Restricted diffusion in photosynthetic membranes

The structural organization of membrane proteins and their linkage by diffusion are topics of much debate. Functional studies in photosynthetic membranes, where rapid equilibration of electron transport between redox centers appears restricted to isolated domains, shed new light on the subject.     

Spherical subunits in photosynthetic membranes of two cyanophyceae and the bacterium rhodospirillum rubrum

Summary The photosynthetic lamellae of Oscillatoria amoena and Pseudanabaena catenata appear as a two-dimensional packing of spherical units following permanganate fixation, ultrathin sectioning and high-resolution electron micrography. The usual three-layered appearance is explained as an optical artifact. Quite similar patterns were observed in chromatophore membranes of Rhodospirillum rubrum. Also, the cytoplasmic membrane of R. rubrum shows an identical pattern.Indication is presented that thylakoids of Cyanophyceae can arise as invaginations of the cytoplasmic membrane.The localization of chloroplast lipids in these organisms and the similarity of spherical units and quantasomes is discussed.     

A new isolate of the rhodospirillum fulvum group and its photosynthetic pigments

Summary The present paper reports the isolation of an obligate phototrophic bacterium which belongs to the Rhodospirillum fulvum-group on the basis of its colour, morphology, nutritional requirements and strictly anaerobic nature. p-Aminobenzoic acid was shown to be the only growth factor required. Rhsp. fulvum synthesizes bacteriochlorophyll a as its only chlorophyllous pigments. The carotenoid composition comprises lycopene (I) and rhodopin (II) as the major components; traces of 1,2,1,2-tetrahydro-1,1-dihydroxy-lycopene (III) were also present.

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Zusammenfassung Es wird die Isolierung eines obligat phototrophen Bakteriums beschrieben, welches auf Grund seiner Farbe, Morphologie, Nährstoff-Bedürfnisse und streng anaeroben Lebensweise in die Rhodospirillum fulvum-Gruppe eingeordnet wird. Als einzigen Wachstumsfaktor benötigt der Stamm p-Aminobenzoesäure. Rhsp. fulvum bildet Bacteriochlorophyll a als einzigen chlorophyllartigen Farbstoff. Die Hauptmenge der Carotinoide besteht aus Lycopin (I) und Rhodopin (II); es sind außerdem Spuren von 1,2,12-Tetrahydro-1,1-dihydroxy-Lycopin (III) nachgewiesen.

     

Shotgun genome sequence of the large purple photosynthetic bacterium Rhodospirillum photometricum DSM122

Here, we present the shotgun genome sequence of the purple photosynthetic bacterium Rhodospirillum photometricum DSM122. The photosynthetic apparatus of this bacterium has been particularly well studied by microscopy. The knowledge of the genome of this oversize bacterium will allow us to compare it with the other purple bacterial organisms to follow the evolution of the photosynthetic apparatus.     

Supplying CO2 to photosynthetic algal cultures by diffusion through gas-permeable membranes

Summary A method of supplying CO₂ to photosynthetic algal cultures by diffusion through a gas-permeable membrane was developed. The diffusion of CO₂ across a silicone membrane could be described by Fick's Laws of Diffusion, with a permeability constant of 1.92x10^-7 m²/min. By the manipulation of tubing dimensions and the partial pressure or static pressure of CO₂ gas within the tubing, the rate of CO₂ supply could be controlled. Pure CO₂ was applied to the cultivation of Chlorella pyrenoidosa with 100% utilization and without the side effect of CO₂ inhibition.     

Distribution of phosphatidylethanolamine in the lipid bilayer of chromatophores of the photosynthetic bacterium rhodospirillum rubrum

The distribution of phosphatidylethanolamine in the two lipid layers of chromatophores ofRhodospirillum rubrum has been analysed by chemical modification of phosphatidylethanolamine (PE) with trinitrobenzenesulfonic acid (TNBA) at low temperatures. Around 45±1% of the total phosphatidylethanolamine is labelled by this procedure independent on chromatophore purity, vesicle size, action of proteases and growth state of the cells. This demonstrates a complete modification of the accessible phosphatidylethanolamine and an asymmetric distribution of phosphatidylethanolamine, with 45% of the phosphatidylethanolamine in the outer part of the bilayer.Abbreviations TNBA 2,4,6 trinitrobenzenesulfonic acid - PE phosphatidylethanolamine - PMS phenazinmethosulfate     

Isolation of detergent-resistant membranes from plant photosynthetic and non-photosynthetic tissues

Microdomains, or lipid rafts, are transient membrane regions enriched in sphingolipids and sterols that have only recently, but intensively, been studied in plants. In this work, we report a detailed, easy-to-follow, and fast procedure to isolate detergent-resistant membranes (DRMs) from purified plasma membranes (PMs) that was used to obtain DRMs from Phaseolus vulgaris and Nicotiana tabacum leaves and germinating Zea mays embryos. Characterized according to yield, ultrastructure, and sterol composition, these DRM preparations showed similarities to analogous preparations from other eukaryotic cells. Isolation of DRMs from germinating maize embryos reveals the presence of microdomains at very early developmental stages of plants.     

Energy transfer and fluorescence mechanisms in photosynthetic membranes

Energy transfer in photosynthetic membranes involves the migration of excitons from light‐harvesting antenna chlorophyll‐protein complexes to the reaction center complexes. Recent efforts have focused on determining the time of arrival of excitons (trapping times) at the reaction centers following excitation with a single picosecond laser pulse. Three different approaches have been utilized: (1) determination of appearance of separated charges within the reaction centers by differential absorbtion spectroscopy, (2) determination of appearance of separated charges by fast photoemf measurements, and (3) kinetics of decay of fluorescence. The first two methods provide more direct information on exciton trapping by reaction centers than fluorescence methods, but are experimentally difficult to realize. Therefore, much activity has centered around the accurate measurement and analysis of fluorescence‐decay profiles by single‐photon counting methods. In green plants, about three different components with lifetimes of about 100 psec, 200 to 500 psec, and >1 nsec, have been reported. The first two components are believed to be related to trapping rates by reaction centers, while the third component is attributed to a charge recombination (Klimov) mechanism. Results from photoemf and exciton‐exciton annihilation experiments are consistent with the interpretation that the first decay component reflects exciton‐trapping rates. A critical analysis and discussion of these fast energy‐transfer phenomena in photosynthetic membranes of green plants are offered in this review.     

FRAP analysis of photosynthetic membranes

Fluorescence Recovery after Photobleaching (FRAP) is a technique widely used in cell biology to observe the dynamics of biological systems, including the diffusion of membrane components. More information is needed on the dynamics of photosynthetic membranes in order to help to understand processes such as photosynthetic electron transport, regulation of light-harvesting, and biogenesis and turnover of the photosynthetic apparatus. FRAP has the potential to provide this information, although applying the technique to photosynthetic membranes is not always straightforward. This review explains the potential and the problems, and gives a brief guide to performing FRAP measurements and analysing the data.     

Plastoquinol diffusion in linear photosynthetic electron transport

The diffusion of plastoquinol and its binding to the cytochrome bf complex, which occurs during linear photosynthetic electron transport and is analogous to reaction sequences found in most energy-converting membranes, has been studied in intact thylakoid membranes. The flash-induced electron transfer between the laterally separated photosystems II and photosystems I was measured by following the sigmoidal reduction kinetics of P-700⁺ after previous oxidation of the intersystem electron carriers. The amount of flash-induced plastoquinol produced at photosystem II was (a) reduced by inhibition with dichlorophenyl-dimethylurea and (b) increased by giving a second saturating flash. These signals were simulated by a new model which combines a deterministic simulation of reaction kinetics with a Monte Carlo approach to the diffusion of plastoquinol, taking into account the known structural features of the thylakoid membrane. The plastoquinol molecules were assumed to be oxidized by either a diffusion-limited or a nondiffusion-limited step in a collisional mechanism or after binding to the cytochrome bf complex. The model was able to account for the experimental observations with a nondiffusion-limited collisional mechanism or with a binding mechanism, giving minimum values for the diffusion coefficient of plastoquinol of 2 × 10^-8 cm²s^-1 and 3 × 10^-7 cm²s^-1, respectively.     

Lateral diffusion in nuclear membranes

Chemical modification of rat liver nuclei with citraconic anhydride selectively removed outer nuclear membrane. This conclusion was based on (a) transmission electron microscopy, (b) lipid analysis, (c) lamin B as an inner membrane-associated marker, and (d) the demonstration of phospholipid lateral mobility on outer membrane-depleted nuclei as a criteria for inner membrane integrity. Addition of urea or N-ethylmaleimide resulted in the additional disruption of inner membrane. Fluorescence photobleaching was used to determine the long range (greater than 4 microns) lateral transport of lectin receptors and a phospholipid analog in both membranes. The diffusion coefficient for wheat germ agglutinin on whole nuclei was 3.9 X 10(-10) cm2/s whereas the diffusion coefficient for wheat germ agglutinin in outer membrane-depleted nuclei was less than or equal to 10(-12) cm2/s. Phospholipid mobilities were the same in whole and outer membrane-depleted nuclei (3.8 X 10(-9) cm2/s). The protein diffusion differences observed between whole and outer membrane-depleted nuclei may be interpreted in the context of two functionally different membrane systems that compose the double bilayer of the nucleus.     

Density and diffusion limited aggregation in membranes

Aggregation of membrane molecules is a crucial phenomenon in developing organisms, a classic example being the aggregation of post-synaptic receptors during synaptogenesis. Our understanding of the molecular events involved is improving, but most models of the aggregation or concentration process do not address binding events on the molecular level. An exception is the study of diffusion limited aggregation, in which the aggregation process is simulated on a molecular level. In this analysis, however, important physical parameters such as molecular size, diffusion constant and initial density are not addressed. Thus no predictions about the rate at which such aggregates will form is possible. In the present work the model of diffusion limited aggregation is extended to incorporate these parameters and make the corresponding predictions.     

Light Harvesting by Lamellar Chromatophores in Rhodospirillum photometricum

Purple photosynthetic bacteria harvest light using pigment-protein complexes which are often arranged in pseudo-organelles called chromatophores. A model of a chromatophore from Rhodospirillum photometricum was constructed based on atomic force microscopy data. Molecular-dynamics simulations and quantum-dynamics calculations were performed to characterize the intercomplex excitation transfer network and explore the interplay between close-packing and light-harvesting efficiency.