Plastocyanin stimulation of whole chain and photosystem I electron transport in inside-out thylakoid vesicles

Inside-out and right-side-out thylakoid vesicles were isolated from spinach chloroplasts by aqueous-polymer two-phase (dextran/polyethylene glycol) partitioning. Externally added plastocyanin stimulated the whole-chain and PSI electron transport rates in the inside-out thylakoid vesicles by about 500 and 350%, respectively, compared to about 50% stimulation for both assays in the fraction enriched in right-side-out vesicles. No apparent stimulation by plastocyanin was observed in unbroken Class II thylakoids. The electron transport between PSII and PSI in inside-out thylakoid vesicles appears to be interrupted due to plastocyanin release from the thylakoids by the Yeda press treatment, but it was restored by externally added plastocyanin. The P700 content of the inside-out membrane preparations, measured by chemical and photochemical methods, was 1 P700 per 1100 to 1500 chlorophylls while it was about 1 P700 per 500 chlorophylls for the right-side-out vesicles. The data presented support the concept of lateral heterogeneity of PS I and II in thylakoid membranes, but does not support a virtual or total absence of PSI in the appressed grana partitions. Further, the heterogeneity does not appear to be as extreme as suggested earlier. Although PSI is somewhat depleted in the appressed grana membrane region, there is adequate photochemically active P700, when sufficient plastocyanin is available, to effectively couple PSI electron transfer with the preponderant PSII in linear electron transport.     

Preparation of highly enriched photosystem II membrane vesicles by a non-detergent method

An improved, non-detergent, method for preparative isolation of PS II membrane vesicles from spinach chloroplasts is presented. Thylakoids (chlorophyll (Chl) a/b ratio 2.8, Chl/P700 435) were fractionated by Yeda press treatment and aqueous two-phase partition to yield inside-out vesicles (1) (chl a/b 2.2, chl/P700 700). These vesicles were subjected a sonication — phase partitioning procedure; steps of sonication of inside-out vesicles, while still present in a dextran-polyethylene glycol two-phase system were alternated by phase partition. These steps selectively removed P700-containing membrane fragments from the inside-out vesicles and yielded a membrane fraction with improved PS II purity (Chl a/b ratio 1.9, Chl/P700 1500) and retained oxygen evolving capacity (295 mol O₂ mg Chl^-1 h^-1).     

Quantification of photosystem I and II in different parts of the thylakoid membrane from spinach

Electron paramagnetic resonance (EPR) was used to quantify Photosystem I (PSI) and PSII in vesicles originating from a series of well-defined but different domains of the thylakoid membrane in spinach prepared by non-detergent techniques. Thylakoids from spinach were fragmented by sonication and separated by aqueous polymer two-phase partitioning into vesicles originating from grana and stroma lamellae. The grana vesicles were further sonicated and separated into two vesicle preparations originating from the grana margins and the appressed domains of grana (the grana core), respectively. PSI and PSII were determined in the same samples from the maximal size of the EPR signal from P700(+) and Y(D)( .-), respectively. The following PSI/PSII ratios were found: thylakoids, 1.13; grana vesicles, 0.43; grana core, 0.25; grana margins, 1.28; stroma lamellae 3.10. In a sub-fraction of the stroma lamellae, denoted Y-100, PSI was highly enriched and the PSI/PSII ratio was 13. The antenna size of the respective photosystems was calculated from the experimental data and the assumption that a PSII center in the stroma lamellae (PSIIbeta) has an antenna size of 100 Chl. This gave the following results: PSI in grana margins (PSIalpha) 300, PSI (PSIbeta) in stroma lamellae 214, PSII in grana core (PSIIalpha) 280. The results suggest that PSI in grana margins have two additional light-harvesting complex II (LHCII) trimers per reaction center compared to PSI in stroma lamellae, and that PSII in grana has four LHCII trimers per monomer compared to PSII in stroma lamellae. Calculation of the total chlorophyll associated with PSI and PSII, respectively, suggests that more chlorophyll (about 10%) is associated with PSI than with PSII.     

Quantification of photosystem I and II in different parts of the thylakoid membrane from spinach

Electron paramagnetic resonance (EPR) was used to quantify Photosystem I (PSI) and PSII in vesicles originating from a series of well-defined but different domains of the thylakoid membrane in spinach prepared by non-detergent techniques. Thylakoids from spinach were fragmented by sonication and separated by aqueous polymer two-phase partitioning into vesicles originating from grana and stroma lamellae. The grana vesicles were further sonicated and separated into two vesicle preparations originating from the grana margins and the appressed domains of grana (the grana core), respectively. PSI and PSII were determined in the same samples from the maximal size of the EPR signal from P700⁺ and Y_D, respectively. The following PSI/PSII ratios were found: thylakoids, 1.13; grana vesicles, 0.43; grana core, 0.25; grana margins, 1.28; stroma lamellae 3.10. In a sub-fraction of the stroma lamellae, denoted Y-100, PSI was highly enriched and the PSI/PSII ratio was 13. The antenna size of the respective photosystems was calculated from the experimental data and the assumption that a PSII center in the stroma lamellae (PSIIβ) has an antenna size of 100 Chl. This gave the following results: PSI in grana margins (PSIα) 300, PSI (PSIβ) in stroma lamellae 214, PSII in grana core (PSIIα) 280. The results suggest that PSI in grana margins have two additional light-harvesting complex II (LHCII) trimers per reaction center compared to PSI in stroma lamellae, and that PSII in grana has four LHCII trimers per monomer compared to PSII in stroma lamellae. Calculation of the total chlorophyll associated with PSI and PSII, respectively, suggests that more chlorophyll (about 10%) is associated with PSI than with PSII.     

The stoichiometry of the two photosystems in higher plants revisited

The stoichiometry of Photosystem II (PSII) to Photosystem I (PSI) reaction centres in spinach leaf segments was determined by two methods, each capable of being applied to monitor the presence of both photosystems in a given sample. One method was based on a fast electrochromic (EC) signal, which in the millisecond time scale represents a change in the delocalized electric potential difference across the thylakoid membrane resulting from charge separation in both photosystems. This method was applied to leaf segments, thus avoiding any potential artefacts associated with the isolation of thylakoid membranes. Two variations of this method, suppressing PSII activity by prior photoinactivation (in spinach and poplar leaf segments) or suppressing PSI by photo-oxidation of P700 (the chlorophyll dimer in PSI) with background far-red light (in spinach, poplar and cucumber leaf segments), each gave the separate contribution of each photosystem to the fast EC signal; the PSII/PSI stoichiometry obtained by this method was in the range 1.5-1.9 for the three plant species, and 1.5-1.8 for spinach in particular. A second method, based on electron paramagnetic resonance (EPR), gave values in a comparable range of 1.7-2.1 for spinach. A third method, which consisted of separately determining the content of functional PSII in leaf segments by the oxygen yield per single turnover-flash and that of PSI by photo-oxidation of P700 in thylakoids isolated from the corresponding leaves, gave a PSII/PSI stoichiometry (1.5-1.7) that was consistent with the above values. It is concluded that the ratio of PSII to PSI reaction centres is considerably higher than unity in typical higher plants, in contrast to a surprisingly low PSII/PSI ratio of 0.88, determined by EPR, that was reported for spinach grown in a cabinet under far-red-deficient light in Sweden [Danielsson et al. (2004) Biochim. Biophys. Acta 1608: 53-61]. We suggest that the low PSII/PSI ratio in the Swedish spinach, grown in far-red-deficient light with a lower PSII content, is not due to greater accuracy of the EPR method of measurement, as suggested by the authors, but is rather due to the growth conditions.     

The stoichiometry of the two photosystems in higher plants revisited

The stoichiometry of Photosystem II (PSII) to Photosystem I (PSI) reaction centres in spinach leaf segments was determined by two methods, each capable of being applied to monitor the presence of both photosystems in a given sample. One method was based on a fast electrochromic (EC) signal, which in the millisecond time scale represents a change in the delocalized electric potential difference across the thylakoid membrane resulting from charge separation in both photosystems. This method was applied to leaf segments, thus avoiding any potential artefacts associated with the isolation of thylakoid membranes. Two variations of this method, suppressing PSII activity by prior photoinactivation (in spinach and poplar leaf segments) or suppressing PSI by photo-oxidation of P700 (the chlorophyll dimer in PSI) with background far-red light (in spinach, poplar and cucumber leaf segments), each gave the separate contribution of each photosystem to the fast EC signal; the PSII/PSI stoichiometry obtained by this method was in the range 1.5-1.9 for the three plant species, and 1.5-1.8 for spinach in particular. A second method, based on electron paramagnetic resonance (EPR), gave values in a comparable range of 1.7-2.1 for spinach. A third method, which consisted of separately determining the content of functional PSII in leaf segments by the oxygen yield per single turnover-flash and that of PSI by photo-oxidation of P700 in thylakoids isolated from the corresponding leaves, gave a PSII/PSI stoichiometry (1.5-1.7) that was consistent with the above values. It is concluded that the ratio of PSII to PSI reaction centres is considerably higher than unity in typical higher plants, in contrast to a surprisingly low PSII/PSI ratio of 0.88, determined by EPR, that was reported for spinach grown in a cabinet under far-red-deficient light in Sweden [Danielsson et al. (2004) Biochim. Biophys. Acta 1608: 53-61]. We suggest that the low PSII/PSI ratio in the Swedish spinach, grown in far-red-deficient light with a lower PSII content, is not due to greater accuracy of the EPR method of measurement, as suggested by the authors, but is rather due to the growth conditions.     

Localization of the reaction side of plastocyanin from immunological and kinetic studies with inside-out thylakoid vesicles

(1) The effect of four active antisera against plastocyanin on Photosystem I-driven electron transport and phosphorylation was investigated in spinach chloroplasts. Partial inhibition of electron transport and stimulation of plastocyanin-dependent phosphorylation were sometimes observed after adding amounts of antibodies which were in large excess and not related to the plastocyanin content of the chloroplasts. This indicates effects of the antibodies on the membrane. (2) The antibodies against plastocyanin neither directly nor indirectly agglutinated unbroken chloroplast membranes. (3) The plastocyanin content of right-side-out and inside-out thylakoid vesicles isolated by aqueous polymer two-phase partition from chloroplasts disrupted by Yeda press treatment was determined by quantitative rocket electroimmunodiffusion. Right-side-out vesicles retained about 25%, inside-out vesicles none of the original amount of plastocyanin. (4) The effect of externally added plastocyanin on the reduction of P-700 was studied by monitoring the absorbance changes at 703 nm after a long flash. In inside-out vesicles P-700 was reduced by the added plastocyanin but not in right-side-out vesicles and class II chloroplasts. These results provide strong evidence for a function of plastocyanin at the internal side of the thylakoid membrane.     

钝顶螺旋藻(Spirulina platensis)光系统Ⅰ的分离表征及稳定性

采用蔗糖密度梯度离心法对钝顶螺旋藻光合膜蛋白(PSI)进行分离纯化,并对其光谱学性质、热稳定性及光合放氧活性进行分析表征。结果发现,采用蔗糖密度精度离心法,可以成功分离出4条色素蛋白质复合体条带,其中最下层条带为完整的PSI三聚体,其每毫克叶绿素a光合放氧活性达到420μmol/h。当温度达到50℃左右时,分离得到的PSI在溶液开始变性失活。     

The effect of the dark interval in intermittent light on thylakoid development: photosynthetic unit formation and light harvesting protein accumulation

Etiolated bean plants were grown in intermittent light with dark intervals of shorter or longer duration, to modulate the rate of chlorophyll accumulation, relative to that of the other thylakoid components formed. We thus produced conditions under which chlorophyll becomes more or less a limiting factor. We then tested whether LHC complexes can be incorporated in the thylakoid. It was found that an equal amount of chlorophyll, formed under the same total irradiation received, may be used for the stabilization of few and large-in-size PS units containing LHC components (short dark-interval intermittent light), or for the stabilization of many and small-in-size PS units with no LHC components (long dark-interval intermittent light). The size of the PS units diminishes as the dark-interval duration is increased, with no further change after 98 minutes. The PSII/cytf ratio remains constant throughout development in intermittent light and equal to that of mature chloroplasts (PSII/cytf = 1) except in the case of very long dark-interval regimes, where about half PSII units per cytf are present. The PSII/PSI ratio was found to be correlated with the PSII unit size (the larger the size, the lower the ratio). The number of PSI units operating on the same electron transfer chain varied depending on the size of the PSII unit (the larger the PSII unit size, the more the PSI units per chain). The results suggest that it is not the chlorophyll content per se which regulates the stabilization of LHC in developing thylakoids and consequently the size of the PS units, but rather the rate by which it is accumulated, relative to that of the other thylakoid components.Abbreviations Chl Chlorophyll - CL Continuous light - CPa the reaction center complex of PSII - CPI the reaction center complex of PSI - CPIa Chlorophyll protein complex containing the CPI and the light harvesting complex of PSI - fr w fresh weight - LDC Light dark cycles - LHC-I Light-harvesting complex of PSI - LHC-II Light harvesting complex of PSII - PS photosystem - PSI photosystem I - PSII photosystem II     

Fragmentation and separation analysis of the photosynthetic membrane from spinach

Membrane vesicles, originating from grana, grana core (appressed grana regions), grana margins and stroma lamellae/end membranes, were analysed by counter current distribution (CCD) using aqueous dextran-polyethylene glycol two-phase systems. Each vesicle population gave rise to distinct peaks in the CCD diagram representing different vesicle subpopulations. The grana vesicles and grana core vesicles each separated into 3 different subpopulations having different chlorophyll a/b ratios and PSI/PSII ratios. Two of the grana core subpopulations had a chlorophyll a/b ratio of 2.0 and PSI/PSII ratio of 0.10 and are among the most PSII enriched thylakoid vesicle preparation obtained so far by a non detergent method. The margin vesicles separated into 3 different populations, with about the same chlorophyll a/b ratios, but different fluorescence emission spectra. The stroma lamellae/end membrane vesicles separated into 4 subpopulations. Plastoglobules, connected to membrane vesicles, were highly enriched in 2 of these subpopulations and it is proposed that these 2 subpopulations originate from stroma lamellae while the 2 others originate from end membranes. Fragmentation and separation analysis shows that the margins of grana constitute a distinct domain of the thylakoid and also allows the estimation of the chlorophyll antenna sizes of PSI and PSII in different thylakoid domains.     

Interactions between thylakoid electron transfer complexes. II. Modification studies with glutaraldehyde

Photosystem I (PSI) and photosystem II (PSII) complexes have been isolated from stacked spinach thylakoid membranes that had been treated with varying amounts of glutaraldehyde. The concentrations of cytochrome f, Q, and P700 have been determined by spectrophotometric methods. It was found that at low concentrations of glutaraldehyde, the amount of cytochrome f associated with either PSII or PSI increased significantly while the amounts of Q and P700 stayed relatively constant. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting analyses indicated the presence of cytochrome f and other components of the cytochrome b6-f complex in the PSII and PSI preparations after glutaraldehyde treatment, but no intermolecular cross-linked polypeptides could be detected. Solubilization of the cytochrome b6-f complex was also inhibited after thylakoid membranes were treated with low concentrations of glutaraldehyde. These results are discussed in relation to current models for the organization of the membrane complexes, and relate to the location of the cytochrome b6-f complex in appressed and nonappressed membrane regions of thylakoids.     

Quantitative separation of spinach thylakoids into Photosystem II-enriched inside-out vesicles and Photosystem I-enriched right-side-out vesicles

Yeda press disruption of thylakoids in the presence of magnesium followed by aqueous polymer two-phase partitioning fractionated the total thylakoid membrane material into two distinctly different fractions. One fraction comprised approx. 60% of the material on a chlorophyll basis and contained inside-out vesicles while the other fraction (40%) contained right-side-out vesicles. The sidedness of the vesicles was determined from the direction of their light-induced proton translocation. The inside-out vesicles showed a pronounced Photosystem (PS) II enrichment as judged by their high PS II and low PS I activities. Moreover, they showed a high ratio between the PS II reaction centre chlorophyll-protein complex and the PS I reaction centre chlorophyll-protein complex (CP I). The chlorophyll $\text{a}\text{b}$ ratio was as low as 2.3 compared to 3.2 for the starting material. In contrast, the right-side-out vesicles showed a pronounced PS I enrichment. Their chlorophyll $\text{a}\text{b}$ ratio was 4.3–4.9. The tight stacking induced by Mg²⁺ allows a quantitative formation of inside-out vesicles from the appressed thylakoid regions while mainly non-appressed thylakoids turn right-side-out. The possibility of fractionating all of the thylakoid material into two sub-populations with markedly different composition with respect to PS I and PS II argues against a close physical association between the two photosystems and in favour of their spatial separation in the plane of the membrane. This fractionation procedure, which can be completed within 1 h and gives high yields of both PS II inside-out thylakoids and PS I right-side-out thylakoids, should be very useful for facilitating and improving studies on both the transverse and lateral organization of the thylakoid membrane.     

Light regulation of photosynthetic membrane structure,organization, and function

The light environment during plant growth determines the structural and functional properties of higher plant chloroplasts, thus revealing a dynamically regulated developmental system. Pisum sativum plants growing under intermittent illumination showed chloroplasts with fully functional photosystem (PS) II and PSI reaction centers that lacked the peripheral chlorophyll (Chi) a/b and Chl a light-harvesting complexes (LHC), respectively. The results suggest a light flux differential threshold regulation in the biosynthesis of the photosystem core and peripheral antenna complexes. Sun-adapted species and plants growing under far-red-depleted illumination showed grana stacks composed of few (3–5) thylakoids connected with long intergrana (stroma) thylakoids. They had a PSII/PSI reaction center ratio in the range 1.3–1.9. Shade-adapted species and plants growing under far-red-enrichcd illumination showed large grana stacks composed of several thylakoids, often extending across the entire chloroplast body, and short intergrana stroma thylakoids. They had a higher PSII/PSI reaction center ratio, in the range of 2.2–4.0. Thus, the relative extent of grana and stroma thylakoid formation corresponds with the relative amounts of PSII and PSI in the chloroplast, respectively. The structural and functional adaptation of the photosynthetic membrane system in response to the quality of illumination involves mainly a control on the rate of PSII and PSI complex biosynthesis.     

Chloroplast structure in normal and pigment-deficient soybeans grown in continuous red or far-red light

Clark L1, a normal green soybean [ Glycine max (L.) Merrill] and Clark y₉y₉, a backross-developed isoline exhibiting pigment deficiency, were grown under continuous red (11 W m⁻² and far-red (9 W m⁻²) light. Chloroplast thylakoids from the unifoliolate leaf (9–10 days old) were isolated and analyzed for pigments, pigment-protein, membrane polypeptides, electron transport and ultrastructural differences. Chloroplasts of soybean plants grown under far-red light have decreased chlorophyll a to chlorophyll b ratio, increased light-harvesting complexes, and grana structure with few stroma-type thylakoids. Photosystem II/photosystem I ratios (PSII/PSI) are higher in far-red due to decreased synthesis of PSI reaction center and/or less antenna associated with PSI.     

Photosystem electron-transport capacity and light-harvesting antenna size in maize chloroplasts

Spectrophotometric and kinetic measurements were applied to yield photosystem (PS) stoichiometries and the functional antenna size of PSI, PSII_α, and PSII_β in Zea mays chloroplasts in situ. Concentrations of PSII and PSI reaction centers were determined from the amplitude of the light-induced absorbance change at 320 and 700 nm, which reflect the photoreduction of the primary electron acceptor Q of PSII and the photooxidation of the reaction center P700 of PSI, respectively. Determination of the functional chlorophyll antenna size (N) for each photosystem was obtained from the measurement of the rate of light absorption by the respective reaction center. Under the experimental conditions employed, the rate of light absorption by each reaction center was directly proportional to the number of light-harvesting chlorophyll molecules associated with the respective photosystem. We determined N_P700 = 195, N_α = 230, N_β = 50 for the number of chlorophyll molecules in the light-harvesting antenna of PSI, PSII_α, and PSII_β, respectively. The above values were used to estimate the PSII/PSI electron-transport capacity ratio (C) in maize chloroplasts. In mesophyll chloroplasts C > 1.4, indicating that, under green actinic excitation when Chl a and Chl b molecules absorb nearly equal amounts of excitation, PSII has a capacity to turn over electrons faster than PSI. In bundle sheath chloroplasts C < 1, suggesting that such chloroplasts are not optimally poised for linear electron transport and reductant generation.     

Redox signalling and the structural basis of regulation of photosynthesis by protein phosphorylation

In photosynthesis in chloroplasts and cyanobacteria, redox control of thylakoid protein phosphorylation regulates distribution of absorbed excitation energy between the two photosystems. When electron transfer through chloroplast photosystem II (PSII) proceeds at a rate higher than that through photosystem I (PSI), chemical reduction of a redox sensor activates a thylakoid protein kinase that catalyses phosphorylation of light-harvesting complex II (LHCII). Phosphorylation of LHCII increases its affinity for PSI and thus redistributes light-harvesting chlorophyll to PSI at the expense of PSII. This short-term redox signalling pathway acts by means of reversible, post-translational modification of pre-existing proteins. A long-term equalisation of the rates of light utilisation by PSI and PSII also occurs: by means of adjustment of the stoichiometry of PSI and PSII. It is likely that the same redox sensor controls both state transitions and photosystem stoichiometry. A specific mechanism for integration of these short- and long-term adaptations is proposed. Recent evidence shows that phosphorylation of LHCII causes a change in its 3-D structure, which implies that the mechanism of state transitions in chloroplasts involves control of recognition of PSI and PSII by LHCII. The distribution of LHCII between PSII and PSI is therefore determined by the higher relative affinity of phospho-LHCII for PSI, with lateral movement of the two forms of the LHCII being simply a result of their diffusion within the membrane plane. Phosphorylation-induced dissociation of LHCII trimers may induce lateral movement of monomeric phospho-LHCII, which binds preferentially to PSI. After dephosphorylation, monomeric, unphosphorylated LHCII may trimerize at the periphery of PSII.     

Differential detergent-solubilization of integral thylakoid membrane complexes in spinach chloroplasts. Localization of photosystem II, cytochrome b6-f complex and photosystem I

Progressive solubilization of spinach chloroplast thylakoids by Triton X-100 was employed to investigate the domain organization of the electron transport complexes in the thylakoid membrane. Triton/chlorophyll ratios of 1:1 were sufficient to disrupt fully the continuity of the thylakoid membrane network, but not sufficient to solubilize either photosystem I (PSI), photosystem II (PSII) or the cytochrome b6-f(Cyt b6-f) complex. Progressive with the Triton concentration increase (Triton/Chl greater than 1:1), a differential solubilization of the three electron transport complexes was observed. Solubilization of the Cyt b6-f complex from the thylakoid membrane preceded that of PSI and apparently occurred early in the solubilization of stroma-exposed segments of the chloroplast lamellae. The initial removal of chlorophyll (up to 40% of the total) occurred upon solubilization of PSI from the stroma-exposed lamella regions in which PSI is localized. The tightly appressed membrane of the grana partition regions was markedly resistant to solubilization by Triton X-100. Thus, solubilization of PSII from this membrane region was initiated only after all Cyt b6-f and PSI complexes were removed from the chloroplast lamellae. The results support the notion of extreme lateral heterogeneity in the organization of the electron transport complexes in higher plant chloroplasts and suggest a Cyt b6-f localization in the membrane of the narrow fret regions which serve as a continuum between the grana and stroma lamellae.     

Photosystem Stoichiometry and Excitation Distribution in Chloroplasts from Surface and Minus 20 Meter Blades of Macrocystis pyrifera, the Giant Kelp

The photochemical apparatus organization in the thylakoid membrane of Macrocystis pyrifera, the giant kelp, was investigated. Chloroplasts were isolated from surface and minus 20 meter blades. Photosynthetic electron-transport complex quantitation revealed ratios of photosystem (PS) II/cytochrome b₆-f/PSI = 1.8:3.3:1.0 in surface and 2.2:2.3:1.0 in minus 20 meter blades. The apparent photosynthetic unit size of chloroplasts from minus 20 meter blades (chlorophyll/P700 = 1485:1) was about 45% larger than that of surface blades (chlorophyll/P700 = 1025:1). The larger photosynthetic unit size of minus 20 meter blades is attributed to the substantially lower intensity of sunlight reaching the minus 20 meter habitat. In different chloroplast preparations, the effective absorption cross section of PSI and PSII to 670 nanometer light (chlorophyll a) and 481 nanometer light (chlorophyll c and fucoxanthin) was investigated. The results showed larger functional antenna size for PSII (about 90%) and for PSI (about 50%) in minus 20 meter than in surface blades. Moreover, the efficiency of utilization of 481 nanometer light by Macrocystis chloroplasts was equal to that of 670 nanometer light. It is concluded that the chlorophyll c-fucoxanthin complex in brown algae enables the highly efficient utilization of blue-green wavelengths of the nearshore marine environment and contributes to the dominance of M. pyrifera in this habitat.     

Dynamics of Photosystem Stoichiometry Adjustment by Light Quality in Chloroplasts

Long-term imbalance in light absorption and electron transport by photosystem I (PSI) and photosystem II (PSII) in chloroplasts brings about changes in the composition, structure, and function of thylakoid membranes. The response entails adjustment in the photosystem ratio, which is optimized to help the plant retain a high quantum efficiency of photosynthesis (W.S. Chow, A. Melis, J.M. Anderson [1990] Proc Nat Acad Sci USA 87: 7502-7506). The dynamics of photosystem ratio adjustment were investigated upon the transfer of pea {Pisum sativum} plants from a predominantly PSI-light to a predominantly PSII-light environment and vice versa. The concentration of functional components (primary electron accepting plastoquinone of PSII [QA], P700) and that of constituent proteins were monitored during acclimation by A difference spectrophotometry and immunoblot analysis, respectively. Fully reversible changes in photosystem ratio occurred with a half-time of about 20 h. They involved closely coordinated changes in the concentration of the QA, reaction center protein D1, D2, and the 9-kD apoprotein of the cytochrome b559 for PSII. Similarly, closely coordinated changes in the relative concentration of P700 and reaction center proteins of PSI were observed. The level of chlorophyll b and that of the light-harvesting complex II changed in accordance with the concentration of PSII in the acclimating thylakoids. Overall, adjustments in the photosystem ratio in response to PSI- or PSII-light conditions appeared to be a well-coordinated reaction in the chloroplast. The response was absent in the chlorophyll b-less chlorina f2 mutant of barley (Hordeum vulgare) and in a phycobilisomeless mutant of Agmenellum quadruplicatum, suggesting that photosystem accessory pigments act as the light-quality perception molecules and that PSI and PSII themselves play a role in the signal transduction pathway.     

Proteins affecting thylakoid morphology – the key to understanding vesicle transport in chloroplasts?

We recently showed that a Rab protein, CPRabA5e (CP = chloroplast localized), is located in chloroplasts of Arabidopsis thaliana where it is involved in various processes, such as thylakoid biogenesis and vesicle transport. Using a yeast two-hybrid method, CPRabA5e was shown to interact with a number of chloroplast proteins, including the CURVATURE THYLAKOID 1A (CURT1A) protein and the light-harvesting chlorophyll a/b binding protein (LHCB1.5). CURT1A has recently been shown to modify thylakoid architecture by inducing membrane curvature in grana, whereas LHCB1.5 is a protein of PSII (Photosystem II) facilitating light capture. LHCB1.5 is imported to chloroplasts and transported to thylakoid membranes using the post-translational Signal Recognition Particle (SRP) pathway. With this information as starting point, we here discuss their subsequent protein-protein interactions, given by the literature and Interactome 3D. CURT1A itself and several of the proteins interacting with CURT1A and LHCB1.5 have relations to vesicle transport and thylakoid morphology, which are also characteristics of cprabA5e mutants. This highlights the previous hypothesis of an alternative thylakoid targeting pathway for LHC proteins using vesicles, in addition to the SRP pathway.