Kinetic behavior of cytochrome f in cyclic and noncyclic electron transport in Porphyridium cruentum.
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Carbon Dioxide↗
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Herbicide-induced changes in charge recombination and redox potential of Q(A) in the T4 mutant of Blastochloris viridis.
To gain new insights into the function of photosystem II (PSII) herbicides DCMU (a urea herbicide) and bromoxynil (a phenolic herbicide), we have studied their effects in a better understood system, the bacterial photosynthetic reaction center of the terbutryn-resistant mutant T4 of Blastochloris (Bl.) viridis. This mutant is uniquely sensitive to these herbicides. We have used redox potentiometry and time-resolved absorption spectroscopy in the nanosecond and microsecond time scale. At room temperature the P(+)(*)Q(A)(-)(*) charge recombination in the presence of bromoxynil was faster than in the presence of DCMU. Two phases of P(+)(*)Q(A)(-)(*) recombination were observed. In accordance with the literature, the two phases were attributed to two different populations of reaction centers. Although the herbicides did induce small differences in the activation barriers of the charge recombination reactions, these did not explain the large herbicide-induced differences in the kinetics at ambient temperature. Instead, these were attributed to a change in the relative amplitude of the phases, with the fast:slow ratio being approximately 3:1 with bromoxynil and approximately 1:2 with DCMU at 300 K. Redox titrations of Q(A) were performed with and without herbicides at pH 6.5. The E(m) was shifted by approximately -75 mV by bromoxynil and by approximately +55 mV by DCMU. As the titrations were done over a time range that is assumed to be much longer than that for the transition between the two different populations, the potentials measured are considered to be a weighted average of two potentials for Q(A). The influence of the herbicides can thus be considered to be on the equilibrium of the two reaction center forms. This may also be the case in photosystem II.
Bacterial Proteins↗
State transitions or delta pH-dependent quenching of photosystem II fluorescence in red algae.
Fluorescence changes attributed to state transitions have been shown to exist in phycobilisome-containing organisms. Contradictory conclusions have been derived from studies about the mechanism of state transitions carried out either in cyanobacteria or in red algae. In this paper, fluorescence changes induced by light 1 and light 2 are reinvestigated in a unicellular red alga, Rhodella violacea, by performing 77 K fluorescence spectra and fluorescence yield measurements at room temperature in the presence of uncouplers and inhibitors of the electron transfer. We show that transfer of light 1-adapted cells to light 2 (green light) induces a large quenching of photosystem II which is suppressed by subsequent incubation in light 1 (far-red or blue light). The level of the photosystem I-related fluorescence does not change during these transfers. We demonstrate that the large quenching of photosystem II induced by low intensities of green light is completely suppressed by addition of NH4Cl, an uncoupler that inhibits ATP synthesis by canceling the delta pH across the membrane. DCCD, which is an inhibitor of the ATPase that swells the delta pH, maintains the quenched state even under light 1 illumination. The opposite effects of DCMU and DBMIB on state transitions are demonstrated to be due to a suppression (by DCMU) or maintenance (by DBMIB) of the delta pH and not to change in the redox state of the plastoquinone. We conclude that, in R. violacea, the fluorescence change commonly associated with state 2 transition is in fact a delta pH-dependent quenching. This type of quenching has always been associated with near-saturating light intensities. Here, we show that very low intensities of a light that activates only the photosystem II induce a delta pH across the membrane that is not dissipated since the ATPase is not activated. The delta pH is dissipated only under conditions in which the photosystem I turns, confirming that the thioredoxin must be reduced to activate the ATPase. We suggest that the fluorescence changes, induced by various light conditions, in cyanobacteria and red algae could be associated with different phenomena.
Ammonium Chloride↗
Redox dependence for photoligation of manganese to the apo-water-oxidizing complex in chloroplasts and photosystem II membranes.
Effects of reducing reagents and redox potentials on photoactivation were studied in Mn-depleted chloroplasts and PSII membranes. Exogenous reducing reagents abolished photoactivation in PSII membranes, while they stimulated photoactivation in chloroplasts. To determine how reducing reagents can have such opposing effects in these preparations, we studied how redox potentials affect photoactivation in the range from 0 mV to +500 mV. In chloroplasts, a modest yield of photoactivation was obtained in the redox potential range of +100 and +330 mV at pH 7.5. The yield of photoactivation decreased at redox potentials above +330 mV, and drastically increased below potentials of +100 mV. Nernst plots of the data show that an n = 1 redox component with an Em7.5 of +374 mV, as well as an n = 2 redox component with an Em7.5 of +61 mV, is involved in photoactivation of chloroplasts isolated from dark-grown spruce seedlings. In the case of PSII membranes, photoactivation decreased sharply on either side of +335 mV at pH 5.5. The n = 1 redox components with Em5.5 of +375 and +319 mV may be involved, both of which showed pH dependences of -60 mV/pH unit. DCMU abolished photoactivation in chloroplasts, but did not affect the dependence of photoactivation on oxidation-reduction potentials in PSII membranes. The component with an Em5.5 of +319 mV involved in photoactivation of PSII membranes was also observed in the dependence of Mn solubilization on oxidation-reduction potentials with PSII membranes lacking extrinsic proteins, suggesting that the reduction of Mn with higher valences to Mn(II) by exogenous reducing reagents reversibly occurs in the intermediates or an active center during photoactivation in PSII membranes. Involvement of such redox components in photoactivation in chloroplasts and PSII membranes is discussed.
Chloroplasts↗
Modification of the water oxidizing complex in leaves of the dgd1 mutant of Arabidopsis thaliana deficient in the galactolipid digalactosyldiacylglycerol.
The primary biochemical defect in the genetically well characterized dgd1 mutant of Arabidopsis thaliana causes a 90% reduction in the relative amount of the galactolipid digalactosyldiacylglycerol (DGDG). To study the effect of this DGDG deficiency on photosystem II (PS II), time-resolved transients of laser-flash-induced changes of the relative fluorescence quantum yield Fvar,rel(t) were measured in whole leaves from wild-type and the dgd1 mutant. The results obtained reveal (i) in untreated leaves the decay kinetics of Fvar, rel(t) reflecting QA.- reoxidation by endogenous plastoquinone are very similar in wild-type and the dgd1 mutant at room temperature, (ii) the Arrhenius plot of the temperature dependence of electron transfer from QA.- to QB exhibits a break point at about 19 degrees C in wild-type and about 12 degrees C in the dgd1 mutant, (iii) in leaves treated with DCMU the slow reoxidation of QA.- by the PS II donor side is blocked to a much higher extent in the dgd1 mutant (about 50%) compared to wild-type (about 10%), and iv) the normalized amplitude of Fvar,rel(t = 1 micros) reflecting the percentage of fast P680.+ reduction by YZ exhibits a characteristic period four oscillation in wild-type while this feature is strongly damped in the dgd1 mutant. Presumably, the severe DGDG deficiency is causing the thermal down shift of a lipid phase transition that affects the QA.- reoxidation by QB. Most strikingly, the properties of the WOC are modified as a result of reduced DGDG content. Thus, the lipid DGDG appears to be of structural relevance for the WOC.
Arabidopsis↗
Room-temperature vibrational difference spectrum for S2QB-/S1QB of photosystem II determined by time-resolved Fourier transform infrared spectroscopy.
Time-resolved FTIR spectroscopy has been used to kinetically characterize the vibrational properties of intact photosystem II-enriched membrane samples undergoing the S1QB-to-S2QB- transition at room temperature. To optimize the experimental conditions for the FTIR measurements, oxygen polarographic and variable chlorophyll a fluorescence measurements were used to define the decay of S2 and QA-, respectively. The flash-induced S2QB-/S1QB difference spectra were measured at a temporal resolution of 4.44 s and a spectral resolution of 4 cm-1. An intense positive band is observed at 1480 cm-1 in the difference spectrum and shows a slow decay with a half time of approximately 13 s. Based on its decay kinetics and analogy to the infrared absorption of QA- of photosystem II and QB- in bacterial reaction centers, we conclude that the 1480 cm-1 band arises from QB- of PSII and tentatively assign it to the upsilon(CO) mode of the semiquinone anion QB-. The infrared spectral features attributed to the S1-to-S2 transition of the Mn cluster at room temperature show striking similarity to the S2/S1 difference spectrum measured at cryogenic temperatures (Noguchi, T., Ono, T.-A., and Inoue, Y. (1995) Biochim. Biophys. Acta 1228, 189-200).
Chloroplasts↗
Stabilization of charge separation and photochemical misses in photosystem II.
Illumination of photosystem II by a saturating short flash results in a stabilized charge separation in only about 90% of the reaction centers. During a series of flashes, the 10% fraction of "photochemical misses" is randomly redistributed among the centers. This phenomenon is investigated in DCMU-inhibited material, eliminating the contribution to misses due to electron transfer equilibrium on the quinone acceptors. Under such conditions, the miss coefficient is about 5% and is enhanced to about 40% in the presence of hydroxylamine at low pH. When a second flash is fired, its efficiency increases as a function of the time delay after the first flash (turnover experiments). This process involves three distinct time domains: <10 micros, 100 micros, and 10 ms. From a study of the 515-nm field-indicating change, it appears that the increased inefficiency caused by hydroxylamine is not due to a lesser amount of initial charge separation but to a recombination process concomitant with the 100 micros phase of the turnover. The slow turnover phase (10 ms) is not associated with a recombination or any other electron transfer event but reflects a modification of open centers during which their probability to achieve charge stabilization rather than recombination is progressively increased. These results are interpreted in terms of an equilibrium between two conformational states of the centers endowed with different stabilization yield ("good" and "bad " stabilizers). The 100 micros turnover phase is due to the reopening of the bad stabilizers by recombination, and the 10 ms phase accompanies the redistribution of these centers among the two conformational states.
Chlorella↗
Effects of carboxyl amino acid modification on the properties of the high-affinity, manganese-binding site in photosystem II.
Our previous work using the "diphenylcarbazide (DPC)-inhibition assay" has identified four amino acid (two carboxyls and two histidyls) ligands to four Mn2+ bound with high affinity on Tris-washed photosystem II (PSII) membrane fragments [Preston and Seibert (1991) Biochemistry 30, 9615-9624, 9625-9633]. One of the ligands binds a photooxidizable Mn, specifically, and the others bind either nonphotooxidizable Mn2+, Zn2+, or Co2+ [Ghirardi et al. (1996) Biochemistry 35, 1820-1828]. The current paper shows the following: (a) the high-affinity photooxidizable Mn, which donates to the oxidized primary PSII donor (YZ*), is bound to a carboxyl residue with a KM = 1.5 microM or Kd = 0.94 microM in the absence of DPC, and a Ki = 1.3 microM in the presence of DPC (both steady-state and flash approaches were used); (b) if this carboxyl is chemically modified using 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide hydrochloride (EDC), Mn2+ is photooxidized at a lower affinity (Kd = 25 microM) site that does not involve carboxyl ligands; (c) low-affinity Mn is photooxidized (possibly by YD*, the oxidized form of the alternative PSII donor) with a KM = 220 microM at a completely different site that also requires a carboxyl ligand; (d) photooxidation of high-affinity DPC by YZ* with a KM of 40-42 microM or Kd of 49-58 microM occurs at a site that does not require carboxyl residues; (e) photooxidation of low-affinity DPC with a KM = 1200 microM occurs at a site (possibly near YD) that is not affected by carboxyl modification with EDC. Due to the similarities between the binding of the high-affinity photooxidizable Mn to EDC-treated membranes and to PSII complexes from Asp170D1 mutants [Nixon and Diner (1992) Biochemistry 31, 942-948], we identify its carboxyl residue ligand as Asp170 on D1, one of the reaction-center proteins. The second carboxyl ligand identified using the DPC-inhibition assay binds Mn (but not a photooxidizable one), Zn, or Co ions. At least one of the two histidyl ligands (either His337 on D1 or another unidentified histidyl) that bind nonphotooxidizable, high-affinity Mn2+ also binds Zn2+ and Co2+.
2,6-Dichloroindophenol↗
Mobility of the primary electron-accepting plastoquinone QA of photosystem II in a Synechocystis sp. PCC 6803 strain carrying mutations in the D2 protein.
Upon introduction of random mutations in a region of the psbDI gene that encodes the D2 protein in the cyanobacterium Synechocystis sp. PCC 6803, an obligate photoheterotrophic mutant was isolated that contained three mutations: V247M, A249T, and M329I. This mutant evolved oxygen in the absence of added electron acceptors, but oxygen evolution was inhibited by micromolar concentrations of several artificial quinones. Complementation analysis showed that the V247M and/or A249T mutations were responsible for this phenotype. Using fluorescence induction and decay measurements, the site of inhibition by the quinones was found to be at the level of the primary electron-accepting quinone in photosystem II, QA. Duroquinone inhibited by blocking reduction of QA, and in the presence of other quinones such as 2,5-dichloro-p-benzoquinone, 2, 5-dimethyl-p-benzoquinone, and p-benzoquinone, QA could be reduced but could not efficiently transfer an electron to QB. To distinguish the effects of the V247M and A249T mutations, single mutants were created. V247M was photoautotrophic and had an essentially normal phenotype. The A249T mutant, although photoautotrophic, was affected by artificial quinones, but less than the mutant carrying both the V247M and A249T changes. The results indicate a decreased plastoquinone affinity at the QA site in the strains carrying a A249T mutation, such that after dark-adaptation a significant percentage of the QA sites is empty or is occupied by an artificial quinone. In light, the percentage of photosystem II centers with plastoquinone bound at the QA site appears to increase, which may be due in part to an increased affinity of the semiquinone versus that of the quinone at the QA site.
Amino Acid Substitution↗
Influence of herbicide binding on the redox potential of the quinone acceptor in photosystem II: relevance to photodamage and phytotoxicity.
Here we show that herbicide binding influences the redox potential (Em) of the plastoquinone QA/QA- redox couple in Photosystem II (PSII). Phenolic herbicides lower the Em by approximately 45 mV, while DCMU raises the Em by 50 mV. These shifts are reflected in changes in the peak temperature of thermoluminescence bands arising from the recombination of charge pairs involving QA-. The herbicide-induced changes in the Em of QA/QA- correlate with earlier work showing that phenolic herbicides increase the sensitivity of PSII to light, while DCMU protects against photodamage. This correlation is explained in terms of the following hypothesis which is based on reactions occurring in the bacterial reaction center. The back-reaction pathway for P680+QA- is assumed to be modulated by the free-energy gap between the P680+QA- and the P680+Ph- radical pairs. When this gap is small (i.e., when the Em of QA/QA- is lowered), a true back-reaction is favored in which P680+Ph- is formed, a state which decays forming a significant yield of P680 triplet. This triplet state of chlorophyll reacts with oxygen, forming singlet oxygen, a species likely to be responsible for photodamage. When the free-energy gap is increased (i.e., when the Em of QA/QA- is raised), the yield of the P680+Ph- is diminished and a greater proportion of the P680+QA- radical pair decays by an alternative, less damaging, route. We propose that at least some of the phytotoxic properties of phenolic herbicides may be explained by the fact that they render PSII ultrasensitive to light due to this mechanism.
Diuron↗
Pulsed EPR studies of doublet signal and singlet-like signal in oriented Ca2+-depleted PS II membranes: location of the doublet signal center in PS II.
Doublet signal and singlet-like signal induced in Ca(2+)-depleted PS II were investigated by pulsed EPR in one-dimensionally oriented photosystem (PS) II membranes. The doublet signal showed marked angular dependent change in its spectrum in term of the applied magnetic field, indicating that the magnetic dipole-dipole interaction is mainly responsible for the doublet signal. The singlet-like signal also showed angular dependence, which was less pronounced than that of the doublet signal. The parameters of dipole and exchange interactions used to simulate the doublet signal indicate that the signal arises from a magnetically coupled organic radical pair. Angular dependence of the doublet signal indicates that the radius vector of the radical pair (r) and the normal of the thylakoid membrane is at an angle of 65 degrees. Pulsed ELDOR studies in the oriented membranes indicate that the vector (R) connecting the doublet-signal center with the Y(D)(*) radical and the plane of the thylakoid membrane are at an angle of 8 degrees. Furthermore, the angle between the projections of the R and r vectors on the plane of the thylakoid membrane was determined to be 64 degrees. The location of the doublet-signal species in PS II is discussed.
Anisotropy↗
Chlorophyll fluorescence imaging of individual algal cells: effects of herbicide on Spirogyra distenta at different growth stages.
Serious environmental degradation of aquatic ecosystems has been caused by eutrophication and by pollutants such as herbicides. Therefore, measurement of in situ algal photosynthetic activity is important for environmental monitoring. With ordinary nonimaging fluorometers, algal chlorophyll fluorescence can be measured easily, but heterogeneity within samples cannot be detected. Effects of a herbicide preparation containing 3-(3,4-dichlorophenyl)-1,1 -dimethylurea (DCMU) on photosynthetic activity at different growth stages of Spirogyra distenta were investigated by using a computer-aided microscopic imaging system for chlorophyll afluorescence. Photosystem II photochemical yield (phiPSII) images were used to diagnose photosynthetic activity of spiral filate chloroplasts in algal cells. The herbicide treatment caused a stronger decline in phiPSII values in younger than in mature algae cells. This result indicated that heterogeneity within algal samples should be considered when algae are used for environmental monitoring. Thus, measurement of chlorophyll fluorescence from young and mature chloroplasts with a microscopic imaging system makes it possible to improve the sensitivity for monitoring the environmental degradation of aquatic ecosystems.
Chlorophyll↗
Herbicide influence on cottonseed oil quality.
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Acetanilides↗
Nitrogen availability and electron transport control the expression of glnB gene (encoding PII protein) in the cyanobacterium Synechocystis sp. PCC 6803.
The glnB gene from Synechocystis sp. PCC 6803 that encodes the PII protein has been cloned by heterologous hybridization using the corresponding glnB gene from Synechococcus sp. PCC 7942. An ORF of 336 nucleotides appeared that potentially coded for a protein of 112 amino acid residues (M(r) 12,397). The deduced amino acid sequence revealed a high identity (higher than 80%) with its cyanobacterial counterparts and a basal level of identity (close to 60%) with other PII proteins. A single mRNA of about 680 nucleotides was found under all growth conditions studied. glnB gene expression was specifically activated under nitrogen deprivation (a 10-fold increase respect to nitrogen-replete conditions). No differences in glnB mRNA levels were observed when using nitrate or ammonium as nitrogen sources. Amount of glnB mRNA decreased to undetectable levels when transferring cells to the dark, but effect was avoided by adding glucose to the culture medium. Primer extension analysis and band-shift assays indicated that expression of the glnB gene, elevated under nitrogen deprivation, might lie under the control of the nitrogen transcriptional regulator NtcA, although constitutive levels of expression were also detected from a sigma 70-dependent Escherichia coli-like promoter.
Amino Acid Sequence↗
Substitution of Ala-251 of the D1 reaction centre polypeptide with a charged residue results in impaired function of photosystem II.
Ala-251 in the membrane-parallel helix in the D-E loop of the D1 polypeptide close to the Q(B) pocket of photosystem II (PS II), was mutated to aspartate (D), lysine (K), leucine (L) or serine (S) in Synechocystis 6803. O2 evolution rates (H2O-->DCBQ; 2,6-dichloro-p-benzoquinone) of A251D, A251L and A251S were lower, being 38, 16, 62 and 70%, respectively, of that of the control, and there was an even more drastic impairment of O2 evolution when measured from H2O to DMBQ (2,5-dimethyl-p-benzoquinone), demonstrating modifications in the Q(B) pocket. However, in all other mutants but A251K, the Q(B) function could sustain O2 evolution at a level high enough to support photosynthetic growth. The mutant A251S, carrying a substitution of alanine for a chemically quite similar residue serine, was less severely affected. Substitution by a positively charged residue drastically delayed chlorophyll a fluorescence relaxation in the non-photosynthetic strain A251K, implying strong impairment of Q(A)-to-Q(B) electron transfer. Delay of fluorescence relaxation was clear in A251D as well, carrying a substitution of alanine for a negatively charged residue. The effects of the substitutions of A251 demonstrate the importance of this residue of the D1 polypeptide in the conformation of the acceptor side of PS II and, accordingly, the effect on the acceptor-side function of PS II was very clear. Nevertheless, the tolerance of PS II activity to high-light-induced photoinhibition in vivo and the subsequent D1 degradation were not much impaired in any of the photosynthetic mutant strains as compared to the control.
Alanine↗
Signals controlling the expression of cytosolic ascorbate peroxidase during pathogen-induced programmed cell death in tobacco.
In plants ascorbate peroxidase (APX) is an important H2O2-detoxifying enzyme. The expression of APX is rapidly induced in response to stresses that result in the accumulation of reactive oxygen species (ROS). We have recently reported that the steady-state level of transcripts encoding cytosolic APX (cAPX) is dramatically induced during the hypersensitive response (HR) of tobacco plants infected with tobacco mosaic virus (TMV). Because cAPX expression is closely linked to the production of ROS in plant cells, studying the regulation cAPX mRNA can reveal some of the signal transduction events associated with ROS metabolism during the HR. Analysis of cAPX mRNA induction during the HR suggested that the expression of cAPX is under the control of the HR signal transduction pathway. The activation of cAPX expression followed signaling events such as changes in protein phosphorylation and induction of ion fluxes. Expression of cAPX was suppressed under conditions of low oxygen pressure, and could only be mimicked by enhancing the intracellular generation of ROS. Interestingly, salicylic acid (SA), which is thought to be involved in ROS metabolism during the HR, did not affect the induction of cAPX mRNA during TMV-induced HR. Using cAPX expression as a marker for the production of ROS, it is suggested that SA may not be involved in the formation of ROS during the HR of tobacco to TMV, and that ROS may not be involved in the induction of the pathogenesis-related protein, PR-1, during this process.
Anti-Infective Agents↗
Transcriptional regulation of the Nia1 gene encoding nitrate reductase in Chlamydomonas reinhardtii: effects of various environmental factors on the expression of a reporter gene under the control of the Nia1 promoter.
The NAD(P)H nitrate reductase (NR) from Chlamydomonas reinhardtii is encoded by the structural gene Nia1. Numerous data from the literature indicate that this enzyme is submitted to complex regulation mechanisms involving multiple controls at transcriptional and post-transcriptional levels. To specifically investigate the regulation of the Nia1 gene at the transcriptional level, NR+ and NR- transformed cells harbouring the Nia1:Ars construct (Nia1 promoter fused to the arylsulfatase (ARS)-encoding Ars reporter gene) were cultivated under various experimental conditions and the ARS activities were recorded. ARS levels were very low in cells grown in the presence of NH4Cl and dramatically increased on agar medium deprived of any nitrogen source or containing nitrate, nitrite, urea, arginine or glutamine. Compared to nitrogen-free medium, a slight positive effect of nitrate in the NR+ strain and a significant negative effect of nitrite in both NR+ and NR- strains were observed. The ARS activities were high in the light and very low in the dark or in the light in the presence of DCMU, indicating that Nia1 transcription is strikingly dependent on photosynthetic activity. Acetate used as a carbon source in the dark did not substitute for light in stimulating Nia1:Ars expression. Inactivation of NR by tungstate treatment of the NR+ strain resulted in a dramatic increase of ARS level suggesting that in Chlamydomonas, like in higher plants, active NR negatively regulates the transcription of the NR structural gene. Deleting the major part of the Nia1 leader sequence still present in the chimeric gene resulted in a decrease of ARS level but did not modify the regulation pattern.
Acetates↗
The Slr0924 protein of Synechocystis sp. strain PCC 6803 resembles a subunit of the chloroplast protein import complex and is mainly localized in the thylakoid lumen.
An isolated 25 kDa protein of Synechocystis sp. PCC 6803 was N-terminally sequenced and assigned to a protein encoded by the ORF slr0924. This ORF shows a certain degree of sequence similarity to a subunit from the protein Translocon at the Inner envelope of pea Chloroplasts (Tic22). The deduced amino acid sequence of Slr0924 has a N-terminal extension, that contains two possible translational start points and two possible cleavage sites for leader peptidases. Immunostaining with an antibody raised to the over-produced protein revealed two cross-reacting forms, which probably correspond to a larger intermediate and the mature protein. Immunogold labelling of thin sections showed that the protein is located mainly in the thylakoid region. This result was verified by thylakoid membrane fractionation indicating that Slr0924 is a lumenal protein. The slr0924 gene product is essential for the viability of Synechocystis sp. PCC 6803 as shown by interposon mutagenesis. The merodiploid strain showed reduced photosynthetic activity compared to the wild-type. Furthermore, growth of the merodiploid strain was found to be completely inhibited after cultivation with glucose. Accordingly, the amount of the slr0924 gene product was regulated by glucose and light intensities in wild-type cells. The potential function of the protein in Synechocystis sp. PCC 6803 will be discussed.
Amino Acid Sequence↗