Intersystem Electron Transport Chain Research Articles

Inorganic Carbon Accumulation Stimulates Linear Electron Flow to Artificial Electron Acceptors of Photosystem I in Air-Grown Cells of the Cyanobacterium Synechococcus UTEX 625.

The effect of inorganic carbon (Ci) transport and accumulation on photosynthetic electron transport was studied in air-grown cells of the cyanobacterium Synechococcus UTEX 625. When the cells were depleted of Ci, linear photosynthetic electron flow was almost completely inhibited in the presence of the photosystem I (PSI) acceptor N,N-dimethyl-p-nitrosoaniline (PNDA). The addition of Ci to these cells, in which CO2 fixation was inhibited with glycolaldehyde, greatly stimulated linear electron flow and resulted in increased levels of photochemical quenching and O2 evolution. In aerobic conditions substantial quenching resulted from methyl viologen (MV) addition and further quenching was not observed upon the addition of Ci. In anaerobic conditions MV addition did not result in quenching until Ci was added. Intracellular Ci pools were formed when MV was present in aerobic or anaerobic conditions or PNDA was present in aerobic conditions. There was no inhibitory effect of Ci depletion on electron flow to 2,6-dimethylbenzoquinone and oxidized diaminodurene, which accept electrons from photosystem II. The degree of stimulation of PNDA-dependent O2 evolution varied with the Ci concentration. The extracellular Ci, concentration required for a half-maximum rate (K1/2) was 3.8 [mu]M and the intracellular K1/2 was 1.4 mM for the stimulation of PNDA reduction. These values agreed closely with the K1/2 values of extracellular and intracellular Ci for O2 photoreduction. Linear electron flow to artificial electron acceptors of PSI was enhanced by intracellular Ci, which appeared to exert an effect on PSI or on the intersystem electron transport chain.

The Inhibition of Photosynthesis after Exposure of Bean Leaves to Various Low Levels of CO2

Mature first leaves of Phaseolus vulgaris L. were exposed to low partial pressures of CO2 (7, 6 and 0 Pa COJ for 24 h. After exposure of leaves to 6 Pa CO2 for 24 h, there was a reduction in the carbon exchange rate (CER) at all partial pressures of CO2 at which measurements were made. After exposure to 7 Pa CO2, the CER decreased only at high partial pressures of CO2. The rates of electron transport from water to methyl viologen, through the whole chain, decreased in parallel with the decrease in CER measured at 90 Pa CO2. One site of inhibition in leaves exposed for 24 h to 6 Pa CO2 appeared to be the intersystem electron-transport chain since there were no significant changes in the activities of PSI and PSII, as determined from the level of P-700 and measurement of fluorescence, respectively. Another inhibitory phenomenon appeared to be a negative change in the activation state of Rubisco, while the level of Rubisco was unaffected by the exposure to 6 Pa CO2. These decreases in photosynthetic activity caused by depletion of CO2 explains at least in part, the inhibition of photosynthesis that is caused by rain treatment [Ishibashi and Terashima (1995) Plant Cell Environ. 18: 431].

Isolation of high-chlorophyll-fluorescence mutants of Arabidopsis thaliana and their characterisation by spectroscopy, immunoblotting and northern hybridisation.

Thirty-four recessive photosynthetic mutants of the high-chlorophyll-fluorescence (hcf) phenotype have been isolated by screening 7700 M2 progenies of ethyl methane sulfonate-treated seeds of Arabidopsis thaliana. Most of the mutants isolated were found to be seedling-lethal, but could be grown on sucrose-supplemented media. Chlorophyll (Chl) fluorescence induction, absorption changes in the reaction-centre chlorophyll of PS I (P700) at 830 nm and Chl a/Chl b ratios were recorded in order to probe the photosynthetic functions and to define the mutational lesion. These studies were complemented by immunoblot and Northern analyses which finally led to the classification of the mutants into six different groups. Four classes of mutants were affected in PS I, PS II (two different classes) or the intersystem electron-transport chain, respectively. A fifth mutant class was of pleiotropic nature and the sixth class comprised a Chl b-deficient mutant. Several of the mutants showed severe deficiencies in the levels of subunits of PS I, PS II or the cytochrome b6/f complex. Thus the mutational lesions could be located precisely. Only one mutant was defective in the transcript patterns of some plastid-encoded photosynthesis genes. Hence most of the mutants isolated appear to be affected in translational and post-translational regulatory processes of thylakoid membrane biogenesis or in structural genes encoding constituent subunits of the thylakoid protein complexes.

Regulation of excitation energy transfer in organisms containing phycobilins

The mechanism of excitation energy redistribution (state transition) in organisms containing phycobilins is reviewed. Recent measurements using time-resolved fluorescence spectroscopy in the picosecond range confirm that the state transition in cyanobacteria and red algae is controlled by changes in the kinetics of energy transfer from PS 2 to PS 1 (spillover) rather than by physical dislocation of the phycobilisome and reassociation between the two photosystems (mobile antenna model). Contrary to the analogous situation in higher plants, there is no compelling evidence for the involvement of a protein phosphorylation event in the rapid time range of the state transition, but a variety of data indicate that a membrane conformational change occurs that might change the relative distance between, and/or orientation of the two photosystems within the thylakoid. The state transition is most probably initiated by the redox state of the intersystem electron transport chain, and the conversion to state 1 is driven by coupled PS1 cyclic electron transport. The cryptomonads also undergo wavelength dependent changes in excitation energy distribution by a mechanism very similar to that observed in the red algae and cyanobacteria. However, the changes in energy distribution in this group are most likely related to a photoprotection mechanism for PS2 rather than to a state transition.

Measuring P700 Absorbance Changes around 830 nm with a New Type of Pulse Modulation System

Abstract A new measuring system for monitoring absorbance changes around 830 n m is described, which was developed by modification of a commercially available pulse modulation fluorometer. All modifications concern the emitter-detector unit of the fluorometer, such that only this unit needs to be exchanged when changing from fluorescence to absorbance measurements and vice versa. The new system is shown to be well-suited for measuring redox changes of P700, the reaction center of photosystem I, in intact leaves and isolated chloroplasts. The observed kinetic changes at 830 nm in response to single turnover or multiple turnover saturating flashes are practically identical to those previously measured around 700 nm . The signal/noise ratio is sufficiently high to give well-resolved kinetics without signal averaging. W h e n P700 is oxidized by far-red background light, valuable information on the state of the intersystem electron transport chain is given by the re-reduction kinetics induced by single or multiple turnover saturating flashes. Such measurements are facilitated by the use of poly-furcated fiberoptics. With intact leaves, almost identical responses are found when measuring through the leaf (transmission mode) or from the leaf surface (remission mode). Modulated chlorophyll fluorescence can be measured in parallel; application of saturation pulses for fluorescence quenching analysis produces transient P700 oxidation without oversaturating the measuring system. The information on the P700 redox state complements that obtained from fluorescence measurements, yielding a new practical tool in plant physiological research.

Cytochrome b6f complex is required for phosphorylation of light‐harvesting chlorophyll a/b complex II in chloroplast photosynthetic membranes

The light-harvesting chlorophyll a/b complex (LHC II) and four photosystem II (PS II) core proteins (8.3, 32, 34 and 44 kDa) become phosphorylated in response to reduction of the intersystem electron transport chain of green plant chloroplasts. Previous studies indicated that reduction of the plastoquinone (PQ) pool is the key event in kinase activation. However, we show here that, unlike PS II proteins, LHC II is phosphorylated only when the cytochrome b6f complex is active. Two lines of evidence support this conclusion. (1) 2,5-Dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) and the 2,4-dinitrophenyl ether of iodonitrothymol (DNP-INT), which are known to block electron flow into the cytochrome complex, selectively inhibit LHC II phosphorylation in spinach thylakoids. (2) The hcf6 mutant of maize, which contains PQ but lacks the cytochrome b6f complex, phosphorylates the four PS II proteins but fails to phosphorylate LHC II in vivo or in vitro.

The interaction of nitrite with photosynthetic electron transport under anaerobic conditions

Chlorella cells were examined in a modulated oxygen polarograph under aerobic and anaerobic conditions. At light intensities below about 600 ergs · cm −2 · s −1 of 650 nm light, the oxygen yield and phase lag are lower under anaerobic conditions. Addition of 25 mM sodium nitrite increases both these parameters to values close to those found in the presence of oxygen. It is proposed that nitrite is reduced by Photosystem I thus diverting electrons from the cyclic electron transport pathway. The intersystem electron transport chain becomes more oxidized and this suppresses a backflow of electrons to the oxidizing side of Photosystem II, hence increasing the oxygen yield and the phase lag. The removal of oxygen from the bathing medium also alters the response of dark adapted Chlorella to a series of saturating light flashes. In terms of the Kok model of Photosystem II (Kok, B., Forbush, B. and McGloin, M. (1970) Photochem. Photobiol. 11, 457–475) there is a large increase in the parameter α. Addition of nitrite reverses this change and virtually restores the response seen in the presence of oxygen. The deactivation of the S 2 state is greatly speeded up in the absence of oxygen but the addition of nitrite again reverses this.