Photosynthetic Unit Research Articles

Leaf photosynthesis, fluorescence response to salinity and the relevance to chloroplast salt compartmentation and anti-oxidative stress in two poplars

In a 16-day study, the effect of increasing soil NaCl on leaf photosynthesis, chlorophyll a fluorescence, chloroplast ion compartmentation, variations of SOD (superoxide dismutase) and POD (peroxidase) isoenzymes and the relevance to salt resistance were investigated in seedlings of Populus euphratica Oliv. (P. euphratica) (salt-resistant) and rooted cuttings of P. “popularis 35–44” (P. popularis) (salt-sensitive). Initial salinity caused a rapid decline of net photosynthetic rate (Pn) and unit transpiration rate (TRN) in P. euphratica, resulting from the NaCl-induced stomatal closure. In a longer-term of salinity, CO2 assimilation in P. popularis was severely reduced whereas stressed P. euphratica maintained a relatively higher and constant level of Pn. Pn–Ci curves showed that salt stress (12 days) reduced CO2 saturation point (CSP), CO2 saturated Pn (CSP n ), and carboxylation efficiency (CE), but increased CO2 compensation point (CCP) in the two genotypes. Similarly, salinity lowered light saturation point (LSP), light saturated Pn (LSP n ), and apparent quantum yield (AQY) in both genotypes but the inhibitory effect of NaCl on light reaction was more pronounced in P. popularis, as compared to P. euphratica. Chlorophyll a fluorescence data indicated that a longer-term of salt stress (12 days) exhibited a marked influence on fluorescence parameters of P. popularis in both dark- and light-adapted states: (a) NaCl inhibited the maximal efficiency of PSII photochemistry (Fv/Fm) due to the salt-induced increase of Fo (the minimal fluorescence) and the marked decline of Fm (the maximal fluorescence); (b) salinity decreased coefficient of photochemical quenching (qP) but markedly elevated coefficient of nonphotochemical quenching (qN) in the light-adapted state. In contrast, there were no corresponding changes of chlorophyll a fluorescence in salinised P. euphratica. X-ray microanalysis results showed that salinity caused salt accumulation in the chloroplasts of P. popularis in which Na+ and Cl− increased up to 42 and 221 mmol dm−3, respectively. Great buildup of Na+ and Cl− in chloroplasts of P. popularis may exhibit direct and indirect restrictions on dark and light reactions. The activity of SOD isoenzymes (CuZn-SOD I and CuZn-SOD II) and POD isoenzymes in P. popularis decreased with increasing exposure period, and leaf malondialdehyde (MDA) content and membrane permeability (MP) increased correspondingly. In contrast to P. popularis, stressed P. euphratica maintained activity of SOD and POD isoenzymes and there was no significant increase of MDA and MP during the period of salt stress. In conclusion, P. euphratica plants exhibited a higher capacity to maintain the activity of anti-oxidant enzymes and restrict salt accumulation in the chloroplasts, the photosynthesis processes were less restricted consequently.

Stochastic parallel processing can shape photosynthesis–irradiance curves in phytoplankton—the Q model

A mechanistic model was formulated that describes the rate of photosynthesis based on an analogy with queuing systems of operational research. The parallel electron processing capacity of the plastoquinone pool was hypothesized to be the key element in the photosynthetic electron transport chain, determining the process of light saturation for phytoplankton. The state of the plastoquinone pool was described mathematically by a continuous-time Markov chain. The model assumes that traditional photosynthesis measurements using incubation under constant irradiance can be regarded as stochastic equilibria. The model was tested on a set of photosynthesis–irradiance measurements taken in Lake Balaton (Hungary). It clearly outperformed the two most common empirical photosynthesis–irradiance models used in limnology by delivering the best-fit in most cases. Thus, the traditional limnological practice of choosing the right empirical, two-parameter photosynthesis–irradiance model that produces the best-fit can be replaced by simple calibration of three parameters, including a new one describing the degree of parallelism in the photosynthetic units. This parameter was found to specify the curvature of the photosynthesis–irradiance function.

Chloroplast Biogenesis 87: Evidence of Resonance Excitation Energy Transfer Between Tetrapyrrole Intermediates of the Chlorophyll Biosynthetic Pathway and Chlorophyll a¶†

The thorough understanding of photosynthetic membrane assembly requires a deeper knowledge of the coordination of chlorophyll (Chl) and thylakoid apoprotein biosynthesis. As a working model for future investigations, we have proposed three Chl–thylakoid apoprotein biosynthesis models, namely, a single-branched Chl biosynthetic pathway (SBP) single-location model, an SBP multilocation model and a multibranched Chl biosynthetic pathway (MBP) sublocation model. Rejection or validation of these models can be probed by determination of resonance excitation energy transfer between various tetrapyrrole intermediates of the Chl biosynthetic pathway and various thylakoid Chl–protein complexes. In this study we describe the detection of resonance energy transfer between protoporphyrin IX (Proto), Mg-Proto and its monomethyl ester (Mp(e)) and divinyl and monovinyl protochlorophyllide a (Pchlide a) and several Chl–protein complexes. Induction of various amounts of tetrapyrrole accumulation in green photoperiodically grown cucumber cotyledons and barley leaves was achieved by dark incubation of excised tissues with delta-aminolevulinic acid (ALA) and various concentrations of 2,2′-dipyridyl for various periods of time. Controls were incubated in distilled water. After plastid isolation, treated and control plastids were diluted in buffered glycerol to the same Chl concentration. Excitation spectra were then recorded at 77 K at emission maxima of about 686, 694 and 738 nm. Resonance excitation energy transfer from Proto, Mp(e) and Pchlide a to Chl–protein complexes emitting at 686, 694 and 738 nm was observed by calculation of treated minus control difference excitation spectra. The occurrence of resonance excitation energy transfer between anabolic tetrapyrroles and Chl–protein complexes appeared as well-defined excitation bands with excitation maxima corresponding to those of Proto, Mp(e) and Pchlide a. Furthermore, it appeared that resonance excitation energy transfer from multiple short-wavelength, medium-wavelength and long-wavelength Proto, Mp(e) and Chlide a sites to various Chl–protein complexes took place. Because resonance excitation transfer from donors to acceptors cannot take place at distances larger than 100 Å, it is proposed that the observed resonance excitation energy transfers are not compatible with the SBP single-location Chl biosynthesis thylakoid membrane biogenesis model. The latter assumes that a single-branched Chl biosynthetic pathway located in the center of a 450 × 130 Å photosynthetic unit generates all of the Chl needed for the assembly of all Chl–protein complexes.

Molecular assembly of artificial photosynthetic antenna core complex on an amino-terminated ITO electrode

Bacterial photosynthetic membrane proteins, light-harvesting antenna complex (LH1), reaction center (RC), and their combined ‘core’ complex (LH1–RC) are functional elements in the primary photosynthetic events, i.e., capturing and transferring light energy and subsequent charge separation. These photosynthetic units (PSUs) isolated from Rhodospirillum rubrum ( Rs. rubrum) were assembled onto an ITO electrode modified with 3-aminopropyltriethoxysilane (APS–ITO). The near IR absorption spectra of PSUs on the assembled electrodes were identical to those of solutions, indicating that the LH1 and LH1–RC core complexes were native on the electrode. Photocurrent response of PSUs on the electrode was examined upon illumination of the LH1 complex at 880 nm. The LH1–RC and a mixed assembly of LH1 and RC exhibited photocurrent response, but not LH1 only, consistent with the function of these PSUs, capturing light energy and transferring electron. This result provides useful methodology for building an artificial fabrication of PSUs on the electrode.

On the advantages of using green light to study fluorescence yield changes in leaves

In photosynthetic chains, the kinetics of fluorescence yield depends on the photochemical rates at the level of both Photosystem I and II and thus on the absorption cross section of the photosynthetic units as well as on the coupling between light harvesting complexes and photosynthetic traps. A new set-up is described which, at variance with the commonly used set-ups, uses of a weakly absorbed light source (light-emitting diodes with maximum output at 520 nm) to excite the photosynthetic electron chain and probe the resulting fluorescence yield changes and their time course. This approach optimizes the homogeneity of the exciting light throughout the leaf and we show that this homogeneity narrows the distribution of the photochemical rates. Although the exciting light is weakly absorbed, the possibility to tune the intensity of the light emitting diodes allows one to reach photochemical rates ranging from 10 4 s − 1 to 0.25 s − 1 rendering experimentally accessible different functional regimes. The variations of the fluorescence yield induced by the photosynthetic activity are qualitatively and quantitatively discussed. When illuminating dark-adapted leaves by a weak light, the kinetics of fluorescence changes displays a pronounced plateau which precedes the fluorescence increase reflecting the full reduction of the plastoquinone pool. We ascribe this plateau to the time delay needed to reduce the photosystem I electron acceptors.

Photosynthetic characteristics of a tropical population of Nitella cernua (Characeae, Chlorophyta)

Photosynthetic characteristics (assessed by chlorophyll fluorescence and O2 evolution) were analysed monthly during one year in a tropical population of Nitella cernua from southeastern Brazil (20º50'32" S, 49º26'15" W). Parameters derived from photosynthesis-irradiance (PI) curves by fluorescence (high values of the photosynthetic parameter, Ik, and lack or low values of photoinhibition, beta) suggested adaptation to high irradiance, whereas those by O2 evolution showed a different pattern (low values of Ik and compensation irradiance, Ic, high values of photosynthetic efficiency, alpha, and photoinhibition). Parameters from PI curves by O2 evolution suggested light acclimation: Ik and maximum photosynthetic rate, Pmax (as rETR, relative electron transfer rate) increased, whereas a became significantly lower under higher irradiance (winter). This pattern is an adjustment of both number and size of photosynthetic units. Photosynthetic performance assessed by fluorescence revealed two seasonal periods: higher values of Pmax (rETR), Ik and non-photochemical quenching from October to March (rainy season), and lower values from April to October (dry season). Temperature responses were observed only in summer, but temperature optima were different between methods: peaks of net photosynthesis occurred at 20ºC, whereas rETR increased towards higher temperatures (up to 30ºC). Dark respiration increased with higher temperatures. Current velocity had a stimulatory effect on photosynthetic rates, as suggested by positive correlations with Pmax (rETR) and alpha. pH experiments revealed highest net photosynthetic rates under pH 4.0, suggesting higher affinity for CO2 than HCO3-. This broad range of responses of photosynthetic characteristics of this N. cernua population to irradiance, temperature, and pH/inorganic carbon reflects a wide tolerance to variations in these environmental variables, which probably contribute to the wide distribution of this species.

Gross photosynthesis and lake community metabolism during the spring phytoplankton bloom

Daily productivity determinations of linear photosynthetic electron transfer and of net and gross inorganic CO2 uptake were determined in situ throughout a 6-week sampling period of the spring phytoplankton bloom in Esthwaite Water in the English Lake District. Photosynthetic electron transfer rates, expressed as gross O2 evolution, were determined from fast repetition rate (FRR) fluorometry and discrete laboratory measurements of the photosynthetic unit size. These gross O2 evolution determinations were also made free from contemporaneous dark-adapted FRR measurements and the need for sample blanks. Net and gross CO2 uptake was determined from changes in total inorganic carbon calculated from in situ pH measurements. Two phases to the bloom were observed. An initial bloom was dominated by the diatom Asterionella formosa, while a secondary phase was characterized by an increase in flagellates and cyanobacteria. For both phases, daily FRR-based gross O2 evolution and pH-based gross CO2 uptake were tightly coupled, suggesting that daily gross O2 production was driving daily changes in CO2 assimilation and thus in community metabolism. This is the first investigation to show closely related rates of gross O2 production and CO2 uptake measured in situ.

Comparison of cultivation‐dependent and molecular methods for studying the diversity of anoxygenic purple phototrophs in sediments of an eutrophic brackish lagoon

Phototrophic anoxygenic purple bacteria play a key role in many aquatic ecosystems by oxidizing sulfur compounds and low-molecular-weight organic compounds using light as energy source. In this study, molecular methods based upon pufM gene (photosynthetic unit forming gene) were compared with culture-dependent methods to investigate anoxygenic purple phototrophic communities in sediments of an eutrophic brackish lagoon. Thirteen strains, belonging to eight different genera of purple phototrophic bacteria were isolated with a large dominance of the metabolically versatile purple non-sulfur bacteria (eight strains), some purple sulfur bacteria (three strains) and two strains belonging to the Roseobacter clade (aerobic phototrophs). The pufM genes amplified from the isolated strains were not detected by the molecular methods [terminal-restriction fragment length polymorphism (T-RFLP)] applied on in situ communities. An environmental clone library of the pufM gene was thus constructed from sediment samples. The results showed that most of the clones probably corresponded to aerobic phototrophic bacteria. Our results demonstrate that the culture-dependent techniques remain the best experimental approach for determining the diversity of phototrophic purple non-sulfur bacteria whereas the molecular approach clearly illustrated the abundance of organisms related to the Roseobacter clade in these eutrophic sediments.

Limitations on microalgal growth at very low photon fluence rates: the role of energy slippage

The lower limits of photosynthetically useable radiation at which growth and photosynthesis can occur establish the lower boundaries for the extent of photolithotrophy in the biosphere. Photolithotrophic growth denotes the capacity to grow with photons as the sole energy input. Slippage in terms of photosynthetic energy conversion implies a less than theoretical stoichiometry of energy-transduction process(es) such as the dissipation of intermediates of O(2) evolution and of ATP synthesis (H(+)/e(-) and H(+)/ATP ratios). Slippage is particularly important in limiting the growth of photolithotrophic organisms at very low photon fluence rates. We found that Dunaliella tertiolecta and Phaeodactylum tricornutum avoid such reductions in photon use efficiency by increasing the size and number of their photosynthetic units, respectively, and by altering Q(A) reduction kinetics on the reducing side of PS II. P. tricornutum is also less susceptible to slippage in terms of the breakdown of intermediates in its O(2) evolution pathway than D. tertiolecta. Minimizing H(+) leakage through the CF(0)-CF(1) ATP synthetase (and other H(+ )porters) is also discussed briefly. In combination, strategies employed by P. tricornutum effectively allow it to grow and photosynthesize at lower rates of energy input than D. tertiolecta, consistent with our observations. Differences in the responses of the photosynthetic apparatus of these two marine microalgae are mechanistic and probably representative of evolutionary divergences associated with strategies for dealing with environmental perturbations.

Growth, Gas Exchange, Abscisic Acid, and Calmodulin Response to Salt Stress in Three Poplars

AbstractIn the present study, we investigated the effects of increasing salinity on growth, gas exchange, abscisic acid (ABA), calmodulin (CaM), and the relevance to salt tolerance in seedlings of Populus euphratica Oliv. and cuttings of P. “pupularis 35–44” (P. popularis) and P. x euramericana cv. I‐214 (P. cv. Italica). The relative growth rates of shoot height (RGRH) for P. cv. Italica and P. popularis were severely reduced by increasing salt stress, whereas the growth reduction was relatively less in P. euphratica. Similarly, P. euphratica maintained higher net photosynthetic rates (Pn) and unit transpiration rates (TRN) than P. cv. Italica and P. popularis under conditions of higher salinity. Salinity caused a significant increase in leaf ABA and CaM in the three genotypes after the onset of stress, but NaCl‐induced ABA and CaM accumulation was more pronounced in P. euphratica, suggesting that P. euphratica plants are more sensitive in sensing soil salinity than the other two poplars. Furthermore, P. euphratica maintained relatively higher ABA and CaM concentrations under conditions of high salinity. The higher capacity to synthesize stress signals, namely ABA and CaM, in P. euphratica and the contribution of this to the salt resistance of P. euphratica are discussed.(Managing editor: Ping He)

Development of a robust marine ecosystem model to predict the role of iron in biogeochemical cycles: A comparison of results for iron-replete and iron-limited areas, and the SOIREE iron-enrichment experiment

A new mixed layer multi-nutrient ecosystem model, incorporating diatoms, non-diatoms and zooplankton, is described that models the role of iron in marine biogeochemical cycles. The internal cell biochemistry of the phytoplankton is modelled using the mechanistic model of Flynn [2001. A mechanistic model for describing dynamic multi-nutrient, light, temperature interactions in phytoplankton. Journal of Plankton Research 23, 977–997] in which the internal cell concentrations of chlorophyll, nitrogen, silica, and iron are all dynamic variables that respond to external nutrient concentrations and light levels. Iron stress in phytoplankton feeds back into chlorophyll synthesis and changes in photosynthetic unit (PSU) size, thereby reducing their growth rate. Because diatom silicon metabolism is inextricably linked with cell division, diatom population density (cell m −3) is modelled as well as C biomass. An optimisation technique was used to fit the model to three time-series datasets at Biotrans (47°N, 20°W) and Kerfix (50°40′S, 68°25′E) and the observations for the Southern Ocean Iron-Release Experiment (SOIREE) iron-enrichment experiment (61°S, 140°E). The model gives realistic simulations of the annual cycles of nutrients, phytoplankton, and primary production at Biotrans and Kerfix and can also accurately simulate an iron fertilisation experiment. Specifically, the model predicts the high values of diatom Si:N and Si:C ratios observed in areas where iron is a limiting factor on algal growth. In addition, the model results at Kerfix confirm previous suggestions that underwater light levels have a more limiting effect on phytoplankton growth than iron supply. The model is also used to calculate C budgets and C and Si export from the mixed layer. The implications of these results for developing biogeochemical models incorporating the role of iron are discussed.

Characterization of a novel Photosystem I–LHCI supercomplex isolated from Chlamydomonas reinhardtii under anaerobic (State II) conditions

A novel supercomplex of Photosystem I (PSI) with light harvesting complex I (LHCI) was isolated from the green alga Chlamydomonas reinhardtii. This novel supercomplex is unique as it is the first stable supercomplex of PSI together with its external antenna. The supercomplex contains 256 chlorophylls per reaction center. The supercomplex was isolated under anaerobic conditions and may represent the State II form of the photosynthetic unit. In contrast to previously reported supercomplexes isolated in State I, which contain only 4 LHC I proteins, this supercomplex contains 10–11 LHC I proteins tightly bound to the PSI core. In contrast to plants, no LHC II is tightly bound to the PSI–LHCI supercomplex in State II. Investigation of the energy transfer from the antenna system to the reaction center core shows that the LHC supercomplexes are tightly coupled to the PSI core, not only structurally but also energetically. The excitation energy transfer kinetics are completely dominated by the fast phase, with a near-complete lack of long-lived fluorescence. This tight coupling is in contrast to all reports of energy transfer in PSI–LHCI supercomplexes (in State I), which have so far been described as weakly coupled supercomplexes with low efficiency for excitation energy transfer. These results indicate that there are large and dynamic changes of the PSI–LHCI supercomplex during the acclimation from aerobic (State I) to anaerobic (State II) conditions in Chlamydomonas.

On the Role of Basic Residues in Adapting the Reaction Centre–LH1 Complex for Growth at Elevated Temperatures in Purple Bacteria

The purple photosynthetic bacterium Thermochromatium tepidum is a moderate thermophile, with a growth optimum of 48-50 degrees C. The X-ray crystal structure of the reaction centre from this organism has been determined, and compared with that from mesophilic bacteria such as Blastochloris viridis and Rhodobacter sphaeroides (Nogi T et al. (2000) Proc Natl Acad Sci USA 97: 13561-13566). Structural features that could contribute to the enhanced thermal stability of the Thermochromatium tepidum reaction centre were discussed, including three arginine residues exposed at the periplasmic side of the membrane that are not present in reaction centres from mesophilic organisms, and potentially could increase the affinity of the complex for the surrounding membrane. In the present report these arginine residues, plus a histidine identified from an extensive sequence alignment, were engineered into structurally homologous positions in the Rhodobacter sphaeroides reaction centre, and the effect on the thermal stability of the Rhodobacter sphaeroides complex was examined. We find that these residues do not enhance the thermal stability of the reaction centre, as assessed by absorbance spectroscopy of the bacteriochlorin cofactors in membrane-bound reaction centres. Possible roles of these residues in the Thermochromatium tepidum reaction centre are discussed, and it is proposed that they facilitate stronger binding of the reaction centre to the encircling LH1 antenna complex, through ionic interactions with acidic residues at the C-terminal end of the LH1 alpha-polypeptide. Such an interaction could enhance the stability of the so-called 'RC-LH1 core' complex that is formed between the reaction centre and the LH1 antenna, and which represents the minimal functional photosynthetic unit in all known purple photosynthetic bacteria. Stronger bonding interactions between the two complexes could also contribute to an increase in the rigidity of the photosynthetic membrane in Thermochromatium tepidum, in accord with the general finding that the cytoplasmic membrane from thermophilic eubacteria is less fluid than its counterpart in mesophilic bacteria.

The First Picoseconds in Bacterial Photosynthesis—Ultrafast Electron Transfer for the Efficient Conversion of Light Energy

In this Minireview, we describe the function of the bacterial reaction centre (RC) as the central photosynthetic energy-conversion unit by ultrafast spectroscopy combined with structural analysis, site-directed mutagenesis, pigment exchange and theoretical modelling. We show that primary energy conversion is a stepwise process in which an electron is transferred via neighbouring chromophores of the RC. A well-defined chromophore arrangement in a rigid protein matrix, combined with optimised energetics of the different electron carriers, allows a highly efficient charge-separation process. The individual molecular reactions at room temperature are well described by conventional electron-transfer theory.

Gas exchange analysis and chlorophyll a fluorescence in cotyledons of the xan1 sunflower mutant with defects in light energy utilization

Mutants with a depigmentated phenotype and characterized by Mendelian inheritance can be useful to analyze the role of nuclear genes in chloroplast biogenesis and photosynthesis. In the important oil crop sunflower, we have recently demonstrated that under dim light conditions the xantha1 ( xan1) mutant develops true leaves unable to express and retain PSII activity. In order to investigate if this mutation induces an organ specific effect on photosynthetic activity, the light energy utilization of cotyledons (7, 15 and 18 days after sowing) was characterized through gas exchange and chlorophyll a fluorescence. The xan1 cotyledons are unable to fix CO 2 at normal rates. However, rubisco specific activity is not different in the wild-type (wt) and the xan1 seedlings, indicating that biochemical reactions of the photosynthetic process were not altered. The photochemical potential of PSII, measured as the dark-adapted quantum efficiency of PSII ( F v/ F m) was strongly decreased in xan1 cotyledons and is attributable to a significant increase in F 0 values. The light-adapted quantum efficiency ( Φ PSII) and the efficiency of excitation capture of PSII ( Φ exc.), were significantly depressed in xan1 cotyledons as compared to the wt. Under light conditions, the most evident result is the strong decrease of Φ PSII, which is linked to a decrease in excitation energy pressure and also to a strong increase in q NP. In summary, with analogy to results previously obtained in leaves, the xan1 cotyledons have a reduced number of photosynthetic units and high sensibility of the PSII reaction centres to photodamage even in low light conditions. We conclude that the loss-of-function of the sunflower XAN1 nuclear gene affects the light energy utilization of chloroplasts in an organ-independent way.

Sequential assembly of photosynthetic units in Rhodobacter sphaeroides as revealed by fast repetition rate analysis of variable bacteriochlorophyll a fluorescence

The development of functional photosynthetic units in Rhodobacter sphaeroides was followed by near infra-red fast repetition rate (IRFRR) fluorescence measurements that were correlated to absorption spectroscopy, electron microscopy and pigment analyses. To induce the formation of intracytoplasmic membranes (ICM) (greening), cells grown aerobically both in batch culture and in a carbon-limited chemostat were transferred to semiaerobic conditions. In both aerobic cultures, a low level of photosynthetic complexes was observed, which were composed of the reaction center and the LH1 core antenna. Interestingly, in the batch cultures the reaction centers were essentially inactive in forward electron transfer and exhibited low photochemical yields F V/ F M, whereas the chemostat culture displayed functional reaction centers with a rather rapid (1–2 ms) electron transfer turnover, as well as a high F V/ F M of ∼0.8. In both cases, the transfer to semiaerobiosis resulted in rapid induction of bacteriochlorophyll a synthesis that was reflected by both an increase in the number of LH1–reaction center and peripheral LH2 antenna complexes. These studies establish that photosynthetic units are assembled in a sequential manner, where the appearance of the LH1–reaction center cores is followed by the activation of functional electron transfer, and finally by the accumulation of the LH2 complexes.

Variable LH2 stoichiometry and core clustering in native membranes of Rhodospirillum photometricum.

The individual components of the photosynthetic unit (PSU), the light-harvesting complexes (LH2 and LH1) and the reaction center (RC), are structurally and functionally known in great detail. An important current challenge is the study of their assembly within native membranes. Here, we present AFM topographs at 12 A resolution of native membranes containing all constituents of the PSU from Rhodospirillum photometricum. Besides the major technical advance represented by the acquisition of such highly resolved data of a complex membrane, the images give new insights into the organization of this energy generating apparatus in Rsp. photometricum: (i) there is a variable stoichiometry of LH2, (ii) the RC is completely encircled by a closed LH1 assembly, (iii) the LH1 assembly around the RC forms an ellipse, (iv) the PSU proteins cluster together segregating out of protein free lipid bilayers, (v) core complexes cluster although enough LH2 are present to prevent core-core contacts, and (vi) there is no cytochrome bc1 complex visible in close proximity to the RCs. The functional significance of all these findings is discussed.

Protein folding in the periplasm in the absence of primary oxidant DsbA: modulation of redox potential in periplasmic space via OmpL porin

Corrigendum29 September 2004free access Protein folding in the periplasm in the absence of primary oxidant DsbA: modulation of redox potential in periplasmic space via OmpL porin Claire Dartigalongue Claire Dartigalongue Search for more papers by this author Hiroshi Nikaido Hiroshi Nikaido Search for more papers by this author Satish Raina Satish Raina Search for more papers by this author Claire Dartigalongue Claire Dartigalongue Search for more papers by this author Hiroshi Nikaido Hiroshi Nikaido Search for more papers by this author Satish Raina Satish Raina Search for more papers by this author Author Information Claire Dartigalongue, Hiroshi Nikaido and Satish Raina The EMBO Journal (2004)23:3907-3907https://doi.org/10.1038/sj.emboj.7600439 This article corrects the following: Protein folding in the periplasm in the absence of primary oxidant DsbA: modulation of redox potential in periplasmic space via OmpL porin15 November 2000 PDFDownload PDF of article text and main figures. ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InMendeleyWechatReddit Figures & Info Correction to: The EMBO Journal (2000) 19, 5980–5988. doi:10.1093/emboj/19.22.5980 During reconstruction of ompL/dsbA double mutants, we observed that null mutation in ompL suppressed the defects of dsbA mutant bacteria in oxidative folding of secreted proteins only partially (about 20–30%). During our initial series of experiments, we observed a complete suppression of dsbA phenotypes upon introduction of the ompL null allele. The discrepancy was repeatedly observed for Escherichia coli strain SR4444Δ (dsbA ompL). We conclude that this strain likely carries a second mutation, which contributes to the complete restoration of oxidative protein folding. Our conclusions for a role of OmpL in modulating the oxidative environment of E. coli periplasmic space gain further support as we have again cloned the ompL gene, which, when overexpressed from a plasmid, confers resistance to sublethal concentrations of oxidizing agents such as diamide. We would like to bring to the attention of readers that it was MC1000 that was used in our studies and is the parent of SR2262 and SR4444, and that in page 5987 temperature used for motility assay was 30°C and the agar concentration was 0.3%. Next ArticlePrevious Article Read MoreAbout the coverClose modalView large imageVolume 23,Issue 19,September 29, 2004Top view of a model membrane containing all the proteins of the photosynthetic unit in Rhodospirillum photometricum: LH2 light harvesting complexes and core complexes, consisting of LH1 light harvesting complexes surrounding reaction centers. The structures of the individual complexes were assembled following the arrangement observed in the native membrane by atomic force microscopy. The image was composed by Simon Scheuring from the Institut Curie, Paris, France, whose accompanying article will appear in one of the next printed issues of The EMBO Journal. Volume 23Issue 1929 September 2004In this issue RelatedDetailsLoading ...

PHYSIOLOGICAL AND PHOTOSYNTHETIC RESPONSES OF SYNECHOCYSTIS AQUATILIS F. AQUATILIS (CYANOPHYCEAE) TO ELEVATED LEVELS OF ZINC1

Physiological and structural changes in cells of Synechocystis aquatilis f. aquatilis acclimated to grow in the presence of high zinc levels (2.20–3.30 mg·L−1) were investigated. Growth of these cells showed a decreased specific growth rate and final yield of about 60% and 50%, respectively, of the values found for cells grown in the presence of 0.21 mg zinc·L−1 (control culture). The higher the zinc concentration in the culture medium, the more pronounced the reduction in the chl a content. Regardless of zinc concentration, S. aquatilis possessed three distinct carotenoids. A decrease in carotenoid content accompanied the decrease of chl a, and the proportions of the pigments to each other were not affected by zinc. The photosynthetic performance of cells cultured in the presence of high zinc levels showed a decline in both the apparent photosynthetic efficiency and the photosynthetic maximal rate. In these cells the PSII reaction centers became partially closed, and the electron transport activity around PSII and PSI was reduced to 61% and 38% of the control values, respectively, which may indicate an altered PSII/PSI stoichiometry. In addition, electron micrographs revealed a reduced amount of thylakoid membranes, indicating that acclimation to high zinc levels led to a decrease in the overall number of photosynthetic units. On the other hand, light microscopic observation of negative‐stained cells revealed the presence of a thick mucilaginous layer surrounding the high zinc‐acclimated cells. This extracellular material could retain high amounts of metal ions from the medium, thus providing the Synechocystis cells a mechanism to circumvent toxic levels of zinc.

Survival Strategy of Photosynthetic Organisms. 1. Variability of the Extent of Light-Harvesting Pigment Aggregation as a Structural Factor Optimizing the Function of Oligomeric Photosynthetic Antenna. Model Calculations

In accordance with our concept of rigorous optimization of photosynthetic machinery by a functional criterion, this series of papers continues purposeful search in natural photosynthetic units (PSU) for the basic principles of their organization that we predicted theoretically for optimal model light-harvesting systems. This approach allowed us to determine the basic principles for the organization of a PSU of any fixed size. This series of papers deals with the problem of structural optimization of light-harvesting antenna of variable size controlled in vivo by the light intensity during the growth of organisms, which accentuates the problem of antenna structure optimization because optimization requirements become more stringent as the PSU increases in size. In this work, using mathematical modeling for the functioning of natural PSUs, we have shown that the aggregation of pigments of model light-harvesting antenna, being one of universal optimizing factors, furthermore allows controlling the antenna efficiency if the extent of pigment aggregation is a variable parameter. In this case, the efficiency of antenna increases with the size of the elementary antenna aggregate, thus ensuring the high efficiency of the PSU irrespective of its size; i.e., variation in the extent of pigment aggregation controlled by the size of light-harvesting antenna is biologically expedient.