 in various preparations. The terminal electron donor in this reaction did not cause large absorbance changes but its oxidized form was revealed by a hitherto unknown electron spin resonance (ESR) signal, which had some properties of the well-known signal II but a linewidth and g-value much nearer to those of signal I. Upon darkening absorbance and ESR changes decayed together in a cyclic or back reaction which was stimulated by 3-(3,4 dichlorophenyl)-1,1-dimethylurea. The donor could be oxidized by ferricyanide in the dark. Illumination in the presence of ferricyanide induced absorbance and ESR changes, rapidly reversed upon darkening, which may be ascribed to the oxidation of a chlorophyll a dimer, possibly the primary electron donor of photosystem II. In addition an ESR signal with 15 to 20 gauss linewidth and a slower dark decay was observed, which may have been caused by a secondary donor.

Further purification of "Triton subchloroplast fraction I" (TSF-I particles). Isolation of a cytochrome-free high-P-700 particle and a complex containing cytochromes f and b6, plastocyanin and iron-sulfur protein(s)

The "Triton Subchloroplast Fraction I" or "TSF-I particles" can be further fractionated into a cytochrome fraction and a P-700-containing fraction essentially free of cytochromes. The cytochrome complex contains cytochromes f and b6 in approx. equimolar amounts, and, in addition, also plastocyanin and one iron-sulfur protein, all in the bound state. Bound plastocyanin was characterized by EPR spectroscopy. The EPR spectrum of the bound iron-sulfur protein resembles that previously detected in Phostosystem I particles under highly reducing conditions at lower than -560 mV. The redox potential of P-700 in the cytochrome-free high-P-700 particles was measured to be +468 mV; those of cytochromes f and b6 are +345 and -140 mV, respectively. Among the four components present in the complex, only cytochrome f can be coupled to a Photosystem I particle and undergoes photooxidation. This coupled photooxidation is totoally inhibited by KCN and only partially inhibited by HgCl2. The similarity of the complex containing cytochromes f and b6, plastocyanin, and an iron-sulfur protein to complexes III and IV of the mitochondrial respiratory redox chain and a possible involvement of the complex in cyclic photophosphorylation are noted and discussed.

Chlorophyll radical cation in photosystem II of chloroplasts. Millisecond decay at low temperature

We compare the absorption changes, in the near infrared and in the green part of the spectrum, induced in spinach chloroplasts suspensions, at -- 170 degrees C, by continuous light and by flashes. (1) Following flash excitation, an absorption increase peaking at 825 nm which reverses rapidly (t 1/2 = 3.0 ms) is not affected by ferricyanide; it is suppressed when chloroplasts are preilluminated in the presence of 3-(3',4'-dichlorophenyl)-1,1'-dimethylurea (DCMU) and hydroxylamine. The reversion of that signal is simultaneous with a partial back reoxidation of C-550 (fully reduced by the flash) and with partial (about 25%) oxidation of cytochrome b559. The magnitude of the signal peaking at 825 nm (that we attribute to the radical cation of the trap chlorophyll of Photosystem II, acting as a primary electron donor) decreases progressively within a series of successive flashes. (2) An absorption increase (40% of which is slowly reversible) with a broad peak around 810 nm is induced by continuous light or by a flash. It is suppressed by pretreatment with ferricyanide, but it is little affected by the treatment with 3-(3',4'-dichlorophenyl)-1,1'-dimethylurea and hydroxylamine. We attribute it to oxidized P700. (3) With chloroplasts pretreated with 10 mM ferricyanide, an absorption increase, whose magnitude is nearly independent of wavelength between 790 and 870 nm, can be induced by continuous light. One saturating flash produces only 20% of the signal. This absorption change (20% of which is reversible in 30 s) might be due to a secondary donor of Photosystem II.

Laser-flash-activated electron paramagnetic resonance studies of primary photochemical reactions in chloroplasts

Electron paramagnetic resonance studies of the primary reactants of Photosystems I and II have been conducted at cryogenic temperatures after laser-flash activation with monochromatic light.P-700 photooxidation occurs irreversibly in chloroplasts and in Photosystem I fragments after activation with a 730 nm laser flash at a temperature of 35 degrees K. Flash activation of chloroplasts or Photosystem II chloroplast fragments with 660 nm light results in the production of a free-radical signal (g = 2.002, linewidth approximately 8 gauss) which decays with a half-time of 5.0 ms at 35 degrees K. The half-time of decay is independent of temperature in the range of 10-77 degrees K. This reversible signal can be eliminated by preillumination of the sample at 35 degrees K with 660 nm light (but not by 730 nm light), by preillumination with 660 nm light at room temperature in the presence of 3-(3',4'-dichlorophenyl)-1,1'-dimethylurea (DCMU) plus hydroxylamine, or by adjustment of the oxidation-reduction potential of the chloroplasts to - 150 mV prior to freezing. In the presence of ferricyanide (20-50 mM), two free-radical signals are photoinduced during a 660 nm flash at 35 degrees K. One signal decays with a half-time of 5 ms, whereas the second signal is formed irreversibly. These results are discussed in terms of a current model for the Photosystem II primary reaction at low temperature which postulates a back-reaction between P-680+ and the primary electron acceptor.