The PsbP Domain Protein 1 Functions in the Assembly of Lumenal Domains in Photosystem I

Photosystem I (PS I) is a multisubunit membrane protein complex that functions as a light-driven plastocyanin-ferredoxin oxidoreductase. The PsbP domain protein 1 (PPD1; At4g15510) is located in the thylakoid lumen of plant chloroplasts and is essential for photoautotrophy, functioning as a PS I assembly factor. In this work, RNAi was used to suppress PPD1 expression, yielding mutants displaying a range of phenotypes with respect to PS I accumulation and function. These PPD1 RNAi mutants showed a loss of assembled PS I that was correlated with loss of the PPD1 protein. In the most severely affected PPD1 RNAi lines, the accumulated PS I complexes exhibited defects in electron transfer from plastocyanin to the oxidized reaction center P₇₀₀⁺. The defects in PS I assembly in the PPD1 RNAi mutants also had secondary effects with respect to the association of light-harvesting antenna complexes to PS I. Because of the imbalance in photosystem function in the PPD1 RNAi mutants, light-harvesting complex II associated with and acted as an antenna for the PS I complexes. These results provide new evidence for the role of PPD1 in PS I biogenesis, particularly as a factor essential for proper assembly of the lumenal portion of the complex.     

Photosystem II in a mutant of Chlamydomonas reinhardtii lacking the 23 kDa psbP protein shows increased sensitivity to photoinhibition in the absence of chloride

The psbP gene product, the so called 23 kDa extrinsic protein, is involved in water oxidation carried out by Photosystem II. However, the protein is not absolutely required for water oxidation. Here we have studied Photosystem II mediated electron transfer in a mutant of Chlamydomonas reinhardtii, the FUD 39 mutant, that lacks the psbP protein. When grown in dim light the Photosystem II content in thylakoid membranes of FUD 39 is approximately similar to that in the wild-type. The oxygen evolution is dependent on the presence of chloride as a cofactor, which activates the water oxidation with a dissociation constant of about 4 mM. In the mutant, the oxygen evolution is very sensitive to photoinhibition when assayed at low chloride concentrations while chloride protects against photoinhibition with a dissociation constant of about 5 mM. The photoinhibition is irreversible as oxygen evolution cannot be restored by the addition of chloride to inhibited samples. In addition the inhibition seems to be targeted primarily to the Mn-cluster in Photosystem II as the electron transfer through the remaining part of Photosystem II is photoinhibited with slower kinetics. Thus, this mutant provides an experimental system in which effects of photoinhibition induced by lesions at the donor side of Photosystem II can be studied in vivo.Abbreviations Chl chlorophyll - DCIP 2,6-dichlorophenolindophenol - DPC 2,2-diphenylcarbonic dihydrazide - HEPES 4-(2-hydroxyethyl)-1-piperazinethanesulfonic acid - P₆₈₀ the primary electron donor to PS II - PpBQ phenyl-p-benzoquinone - PS II Photosystem II - Q_A the first quinone acceptor of PS II - Q_B the second quinone acceptor of PS II - SDS sodium dodecyl sulfate - Tris tris(hydroxymethyl)aminomethane - Tyr_D accessory electron donor on the D₂-protein - Tyr_Z tyrosine residue, acting as electron carrier between P₆₈₀ and the water oxidizing system     

Ultrastructure of cell wall and photosynthetic apparatus of the phycobilisome-less Synechocystis sp. strain BO 8402 and phycobilisome-containing derivative strain BO 9201

The ultrastructures of two closely related strains of a novel diazotrophic cyanobacterium, Synechocystis sp. BO 8402 and BO 9201, were examined using ultrathin sections and freeze-fracture electron microscopy. Cells of both strains were surrounded by an unusual thick peptidoglycan layer. Substructures in the layer indicated the presence of microplasmodesmata aligned perpendicular to the free cell surface and in the septum of dividing cells. Synechocystis sp. strain BO 8402 contained lobed, electronopaque, highly fluorescent inclusion bodies consisting of phycocyanin-linker complexes. The thylakoids lacked phycobilisomes and accommodated, in addition to randomly distributed exoplasmic freeze-fracture particles, patches of two-dimensionally ordered arrays of dimeric photosystem II particles in the exoplasmic fracture face. Determination of photosystem I and photosystem II suggested an increase of photosystem II in strain BO 8402. Strain BO 9201 performed phycobilisome-supported photosynthesis and showed rows of dimeric photosystem II particles in the exoplasmic fracture face. Corresponding particle-free grooves in the protoplasmic fracture face were lined by a class of large particles tentatively assigned as trimers of photosystem I. The different lateral organization of protein complexes in the thylakoid membranes and the fine structure of the cell wall are discussed with respect to absorption cross-section of photosynthesis and nitrogen fixation.Abbreviations EF Exoplasmic freeze-fracture face - P 700 Reaction centre chlorophyll of photosystem I - PF Protoplasmic freeze-fracture face - PS I Photosystem I - PS II Photosystem II     

Prolonged high light treatment of plant cells results in changes of the amount,the localization and the electrophoretic behavior of several thylakoid membrane proteins

The effect of a 30 h high light treatment on the amount and the localization of thylakoid proteins was analysed in low light grown photoautotrophic cells of Marchantia polymorpha and Chenopodium rubrum. High light treatment resulted in a net loss of D1 protein which was accompanied by comparable losses of other proteins of the PS II core (reaction center with inner antenna). LHC II proteins were not reduced correspondingly, indicating that these complexes are less affected by prolonged high light. High light influenced the distribution of PS II components between the grana and the stroma region of the thylakoid membrane, probably by translocation of the respective PS II proteins. Additionally, modifications of several thylakoid proteins were detected in high light treated cells of C. rubrum. These effects are discussed in relation to photoinhibitory damage and repair processes.Abbreviations BCA bioinchonic acid - chl chlorophyll - CF₁ coupling factor - CYC cycloheximide - GT grana thylakoids - HL high light - LL low light - PAGE polyacrylamide gel electrophoresis - PFD photon flux density - PS I Photosystem I - PS II Photosystem II - RC reaction center - SDS sodium dodecylsulfate - ST stroma thylakoids - Thyl unfractionated thylakoids     

Biogenesis of thylakoid membranes with emphasis on the process in Chlamydomonas

Recent results obtained by electron microscopic and biochemical analyses of greening Chlamydomonas reinhardtii y1 suggest that localized expansion of the plastid envelope is involved in thylakoid biogenesis. Kinetic analyses of the assembly of light-harvesting complexes and development of photosynthetic function when degreened cells of the alga are exposed to light suggest that proteins integrate into membrane at the level of the envelope. Current information, therefore, supports the earlier conclussion that the chloroplast envelope is a major biogenic structure, from which thylakoid membranes emerge. Chloroplast development in Chlamydomonas provides unique opportunities to examine in detail the biogenesis of thylakoids.Abbreviations Rubisco ribulose bisphosphate carboxylase/oxygenase - CAB Chl a/b-binding (proteins) - Chlide chlorophyllide - LHC I light-harvesting complex of PS I - LHC II light-harvesting complex of PS II - Pchlide protochlorophyllide     

Autotrophic cells of the <Emphasis Type="Italic">Synechocystis psbH</Emphasis> deletion mutant are deficient in synthesis of CP47 and accumulate inactive PS II core complexes

Cells of the psbH deletion mutant IC7 of the cyanobacterium Synechocystis PCC 6803 grown in the absence of glucose contain strongly reduced levels of chlorophyll when compared with cells grown in the presence of glucose, or compared with wild-type (WT) cells. Low-temperature fluorescence emission spectra revealed decreased content of both active PS II (Photosystem II) and PS I (Photosystem I) complexes. Analysis of thylakoid membrane complexes of IC7 by native electrophoresis showed a similar set of chlorophyll–proteins, namely a PS II core complex and trimeric and monomeric PS II complexes, as in WT. However, in contrast to WT, the ³⁵S-methionine protein labeling pattern of the mutant exhibited no preferential labeling of the D1 protein in the PS II core complexes, and the labeled D1 and D2 proteins accumulated predominantly in the PS II reaction center lacking CP47. The results show that in autotrophically grown cells of the psbH deletion mutant, selective D1 turnover is inhibited and synthesis of CP47 becomes a limiting step in the PS II assembly.     

Solubilization of green plant thylakoid membranes with n-dodecyl-α,D-maltoside. Implications for the structural organization of the Photosystem II,Photosystem I,ATP synthase and cytochrome b6 f complexes

A biochemical and structural analysis is presented of fractions that were obtained by a quick and mild solubilization of thylakoid membranes from spinach with the non-ionic detergent n-dodecyl-α,D-maltoside, followed by a partial purification using gel filtration chromatography. The largest fractions consisted of paired, appressed membrane fragments with an average diameter of about 360 nm and contain Photosystem II (PS II) and its associated light-harvesting antenna (LHC II), but virtually no Photosystem I, ATP synthase and cytochrome b ₆ f complex. Some of the membranes show a semi-regular ordering of PS II in rows at an average distance of about 26.3 nm, and from a partially disrupted grana membrane fragment we show that the supercomplexes of PS II and LHC II represent the basic structural unit of PS II in the grana membranes. The numbers of free LHC II and PS II core complexes were very high and very low, respectively. The other macromolecular complexes of the thylakoid membrane occurred almost exclusively in dispersed forms. Photosystem I was observed in monomeric or multimeric PS I-200 complexes and there are no indications for free LHC I complexes. An extensive analysis by electron microscopy and image analysis of the CF₀F₁ ATP synthase complex suggests locations of the δ (on top of the F₁ headpiece) and ∈ subunits (in the central stalk) and reveals that in a substantial part of the complexes the F₁ headpiece is bended considerably from the central stalk. This kinking is very likely not an artefact of the isolation procedure and may represent the complex in its inactive, oxidized form. This revised version was published online in June 2006 with corrections to the Cover Date.     

The cyanobacterium Synechococcus modulates Photosystem II function in response to excitation stress through D1 exchange

In this minireview we discuss effects of excitation stress on the molecular organization and function of PS II as induced by high light or low temperature in the cyanobacterium Synechococcus sp. PCC 7942. Synechococcus displays PS II plasticity by transiently replacing the constitutive D1 form (D1:1) with another form (D1:2) upon exposure to excitation stress. The cells thereby counteract photoinhibition by increasing D1 turn over and modulating PS II function. A comparison between the cyanobacterium Synechococcus and plants shows that in cyanobacteria, with their large phycobilisomes, resistance to photoinhibition is mainly through the dynamic properties (D1 turnover and quenching) of the reaction centre. In contrast, plants use antenna quenching in the light-harvesting complex as an important means to protect the reaction center from excessive excitation.Abbreviations D1 reaction center protein of Photosystem II - P₆₈₀ the reaction center of Photosystem II - Q_A the primary quinone acceptor of Photosystem II - Tyr_Z tyrosine electron donor to P₆₈₀     

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.     

Effects of dehydration on the electron transport of Chlorella. An in vivo fluorescence study

Chlorella was used to study the effects of dehydration on photosynthetic activities. The use of unicellular green algae assured that the extent of dehydration was uniform throughout the whole cell population during the course of desiccation. Changes in the activities of the cells were monitored by measurements of fluorescence induction kinetics. It was found that inhibition of most of the photosynthetic activities started at a similar level of cellular water content. They included CO₂ fixation, photochemical activity of Photosystem II and electron transport through Photosystem I. The blockage of electron flow through Photosystem I was complete and the whole transition occurred within a relative short time of dehydration. On the other hand, the suppression of Photosystem II activity was incomplete and the transition took a longer time of dehydration. Upon rehydration, the inhibition of Photosystem II activity was fully reversible when samples were in the middle of the transition, but was not thereafter. The electron transport through Photosystem I was also reversible during the transition, but was only partially afterward.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - F_m maximum fluorescence yield - F₀ non-variable fluorescence level emitted when all PS II centers are open - F_v variable part of fluorescence - PS photosystem - Q_A primary quinone acceptor of Photosystem II     

Pigment protein complexes and the concept of the photosynthetic unit: Chlorophyll complexes and phycobilisomes

The photosynthetic unit includes the reaction centers (RC 1 and RC 2) and the light-harvesting complexes which contribute to evolution of one O₂ molecule. The light-harvesting complexes, that greatly expand the absorptance capacity of the reactions, have evolved along three principal lines. First, in green plants distinct chlorophyll (Chl) a/b-binding intrinsic membrane complexes are associated with RC 1 and RC 2. The Chl a/b-binding complexes may add about 200 additional chromophores to RC 2. Second, cyanobacteria and red algae have a significant type of antenna (with RC 2) in the form of phycobilisomes. A phycobilisome, depending on the size and phycobiliprotein composition adds from 700 to 2300 light-absorbing chromophores. Red algae also have a sizable Chl a-binding complex associated with RC 1, contributing an additional 70 chromophores. Third, in chromophytes a variety of carotenoid-Chl-complexes are found. Some are found associated with RC 1 where they may greatly enhance the absorptance capacity. Association of complexes with RC 2 has been more difficult to ascertain, but is also expected in chromophytes. The apoprotein framework of the complexes provides specific chromophore attachment sites, which assures a directional energy transfer whithin complexes and between complexes and reaction centers. The major Chl-binding antenna proteins generally have a size of 16–28 kDa, whether of chlorophytes, chromophytes, or rhodophytes. High sequence homology observed in two of three transmembrane regions, and in putative chlorophyll-binding residues, suggests that the complexes are related and probably did not evolve from widely divergent polyphyletic lines.Abbreviations APC allophycocyanin - B phycoerythrin-large bangiophycean phycoerythrin - Chl chlorophyll - L_CM linker polypeptide in phycobilisome to thylakoid - FCP fucoxanthin Chl a/c complex - LHC(s) Chl-binding light harvesting complex(s) - LHC I Chl-binding complex of Photosystem I - LHC II Chl-binding complex of Photosystem II - PC phycocyanin - PCP peridinin Chl-binding complex - P700 photochemically active Chl a of Photosystem I - PS I Photosystem I - PS II Photosystem II - RC 1 reaction center core of PS I - RC 2 reaction center core of PS II - R phycoerythrin-large rhodophycean phycoerythrin - sPCP soluble peridinin Chl-binding complex     

Characterization of photosystem II in stroma thylakoid membranes

The functional state of the PS II population localized in the stroma exposed non-appressed thylakoid region was investigated by direct analysis of the PS II content of isolated stroma thylakoid vesicles. This PS II population, possessing an antenna size typical for PS II, was found to have a fully functional oxygen evolving capacity in the presence of an added quinone electron acceptor such as phenyl-p-benzoquinone. The sensitivity to DCMU for this PS II population was the same as for PS II in control thylakoids. However, under more physiological conditions, in the absence of an added quinone acceptor, no oxygen was evolved from stroma thylakoid vesicles and their PS II centers were found to be incapable to pass electrons to PS I and to yield NADPH. By comparison of the effect of a variety of added quinone acceptors with different midpoint potentials, it is concluded that the inability of PS II in the stroma thylakoid membranes to contribute to NADPH formation probably is due to that Q_A of this population is not able to reduce PQ, although it can reduce some artificial acceptors like phenyl-p-benzoquinone. These data give further support to the notion of a discrete PS II population in the non-appressed stroma thylakoid region, PS II, having a higher midpoint potential of Q_A than the PS II population in the appressed thylakoid region, PS II. The physiological significance of a PS II population that does not produce any NADPH is discussed.Abbreviations pBQ p-benzoquinone - Chl chlorophyll - DCBQ 2,6-dichloro-p-benzoquinone - DCIP 2,6-dichloroindophenol - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DMBQ 2,5-dimethyl-p-benzoquinone - DQ duroquinone(tetramethyl-p-benzoquinone) - FeCN ferricyanide (potassium hexacyanoferrat) - MV methylviologen - NADPH,NADP⁺ reduced or oxidized form of nicotinamide adenine dinucleotide phosphate respectively - PpBQ phenyl-p-benzoquinone - PQ plastoquinone - PS II photosystem II - PS I photosystem I - Q_A primary quinone acceptor of PS II - Q_B secondary quinone acceptor of PS II - E microEinstein     

Reduced levels of cytochrome b 6/f in transgenic tobacco increases the excitation pressure on Photosystem II without increasing sensitivity to photoinhibition in vivo

We have examined tobacco transformed with an antisense construct against the Rieske-FeS subunit of the cytochromeb ₆ f complex, containing only 15 to 20% of the wild-type level of cytochrome f. The anti-Rieske-FeS leaves had a comparable chlorophyll and Photosystem II reaction center stoichiometry and a comparable carotenoid profile to the wild-type, with differences of less than 10% on a leaf area basis. When exposed to high irradiance, the anti-Rieske-FeS leaves showed a greatly increased closure of Photosystem II and a much reduced capacity to develop non-photochemical quenching compared with wild-type. However, contrary to our expectations, the anti-Rieske-FeS leaves were not more susceptible to photoinhibition than were wild-type leaves. Further, when we regulated the irradiance so that the excitation pressure on photosystem II was equivalent in both the anti-Rieske-FeS and wild-type leaves, the anti-Rieske-FeS leaves experienced much less photoinhibition than wild-type. The evidence from the anti-Rieske-FeS tobacco suggests that rapid photoinactivation of Photosystem II in vivo only occurs when closure of Photosystem II coincides with lumen acidification. These results suggest that the model of photoinhibition in vivo occurring principally because of limitations to electron withdrawal from photosystem II does not explain photoinhibition in these transgenic tobacco leaves, and we need to re-evaluate the twinned concepts of photoinhibition and photoprotection.Abbreviations Chl chlorophyll - DCMU 3-(3,4-dichlophenyl)-1,-dimethylurea - Fo and Fo minimal fluorescence when all PS II reaction centers are open in dark- and light-acclimated leaves, respectively - Fm and Fm maximal fluorescence when all PS II reaction centers are closed in dark- and light-acclimated leaves, respectively - Fv variable fluorescence (Fm-Fo) in dark acclimated leaves - Fv variable fluorescence (Fm-Fo) in lightacclimated leaves - NPQ non-photochemical quenching of fluorescence - PS I and PS II Photosystem I and II - P680 primary electron donor of the reaction center of PS II - PFD photosynthetic flux density - Q_A primary acceptor quinone of PS II - q_p photochemical quenching of fluorescence - V+A+Z violaxanthin+antheraxanthin+zeaxanthin     

Effects of natural shade on soybean thylakoid membrane composition

The effect of natural shade on chloroplast thylakoid membrane activity and composition was examined for soybean (Glycine Max. cv. Young) grown under field conditions. Plots with high (10 plants m^–1 row) or low (1 plant m^–1 row) plant density were established. Expanding leaves were tagged at 50, 58 and 65 days after planting (DAP). At 92 DAP, tagged leaves were used as reference points to characterize canopy light environments and isolate thylakoid membranes. Light environments ranged from a photosynthetic photon flux density (PPFD) of 87% of full sun to a PPFD of 10% of full sun. The decline in PPFD was accompanied by an increase in the far-red/red (735 nm/645 nm) ratio from 0.9 to approximately six. The major effects of shade on chloroplast thylakoid membranes were a reduction in chloroplast coupling factor and a shift in light-harvesting capacity from Photosystem I to Photosystem II. Photosynthetic electron transport capacity was not affected by differences in PPFD, but was 20 to 30% higher in the 1 plant m^–1 row treatment. The plant density effect on electron transport was associated with differences in plastocyanin concentration, suggesting that plastocyanin is a limiting factor in soybean. Shade did not have a significant effect on the concentration of Photosystem II, Cyt b₆f, or Photosystem I complexes.Abbreviations CF₁ chloroplast coupling factor - DAP days after planting - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCIP 2,6-dichlorophenolindophenol - FR/R far-red/red - PBS 10 mM sodium phosphate (pH 7.0), 150 mM NaCl - PPFD photosynthetic photon flux density - PS I Photosystem I - PS II Photosystem II - P700 reaction center of Photosystem I - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - TBS 20 mM Tris-HCl (pH 7.5), 500 mM NaCl - TTBS 20 mM Tris-HCl (pH 7.5), 500 mM NaCl, 0.05% (w/v) polyoxyethylenesorbitan monolaurate (Tween-20) The US Government right to retain a non-exclusive, royalty-free licence in and to any copyright is acknowledged.The US Government right to retain a non-exclusive, royalty-free licence in and to any copyright is acknowledged.     

Chimaeric CP47 mutants of the cyanobacterium Synechocystis sp. PCC 6803 carrying spinach sequences: Construction and function

Chimaeric mutants of the cyanobacterium Synechocystis sp. PCC 6803 have been generated carrying part or all of the spinach psbB gene, encoding CP47 (one of the chlorophyll-binding core antenna proteins in Photosystem II). The mutant in which the entire psbB gene had been replaced by the homologous gene from spinach was an obligate photoheterotroph and lacked Photosystem II complexes in its thylakoid membranes. However, this strain could be transformed with plasmids carrying selected regions of Synechocystis psbB to give rise to photoautotrophs with a chimaeric spinach/cyanobacterial CP47 protein. This process involved heterologous recombination in the cyanobacterium between psbB sequences from spinach and Synechocystis 6803; which was found to be reasonably effective in Synechocystis. Also other approaches were used that can produce a broad spectrum of chimaeric mutants in a single experiment. Functional characterization of the chimaeric photoautotrophic mutants indicated that if a decrease in the photoautotrophic growth rates was observed, this was correlated with a decrease in the number of Photosystem II reaction centers (on a chlorophyll basis) in the thylakoid membrane and with a decrease in oxygen evolution rates. Remaining Photosystem II reaction centers in these chimaeric mutants appeared to function rather normally, but thermoluminescence and chlorophyll a fluorescence measurements provided evidence for a destabilization of Q_B ^–. This illustrates the sensitivity of the functional properties of the PS II reaction center to mild perturbations in a neighboring protein.Abbreviations diuron 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F_v variable chlorophyll a fluorescence - HEPES N-(2-hydroxyethyl)piperazine-N-(2-ethanesulfonic acid) - (k)bp (kilo)base pairs - PS II Photosystem II - Q_A primary electron-accepting plastoquinone in Photosystem II - Q_B secondary electron-accepting plastoquinone in Photosystem II - SDS sodium dodecyl sulfate     

Determination of the primary charge separation rate in Photosystem II reaction centers at 15 K

We have measured the rate constant for the formation of the oxidized chlorophyll a electron donor (P680⁺) and the reduced electron acceptor pheophytin a ^– (Pheo a ^–) following excitation of isolated Photosystem II reaction centers (PS II RC) at 15 K. This PS II RC complex consists of D₁, D₂, and cytochrome b-559 proteins and was prepared by a procedure which stabilizes the protein complex. Transient absorption difference spectra were measured from 450–840 nm as a function of time with 500fs resolution following 610 nm laser excitation. The formation of P680⁺-Pheo a ^– is indicated by the appearance of a band due to P680⁺ at 820 nm and corresponding absorbance changes at 490, 515 and 546 nm due to the formation of Pheo a ^–. The appearance of the 490 nm and 820 nm bands is monoexponenital with =1.4±0.2 ps. Treatment of the PS II RC with sodium dithionite and methyl viologen followed by exposure to laser excitation results in accumulation of Pheo a ^–. Laser excitation of these prereduced RCs at 15 K results in formation of a transient absorption spectrum assigned to ^1*P680. We observe wavelength-dependent kinetics for the recovery of the transient bleach of the Q_y absorption bands of the pigments in both untreated and pre-reduced PS II RCs at 15K. This result is attributed to an energy transfer process within the PS II RC at low temperature that is not connected with charge separation.Abbreviations PS I Photosystem I - PS II Photosystem II - RC reaction center - P680 primary electron donor in Photosystem II - Chl a chlorophyll a - Pheo a pheophytin a     

Mechanism of action of anions on the electron transport chain in thylakoid membranes of higher plants

With an aim to improve our understanding of the mechanisms behind specific anion effects in biological membranes, we have studied the effects of sodium salts of anions of varying valency in thylakoid membranes. Rates of electron transport of PS II and PS I, 77K fluorescence emission and excitation spectra, cyclic electron flow around PS I and circular dichroism (CD) spectra were measured in thylakoid membranes in order to elucidate a general mechanism of action of inorganic anions on photosynthetic electron transport chain. Re-distribution of absorbed excitation energy has been observed as a signature effect of inorganic anions. In the presence of anions, such as nitrite, sulphate and phosphate, distribution of absorbed excitation energy was found to be more in favor of Photosystem I (PS I). The amount of energy distributed towards PS I depended on the valency of the anion. In this paper, we propose for the first time that energy re-distribution and its valence dependence may not be the effect of anions per se. The entry of negative charge (anion) is accompanied by influx of positive charge (protons) to maintain a balance of charge across the thylakoid membranes. As reflected by the CD spectra, the observed energy re-distribution could be a result of structural rearrangements of the protein complexes of PS II caused by changes in the ionic environment of the thylakoid lumen.     

A novel P700 redox kinetics probe for rapid,non‐intrusive and whole‐tissue determination of photosystem II functionality,and the stoichiometry of the two photosystems in vivo

We sought a rapid, non‐intrusive, whole‐tissue measure of the functional photosystem II (PS II) content in leaves. Summation of electrons, delivered by a single‐turnover flash to P700⁺ (oxidized PS I primary donor) in continuous background far‐red light, gave a parameter S in absorbance units after taking into account an experimentally determined basal electron flux that affects P700 redox kinetics. S was linearly correlated with the functional PS II content measured by the O₂ yield per single‐turnover repetitive flash in Arabidopsis thaliana expressing an antisense construct to the PsbO (manganese‐stabilizing protein in PS II) proteins of PS II (PsbO mutants). The ratio of S to z_max (total PS I content in absorbance units) was comparable to the PS II/PS I reaction‐center ratio in wild‐type A. thaliana and in control Spinacea oleracea. Both S and S/z_max decreased in photoinhibited spinach leaf discs. The whole‐tissue functional PS II content and the PS II/photosystem I (PS I) ratio can be non‐intrusively monitored by S and S/z_max, respectively, using a quick P700 absorbance protocol compatible with modern P700 instruments.     

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. -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.Abbreviations DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea - DCPIP dichlorphenolindophenol - MV methylviologen (paraquat) - PS I photosystem I - PS II photosystem II - DPC diphenyl-carbazide