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PubMed1998

Fluorescence and absorption spectroscopy of the weakly fluorescent chlorophyll a in cytochrome b6f of Synechocystis PCC6803.

A spectroscopic characterization of the chlorophyll a (Chl) molecule in the monomeric cytochrome b6f complex (Cytb6f) isolated from the cyanobacterium Synechocystis PCC6803 is presented. The fluorescence lifetime and quantum yield have been determined, and it is shown that Chl in Cytb6f has an excited-state lifetime that is 20 times smaller than that of Chl in methanol. This shortening of the Chl excited state lifetime is not caused by an increased rate of intersystem crossing. Most probably it is due to quenching by a nearby amino acid. It is suggested that this quenching is a mechanism for preventing the formation of Chl triplets, which can lead to the formation of harmful singlet oxygen. Using site-selected fluorescence spectroscopy, detailed information on vibrational frequencies in both the ground and Qy excited states has been obtained. The vibrational frequencies indicate that the Chl molecule has one axial ligand bound to its central magnesium and accepts a hydrogen bond to its 13(1)-keto carbonyl. The results show that the Chl binds to a well-defined pocket of the protein and experiences several close contacts with nearby amino acids. From the site-selected fluorescence spectra, it is further concluded that the electron-phonon coupling is moderately strong. Simulations of both the site-selected fluorescence spectra and the temperature dependence of absorption and fluorescence spectra are presented. These simulations indicate that the Huang-Rhys factor characterizing the electron-phonon coupling strength is between 0.6 and 0.9. The width of the Gaussian inhomogeneous distribution function is 210 +/- 10 cm-1.

Binding Sites↗
PubMed1995

Etiolated cells of Chlamydomonas reinhardtii: choice material for characterization of mitochondrial membrane polypeptides.

We have investigated a light-conditional mutant of Chlamydomonas reinhardtii (J12) that is unable to synthesize chlorophyll in the dark with the aim of characterizing the mitochondrial membrane polypeptides of this alga. A crude membrane fraction derived from etiolated cells was analyzed by gel electrophoresis, immunoblot analysis, and pulse-labeling in the presence of specific protein synthesis inhibitors. This fraction contained both mitochondrial and etioplast membranes, and the latter contained appreciable amounts of subunits of the cytochrome b6f complex. The mitochondria-encoded subunit 1 of cytochrome-c oxidase called COX1 was identified, and its synthesis was detected in this membrane fraction. The redox-difference spectra of mitochondrial cytochromes were studied in whole cells and membrane fractions, in both respiratory-competent and -deficient strains. Mitochondrial membranes could be further purified after sucrose gradient centrifugation. The use of etiolated cells and their membrane extracts, in association with appropriate methodologies, opens ways to study the molecular genetics of mitochondria in C. reinhardtii and allows us to address the question of the cooperation established between the three genetic compartments of a plant cell.

Journal Article↗
PubMed1987

The site of inhibition of the chloroplast electron-transport system by 2,3-dithiopropan-1-ol (BAL).

BAL (2,3-dithiopropan-1-ol) treatment of chloroplasts has previously been reported to induce a block in electron transport from water to NADP+ at a site preceding plastocyanin [Belkin et al. (1980) Biochim. Biophys. Acta 766, 563-569]. In the present work the block was further characterized. The following properties of BAL treatment are described. Inhibition of electron transport from water to lipophilic acceptors but not to silicomolybdate. Inhibition of the slow, sigmoidal phase of chlorophyll a fluorescence induction. Inability of N,N,N',N',-tetramethyl-p-phenylenediamine to bypass the inhibition of NADP+ photoreduction with water as the electron donor. Inhibition of electron transport from externally added quinols to NADP+. Inhibition of cytochrome f reduction by photosystem II, but not its oxidation by photosystem I. Inhibition of cytochrome b6 turnover and cytochrome f rereduction after single-turnover flash illumination under cyclic electron-flow conditions. The BAL-induced block is therefore located between the secondary quinone acceptor (QB) and the cytochrome b6f complex. It was further found that (a) the isolated cytochrome complex is not inhibited after BAL treatment; (b) BAL-reacted plastoquinone-1 inhibits electron transport in chloroplasts; (c) BAL does not inhibit electron transport in chromatophores of Rhodospirilum rubrum or Rhodopseudomonas capsulata. It is suggested that the inhibition of electron transport in chloroplasts results from specific reaction of BAL with the endogenous plastoquinone.

Bacterial Chromatophores↗
PubMed1995

Conversion of cytochrome f to a soluble form in vivo in Chlamydomonas reinhardtii.

We introduced a stop codon in place of the ATT codon encoding Ile283 (numbered from the Met initiation codon) in the petA gene from Chlamydomonas reinhardtii. The resulting protein was expected to be truncated on its carboxy-terminus end, lacking the last 35 amino acids. This region of the polypeptide sequence encompasses a hydrophobic stretch assumed to anchor the protein in the thylakoid membrane. Once introduced in whole cells of C. reinhardtii by chloroplast transformation, the modified petA gene expressed a truncated apoprotein which was efficiently converted to a truncated holocytochrome f. This protein accumulated in the lumen of the thylakoids in a soluble form. Thus the conversion of preapocytochrome f to holocytochrome f does not require an interaction with the membrane through its C-terminus anchor. We show that the rest of the cytochrome b6f complex failed to accumulate in the transformants, most probably because of a lack of interaction between soluble cytochrome f and the other cytochrome b6f subunits. However, soluble cytochrome f was still able to donate electrons to photosystem I, which is indicative of its ability to maintain interactions with plastocyanin. The control of the rate of synthesis of cytochrome f by the neighboring subunit, suIV (Kuras & Wollman (1994) EMBO J. 13, 1019-1027), was not observed with the truncated cytochrome f. This observation suggests that either the transmembrane anchor of cytochrome f contains a target for the regulation of cytochrome f translation by suIV or there is a transient form of membrane-bound cytochrome f which is highly sensitive to proteolysis at an early post-translational stage.

Amino Acid Sequence↗
PubMed1990

Purification and characterization of photosystem I and photosystem II core complexes from wild-type and phycocyanin-deficient strains of the cyanobacterium Synechocystis PCC 6803.

Highly photoactive Photosystem I (PS I) and Photosystem II (PS II) core complexes have been isolated from the cyanobacterium Synechocystis Pasteur Culture Collection (PCC) 6803 and a phycocyanin-deficient mutant, enriched in PS II. Cell breakage using glass beads was followed by sucrose density gradient centrifugation and two high-performance liquid chromatography steps involving anion-exchange and hydroxyapatite. The PS I core complex has an apparent molecular mass of 300 +/- 20 kDa (including a detergent shell of about 50 kDa) and contains subunits of approximately 60, approximately 60, 18.5, 18.5, 16, 15, 10.5, 9.5, and 6.5 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblots; its antenna size is 75 +/- 5 chlorophyll/P-700. The PS II core complex has an apparent molecular mass of 310 +/- 20 kDa (including the detergent shell); subunits of 43, 37, 33, 29, and 10-11 kDa were identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting. The antenna size of the average PS II complex is 45 +/- 5 chlorophyll/primary quinone electron acceptor (QA). This preparation procedure also yields, as a byproduct, a highly purified cytochrome b6f complex. This complex contains four subunits of 38, 24, 19, and 15 kDa and b- and c-type cytochromes in a ratio of 2:1. Its apparent molecular mass of 180 +/- 20 kDa (including the detergent shell) is consistent with a monomeric complex.

Bacterial Proteins↗
PubMed2002

In vitro reconstitution of the spinach chloroplast cytochrome b6 protein from a fusion protein expressed in Escherichia coli.

We have developed a strategy for overproduction of spinach apocytochrome b6 as a fusion protein to maltose-binding protein (MBP) in Escherichia coli, using the expression vector pMal-c2. The fusion protein was purified to virtual homogeneity by gel filtration chromatography and the method of insertion of hemes into fusion protein was elaborated. The ambient and low-temperature absorption spectra of the reconstituted cytochrome b6 were similar to those of cytochrome b6 spectra in isolated proteins or cytochrome b6f complexes and are typical for bis-histidine ligated b-type cytochromes. Optical circular dichroism (CD) spectra of the visible region further confirmed the appropriate binding of hemes by the apocytochrome b6 protein. We found that the incorporation of hemes was required for the refolding of the cytochrome b6 protein into the more compact structure found in the native cytochrome protein. Heme staining experiments suggested that the two hemes in the reconstituted cytochrome b6 protein are bound with different affinities. The reconstituted cytochrome b6 protein was cleaved by Xa factor proteolysis from fusion protein and separated for characterization. The procedure presented in this work for reconstitution of hemes into the cytochrome b6 protein should provide an important tool for structure/function studies of membrane-bound cytochrome proteins.

ATP-Binding Cassette Transporters↗
PubMed1984

The effects of quinone analogues on cytochrome b6 reduction and oxidation in a reconstituted system.

The reconstituted system containing Photosystem I, plastocyanin and the cytochrome b6-f complex is used to study the effects of various quinone analogues on the redox behavior of cytochrome b6. The effects of DBMIB, DNP-INT and HQNO are compared in an attempt to discern the modes of action of these quinone analogues. Both DBMIB and DNP-INT are potent inhibitors of the plastocyanin reductase activity of the isolated cytochrome complex. However, while DBMIB abolished the oxidant-induced reduction of cytochrome b6, DNP-INT only inhibited about 25% of the net reduction. On the other hand, HQNO does not show any significant inhibition of plastocyanin reductase activity of the isolated cytochrome complex at concentrations up to 20 microM. An enhancement of the net amount of cytochrome b6 reduced is observed in the presence of HQNO. Both DNP-INT and HQNO inhibited the dark oxidation rate of cytochrome b6. The possible identity of the oxidant for cytochrome b6 is discussed. Plastoquinone is concluded to be the most likely candidate. DNP-INT is concluded to have at least two sites of inhibition in the cytochrome complex. The implications of these findings on quinone functions in the cytochrome b6-f complex are discussed.

Cytochrome b Group↗
PubMed1995

Purification and physicochemical properties of the low-potential cytochrome C549 from the cyanobacterium Synechocystis sp. PCC 6803.

A soluble low-potential cytochrome c549 has been purified in milligram quantities from the cyanobacterium Synechocystis sp. PCC 6803. The protein exhibits an acid isoelectric point of 3.9, a molecular mass of 15.8 kDa, and a midpoint redox potential value of -250 mV at pH 7.0 EPR and 1H NMR studies suggest a low-spin heme iron with bis-histidine coordination at the fifth and sixth positions. EDTA-photoreduced 5-deazariboflavin has been used as the electron-donating system to study, by laser flash absorption spectroscopy, the electron transfer reactions between Synechocystis cytochrome c549 and redox proteins involved in the cyclic electron flow around photosystem I. The second-order rate constants (k2) obtained for ferredoxin (or flavodoxin) oxidation by Synechocystis cytochrome c549 are rather low (ca. 10(5) M-1 s-1), thus suggesting that this low-potential heme-protein does not operate as the primary electron carrier for either transferring electrons to the cytochrome b6f complex in cyclic photophosphorylation or to hydrogenase during anaerobic metabolism. The k2 values for plastocyanin reduction by cytochrome c549 are about 100 times higher (ca. 10(7) M-1 s-1), but it remains to be determined whether or not this reaction actually reflects a physiological process.

Chemical Phenomena↗
PubMed2003

Deletion of the tobacco plastid psbA gene triggers an upregulation of the thylakoid-associated NAD(P)H dehydrogenase complex and the plastid terminal oxidase (PTOX).

We have constructed a tobacco psbA gene deletion mutant that is devoid of photosystem II (PSII) complex. Analysis of thylakoid membranes revealed comparable amounts, on a chlorophyll basis, of photosystem I (PSI), the cytochrome b6f complex and the PSII light-harvesting complex (LHCII) antenna proteins in wild-type (WT) and DeltapsbA leaves. Lack of PSII in the mutant, however, resulted in over 10-fold higher relative amounts of the thylakoid-associated plastid terminal oxidase (PTOX) and the NAD(P)H dehydrogenase (NDH) complex. Increased amounts of Ndh polypeptides were accompanied with a more than fourfold enhancement of NDH activity in the mutant thylakoids, as revealed by in-gel NADH dehydrogenase measurements. NADH also had a specific stimulating effect on P700+ re-reduction in the DeltapsbA thylakoids. Altogether, our results suggest that enhancement of electron flow via the NDH complex and possibly other alternative electron transport routes partly compensates for the loss of PSII function in the DeltapsbA mutant. As mRNA levels were comparable in WT and DeltapsbA plants, upregulation of the alternative electron transport pathways (NDH complex and PTOX) occurs apparently by translational or post-translational mechanisms.

FMN Reductase↗
PubMed1997

Minor constituents of photosystem II core complexes: possible regulators of photosystem II protein kinase.

We recently identified a 58-kDa protein kinase, PS II-PK, closely associated in substoichiometric abundance with a core complex of photosystem II and capable of phosphorylating both the photosystem and its associated light-harvesting proteins. Oxidizing species, produced during aerobic illumination, inhibited the kinase, and the irreversibility of this process is now demonstrated. Other substoichiometric protein constituents of the core preparation were identified as probably originating in the grana margins. These include the cytochrome b6/f complex, the apocytochrome f precursor, and membrane-bound ferredoxin:NADP+ oxidoreductase. PS II-PK was successfully resolved from photosystem II cores and shown to be active in the absence of cytochrome complex.

Cell Compartmentation↗
PubMed2000

Resolution of mitochondrial and chloroplast membrane protein complexes from green leaves of potato on blue-native polyacrylamide gels.

Purification of mitochondria and mitochondrial protein complexes from green tissues is often severely impaired by the presence of chloroplasts and their proteins. Here we present a method which allows analysis of respiratory protein complexes from potato leaves. The procedure includes the preparation of an organellar fraction specifically enriched in mitochondria and the separation of organellar protein complexes by blue-native polyacrylamide gel electrophoresis (BN-PAGE). For the first time mitochondrial and chloroplast protein complexes have been resolved simultaneously in a native gel. BN-PAGE allowed the separation of eleven bands, including the mitochondrial NADH-dehydrogenase, the bc1 complex and the mitochondrial F1-ATP synthase as well as the chloroplast F1-ATP synthase, the cytochrome b6f complex, the two photosystems and the light harvesting complex. The resolution of the protein complexes in the first dimension was good enough to allow identification of all subunits of individual complexes in the second dimension under denaturing conditions. Thus, BN-PAGE offers an opportunity to analyze mitochondrial and chloroplast protein complexes from a single preparation from very small amounts of tissue. The implications of our findings, for studies on protein expression and turnover in different tissues and developmental stages, are discussed.

Chloroplasts↗
PubMed1992

Forces involved in the assembly and stabilization of membrane proteins.

Hydrophobic organization: Determination of the structure of the bacterial photosynthetic reaction center, bacterial porins, and bacteriorhodopsin allows a comparison of the basic structural features of integral membrane proteins. Structure parameters of membrane- and water-soluble proteins are surprisingly similar, given the different dielectric environments, except for the polarity of residues on the protein surface. Hydrophobic and electrostatic forces: 1) Intramembrane helix-helix interactions that are sensitive to small structure changes can dictate assembly of membrane proteins, as indicated by reconstitution of bacteriorhodopsin from proteolytic fragments and specific dimer formation of the human erythrocyte sialoglycoprotein glycophorin A. 2) Electrostatic interactions have an important role in determining the trans-membrane orientation of integral membrane proteins of the bacterial inner membrane, as expressed by the "positive-inside" rule for the distribution of basic residues on the cis relative to the trans side of the membrane-spanning alpha-helices. The use of this charge asymmetry rule, in conjunction with a hydrophobicity algorithm for prediction of membrane-spanning domains, allows accurate prediction of the folding patterns of such polypeptides across the membrane. A role of electrostatic interactions in assembly and maintenance of the structure of oligomeric integral membrane protein complexes is also implied by the separation and extrusion from the membrane, at high pH, of the major hydrophobic subunits of the cytochrome b6f complex from the chloroplast thylakoid membrane. It is inferred that the hydrophobic helix-helix interactions between the subunits of this complex, whose function is electron transfer and proton translocation, are relatively weak compared to those in bacteriorhodopsin.

Crystallization↗
PubMed1987

Large-scale purification of active cytochrome b6/f complex from spinach chloroplasts.

A preparation is described through which large quantities of pure, active cytochrome b6/f complex can be isolated from spinach chloroplasts. The resulting complex is at least 90% pure with respect to the maximum content of redox centers, consists of four polypeptides according to polyacrylamide gel electrophoresis, and lacks both ferredoxin: NADP+ oxidoreductase and the high molecular weight form of cytochrome f seen in some other preparations. The complex contains 2 mol b6 and 2 atoms of nonheme iron per mole of cytochrome f, and possesses a high plastoquinol-plastocyanin oxidoreductase activity (Cyt f turnover no. 20-35 s-1). The present preparation should be helpful in the effort to crystallize the cytochrome b6/f complex.

Chloroplasts↗
PubMed2000

The proton to electron stoichiometry of steady-state photosynthesis in living plants: A proton-pumping Q cycle is continuously engaged.

A noninvasive technique is introduced with which relative proton to electron stoichiometries (H(+)/e(-) ratios) for photosynthetic electron transfer can be obtained from leaves of living plants under steady-state illumination. Both electron and proton transfer fluxes were estimated by a modification of our previously reported dark-interval relaxation kinetics (DIRK) analysis, in which processes that occur upon rapid shuttering of the actinic light are analyzed. Rates of turnover of linear electron transfer through the cytochrome (cyt) b(6)f complex were estimated by measuring the DIRK signals associated with reduction of cyt f and P(700). The rates of proton pumping through the electron transfer chain and the CF(O)-CF(1) ATP synthase (ATPase) were estimated by measuring the DIRK signals associated with the electrochromic shifting of pigments in the light-harvesting complexes. Electron transfer fluxes were also estimated by analysis of saturation pulse-induced changes in chlorophyll a fluorescence yield. It was shown that the H(+)/e(-) ratio, with respect to both cyt b(6)f complex and photosystem (PS) II turnover, was constant under low to saturating illumination in intact tobacco leaves. Because a H(+)/e(-) ratio of 3 at a low light is generally accepted, we infer that this ratio is maintained under conditions of normal (unstressed) photosynthesis, implying a continuously engaged, proton-pumping Q cycle at the cyt b(6)f complex.

Chlorophyll↗
PubMed1992

An archaebacterial terminal oxidase combines core structures of two mitochondrial respiratory complexes.

The operon coding for a respiratory quinol oxidase was cloned from thermoacidophilic archaebacterium Sulfolobus acidocaldarius. It contains three genes, soxA, soxB and soxC. The first two genes code for proteins related to the cytochrome c oxidase subunits II and I, respectively. soxC encodes a protein homologous to cytochrome b, which is a subunit of the mitochondrial and bacterial cytochrome c reductases and the chloroplast cytochrome b6f complex. soxA is preceded by a promoter and the genes are cotranscribed into a 4 kb mRNA. Their protein products form a complex which has been partially purified and has quinol oxidase activity. The reduced minus oxidized absorption spectrum of the complex has two maxima at 586 and 606 nm. The latter is typical of cytochrome c oxidase. The complex contains four haems A. Two haems belong to the 'cytochrome oxidase' part of the complex and two are probably bound to be apocytochrome b (SoxC) and responsible for the 586 nm absorption peak. The homology between the sox gene products and their mitochondrial counterparts suggests that energy conservation coupled to the quinol oxidation catalysed either by the Sulfolobus oxidase or two mitochondrial respiratory enzymes may have a similar mechanism.

Amino Acid Sequence↗
PubMed1987

Anomalous uncoupling of photophosphorylation by palmitic acid and by gramicidin D.

Palmitic acid and gramicidin D at low concentrations uncouple photophosphorylation in a mechanism that is inconsistent with classical uncoupling in the following properties: (1) delta pH, H+ uptake, or the transmembrane electric potential is not inhibited. (2) O2 evolution is stimulated under nonphosphorylating conditions but slightly inhibited in the presence of adenosine 5'-diphosphate + inorganic phosphate (Pi). (3) Light-triggered adenosine 5'-triphosphate (ATP)-Pi exchange is hardly affected, and ATPase activity is only slightly stimulated. (4) ATP-induced delta pH formation is selectively inhibited. This characteristic uncoupling is observed only when the native coupling sites of the electron transport system are used for energization such as for methylviologen-coupled phosphorylation. With pyocyanine, which creates an artificial coupling site, 1000-fold higher gramicidin D and higher palmitic acid concentrations are required for inhibition, and the inhibition is accompanied by a decrease in delta pH. Moreover, comparison between photosystem 1 and photosystem 2 electron transport and the effects of membrane unstacking suggest that low gramicidin D preferentially inhibits photosystem 2, while palmitic acid inhibits more effectively photosystem 1 coupling sites. The inhibitory capacity of fatty acids significantly drops when the chain length is reduced below 16 hydrocarbons or upon introduction of a single double bond in the hydrocarbon chain. It is suggested that palmitic acid and gramicidin D interfere with a direct H+ transfer between specific electron transport and the ATP synthase complexes, which provides an alternative coupling mechanism in parallel with bulk to bulk delta microH+. The sites of inhibition seem to be located in chloroplast ATP synthase, photosystem 2, and the cytochrome b6f complexes.

Chloroplasts↗
PubMed1996

Bacillus stearothermophilus qcr operon encoding rieske FeS protein, cytochrome b6, and a novel-type cytochrome c1 of quinol-cytochrome c reductase.

The gcr of Bacillus stearothermophilus K1041 encoding three subunits of the quinol-cytochrome c oxidoreductase (cytochrome reductase, b6c1 complex) was cloned and sequenced. The gene (qcrA) for a Rieske FeS protein of 19,144 Da with 169 amino acid residues, and the gene (qcrC) for cytochrome c1 of 27,342 Da with 250 amino acid residues were found at adjacent upstream and downstream sides of the previously reported qcrB (petB) for cytochrome b6 of subunit 25,425 Da with 224 residues (Sone, N., Sawa, G., Sone, T., and Noguchi, S. (1995) J. Biol. Chem. 270, 10612-10617). The three structural genes for thermophilic Bacillus cytochrome reductase form a transcriptional unit. In the deduced amino acid sequence for the FeS protein, the domain including four cysteines and two histidines binding the 2Fe-2S cluster was conserved. Its N-terminal part more closely resembled the cyanobacteria-plastid type than the proteobacteria-mitochondria type when their sequences were compared. The amino acid sequence of cytochrome c1 was not similar to either type; the thermophilic Bacillus cytochrome c1 is composed of an N-terminal part corresponding to subunit IV with three membrane-spanning segments, and a C-terminal part of cytochrome c reminiscent of cytochrome c-551 of thermophilic Bacillus. The subunit IV in the enzyme of cyanobacteria and plastids is the counterpart of C-terminal part of cytochrome b of proteobacteria and mitochondria. These characteristics indicate that Bacillus cytochrome b6c1 complex is unique.

Amino Acid Sequence↗