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PubMed1998

Translation of cytochrome f is autoregulated through the 5' untranslated region of petA mRNA in Chlamydomonas chloroplasts.

A process that we refer to as control by epistasy of synthesis (CES process) occurs during chloroplast protein biogenesis in Chlamydomonas reinhardtii: the synthesis of some chloroplast-encoded subunits, the CES subunits, is strongly attenuated when some other subunits from the same complex, the dominant subunits, are missing. Herein we investigate the molecular basis of the CES process for the biogenesis of the cytochrome b6f complex and show that negative autoregulation of cytochrome f translation occurs in the absence of other complex subunits. This autoregulation is mediated by an interaction, either direct or indirect, between the 5' untranslated region of petA mRNA, which encodes cytochrome f, and the C-terminal domain of the unassembled protein. This model for the regulation of cytochrome f translation explains both the decreased rate of cytochrome f synthesis in vivo in the absence of its assembly partners and its increase in synthesis when significant accumulation of the C-terminal domain of the protein is prevented. When expressed from a chimeric mRNA containing the atpA 5' untranslated region, cytochrome f no longer showed an assembly-dependent regulation of translation. Conversely, the level of antibiotic resistance conferred by a chimeric petA-aadA-rbcL gene was shown to depend on the state of assembly of cytochrome b6f complexes and on the accumulation of the C-terminal domain of cytochrome f. We discuss the possible ubiquity of the CES process in organellar protein biogenesis.

Animals↗
PubMed1997

Lumenal proteins involved in respiratory electron transport in the cyanobacterium Synechocystis sp. PCC6803.

Cyanobacterial thylakoids catalyze both photosynthetic and respiratory activities. In a photosystem I-less Synechocystis sp. PCC 6803 strain, electrons generated by photosystem II appear to be utilized by cytochrome oxidase. To identify the lumenal electron carriers (plastocyanin and/or cytochromes c553, c550, and possibly cM) that are involved in transfer of photosystem II-generated electrons to the terminal oxidase, deletion constructs for genes coding for these components were introduced into a photosystem I-less Synechocystis sp. PCC 6803 strain, and electron flow out of photosystem II was monitored in resulting strains through chlorophyll fluorescence yields. Loss of cytochrome c553 or plastocyanin, but not of cytochrome c550, decreased the rate of electron flow out of photosystem II. Surprisingly, cytochrome cM could not be deleted in a photosystem I-less background strain, and also a double-deletion mutant lacking both plastocyanin and cytochrome c553 could not be obtained. Cytochrome cM has some homology with the cytochrome c-binding regions of the cytochrome Caa3-type cytochrome oxidase from Bacillus spp. and Thermus thermophilus. We suggest that cytochrome cM is a component of cytochrome oxidase in cyanobacteria that serves as redox intermediate between soluble electron carriers and the cytochrome aa3 complex, and that either plastocyanin or cytochrome c553 can shuttle electrons from the cytochrome b6f complex to cytochrome cM.

Amino Acid Sequence↗
PubMed2005

Structure of the intermolecular complex between plastocyanin and cytochrome f from spinach.

In oxygenic photosynthesis, plastocyanin shuttles electrons between the membrane-bound complexes cytochrome b6f and photosystem I. The homologous complex between cytochrome f and plastocyanin, both from spinach, is the object of this study. The solution structure of the reduced spinach plastocyanin was determined using high field NMR spectroscopy, whereas the model structure of oxidized cytochrome f was obtained by homology modeling calculations and molecular dynamics. The model structure of the intermolecular complex was calculated using the program AUTODOCK, taking into account biological information obtained from mutagenesis experiments. The best electron transfer pathway from the heme group of cytochrome f to the copper ion of plastocyanin was calculated using the program HARLEM, obtaining a coupling decay value of 1.8 x 10(-4). Possible mechanisms of interaction and electron transfer between plastocyanin and cytochrome f were discussed considering the possible formation of a supercomplex that associates one cytochrome b6f, one photosystem I, and one plastocyanin.

Algorithms↗
PubMedDate not supplied

Stabilization of integral membrane proteins in aqueous solution using fluorinated surfactants.

Surfactants carrying either a hydrocarbon or a fluorocarbon alkyl chain have been synthesized. The polar head was either tris(hydroxymethyl)acrylamidomethane (THAM), telomerized THAM, or a glycosylated THAM moiety. The aqueous solubility of some of these molecules was increased by oxidizing to a sulfoxide the thioether function that associates their hydrophobic and hydrophilic moieties. In all cases, the critical micellar concentration was principally determined by the length and chemical nature of the alkyl chain. The usefulness of these surfactants in handling integral membrane proteins in solution has been examined using as test materials chloroplast thylakoid membranes and the photosynthetic complex cytochrome b6f. In keeping with earlier observations in other systems, none of the fluorinated surfactants was able to solubilize thylakoid membranes. Transfer to a solution of fluorinated surfactant of b6f complexes that had been solubilized and purified in the presence of a classical detergent usually resulted in aggregation and precipitation of the protein, while most homologous molecules with hydrocarbon chains did keep the b6f complex soluble. Two of the fluorinated surfactants, however, proved able to maintain the b6f complex water-soluble, intact, and enzymatically active. Because of their limited affinity for lipid alkyl chains and other hydrocarbon surfaces, fluorinated surfactants appear as potentially interesting tools for the study of membrane proteins that do not stand well exposure to classical detergents.

Acrylates↗
PubMed1998

Glu78, from the conserved PEWY sequence of subunit IV, has a key function in cytochrome b6f turnover.

We have investigated the structure to function relationship at the Qo site in cytochrome b6f complexes in vivo. To this end, we created site-directed mutants of Chlamydomonas reinhardtii, at position 78 in the sequence of subunit IV. The target glutamic acid, present in the highly conserved 77PEWY80 sequence, was changed to residues of different polarities which did not prevent the functional assembly of cytochrome b6f complexes. Spectroscopic analysis performed in anaerobic conditions in vivo revealed distinct alterations in cytochrome b6f function, depending on the nature of the substituted residue. The semiconservative E78D substitution, in which only the length of the side chain is reduced, retained the functional features of the wild-type configuration. The E78K and E78L substitutions caused a significant decrease, by factors of 3 and 5, respectively, in the rate of the concerted oxidation process at the Qo site without a change in the affinity of Qo for reduced plastoquinones. The E78Q and E78N substitutions modified the characteristics of cytochrome b6f turnover under repetitive flash illumination. They caused a large increase in the electrogenicity of the electron-transfer reactions through the mutated cytochrome b6f complex. This increase was specifically sensitive to the electrical component of the proton-motive force. Surprisingly, despite the larger number of charges translocated across the membrane per charge injected in the high potential chain, the reduction phase for cytochrome b6 became barely detectable in the mutants, unless inhibitors at the Qi site were present. We show that similar functional characteristics can be observed with the cytochrome b6f complex in the wild-type in anaerobic conditions, provided a single flash illumination regime is used. These observations suggest that cytochrome b6f turnover may involve a mechanism implying an extra proton pumping activity.

Amino Acid Sequence↗
PubMed1998

In vivo characterization of the electrochemical proton gradient generated in darkness in green algae and its kinetic effects on cytochrome b6f turnover.

When unicellular algal cells are placed under anaerobic conditions, a large electrochemical gradient is built in darkness across the thylakoid membranes. We have estimated, in vivo, the amplitude of the Delta pH component of this transmembrane potential and shown that the Delta pH is twice as large as the Delta Psi. The amplitude of the Delta mu tildeH+ (approximately 110-140 mV) fits well with estimations based on the ATP/ADP ratio measured in green algae under the same conditions, suggesting that an equilibrium state is established across the thylakoid membrane. Therefore, under anaerobic dark incubation of algae, the electrochemical transmembrane potential is determined only by the cellular ATP content. The existence of this Delta mu tildeH+ is expected to result in a constitutive amount of activated CFo-CF1 ATPase, thereby facilitating ATP synthesis under low light intensity illumination. We report also on the effects of this dark-existing electrochemical gradient on the cytochrome b6f complex turnover kinetics. We show that they are largely slowed by the presence of this electrochemical transmembrane potential. The pH component is mainly responsible for the kinetic slowing down of cytochrome b6f complex turnover, despite the fact that electrogenicity is associated with the reactions taking place within this complex. Therefore, in vivo, owing to the low lumenal pH, the oxidation of plastoquinol at the Qo site is limiting the turnover of the cytochrome b6f complex in the presence of the Delta pH, while in its absence the oxidation rate of the b6 hemes becomes rate-limiting.

Adenosine Triphosphate↗
PubMed2002

Involvement of state transitions in the switch between linear and cyclic electron flow in Chlamydomonas reinhardtii.

The energetic metabolism of photosynthetic organisms is profoundly influenced by state transitions and cyclic electron flow around photosystem I. The former involve a reversible redistribution of the light-harvesting antenna between photosystem I and photosystem II and optimize light energy utilization in photosynthesis whereas the latter process modulates the photosynthetic yield. We have used the wild-type and three mutant strains of the green alga Chlamydomonas reinhardtii--locked in state I (stt7), lacking the photosystem II outer antennae (bf4) or accumulating low amounts of cytochrome b6f complex (A-AUU)--and measured electron flow though the cytochrome b6f complex, oxygen evolution rates and fluorescence emission during state transitions. The results demonstrate that the transition from state 1 to state 2 induces a switch from linear to cyclic electron flow in this alga and reveal a strict cause-effect relationship between the redistribution of antenna complexes during state transitions and the onset of cyclic electron flow.

Animals↗
PubMed1996

The heme redox center of chloroplast cytochrome f is linked to a buried five-water chain.

The crystal structure of the 252-residue lumen-side domain of reduced cytochrome f, a subunit of the proton-pumping integral cytochrome b6f complex of oxygenic photosynthetic membranes, was determined to a resolution of 1.96 A from crystals cooled to -35 degrees. The model was refined to an R-factor of 15.8% with a 0.013-A RMS deviation of bond lengths from ideality. Compared to the structure of cytochrome f at 20 degrees, the structure at -35 degrees has a small change in relative orientation of the two folding domains and significantly lower isotropic temperature factors for protein atoms. The structure revealed an L-shaped array of five buried water molecules that extend in two directions from the N delta 1 of the heme ligand His 25. The longer branch extends 11 A within the large domain, toward Lys 66 in the prominent basic patch at the top of the large domain, which has been implicated in the interaction with the electron acceptor, plastocyanin. The water sites are highly occupied, and their temperature factors are comparable to those of protein atoms. Virtually all residues that form hydrogen bonds with the water chain are invariant among 13 known cytochrome f sequences. The water chain has many features that optimize it as a proton wire, including insulation from the protein medium. It is suggested that this chain may function as the lumen-side exit port for proton translocation by the cytochrome b6f complex.

Chloroplasts↗
PubMed1997

Mutations of cytochrome b6 in Chlamydomonas reinhardtii disclose the functional significance for a proline to leucine conversion by petB editing in maize and tobacco.

We have introduced a proline codon in place of a leucine codon at position 204 of the petB gene of Chlamydomonas reinhardtii. This gene modification mimics the presence of proline codons at the same position in the petB genes of maize and tobacco, which are subsequently edited to leucine codons at the RNA level. Following transformation, we observed no editing at this position in C. reinhardtii, independent of the type of proline codon we have used: the CCA codon, edited in maize, or a CCT codon. Strains carrying the introduced mutation were non phototrophic and displayed a block in photosynthetic electron transfer, consistent with a lack of cytochrome b6f activity. Thus the presence of a proline residue at position 204 in cytochrome b6 is detrimental to photosynthesis. We show that the mutant phenotype arose from a defective assembly of cytochrome b6f complexes and not from altered electron transfer properties in the assembled protein complex. Biochemical comparison of the proline-containing transformants with a cytochrome b6 mutant deficient in heme-attachment indicates that their primary defect is at the level of assembly of apocytochrome b6 with the bh heme, thereby preventing assembly of the whole cytochrome b6f complex.

Animals↗
PubMed1997

Biological identity and diversity in photosynthesis and respiration: structure of the lumen-side domain of the chloroplast Rieske protein.

BACKGROUND: The cytochrome b6f complex functions in oxygenic photosynthesis as an integral membrane protein complex that mediates coupled electron transfer and proton translocation. The Rieske [2Fe-2S] protein subunit of the complex functions at the electropositive (p) membrane interface as the electron acceptor for plastoquinol and donor for the cytochrome f subunit, and may have a dynamic role in catalyzing electron and proton transfer at the membrane interface. There are significant structure/function similarities to the cytochrome bc1 complex of the respiratory chain. RESULTS: The 1.83 A crystal structure of a 139-residue C-terminal fragment of the Rieske [2Fe-2S] protein, derived from the cytochrome b6f complex of spinach chloroplasts, has been solved by multiwavelength anomalous diffraction. The structure of the fragment comprises two domains: a small 'cluster-binding' subdomain and a large subdomain. The [2Fe-2S] cluster-binding subdomains of the chloroplast and mitochondrial Rieske proteins are virtually identical, whereas the large subdomains are strikingly different despite a common folding topology. A structure-based sequence alignment of the b6f and bc1 groups of Rieske soluble domains is presented. CONCLUSIONS: The segregation of structural conservation and divergence in the cluster-binding and large subdomains of the Rieske protein correlates with the overall relatedness of the cytochrome b6f and bc1 complexes, in which redox domains in the aqueous p phase are dissimilar and those within the membrane are similar. Distinct sequences and surface charge distributions among Rieske large subdomains may provide a signature for interaction with the p-side oxidant protein and for the pH of the intraorganelle compartment.

Amino Acid Sequence↗
PubMed1994

Crystal structure of chloroplast cytochrome f reveals a novel cytochrome fold and unexpected heme ligation.

BACKGROUND: Cytochrome f is the high potential electron acceptor of the chloroplast cytochrome b6f complex, and is the electron donor to plastocyanin. The 285-residue cytochrome f subunit is anchored in the thylakoid membrane of the chloroplast by a single membrane-spanning segment near the carboxyl terminus. A soluble redox-active 252-residue lumen-side polypeptide with native spectroscopic and redox properties, missing the membrane anchor and carboxyl terminus, was purified from turnip chloroplasts for structural studies. RESULTS: The crystal structure of cytochrome f, determined to 2.3 A resolution, has several unexpected features. The 252-residue polypeptide is organized into one large and one small domain. The larger heme-binding domain is strikingly different from known structures of other c-type cytochromes and has the same fold as the type III domain of the animal protein, fibronectin. Cytochrome f binds heme with an unprecedented axial heme iron ligand: the amino terminus of the polypeptide. CONCLUSION: The first atomic structure of a subunit of either the cytochrome b6f complex or of the related cytochrome bc1 complex has been obtained. The structure of cytochrome f allows prediction of the approximate docking site of plastocyanin and should allow systematic studies of the mechanism of intra- and inter-protein electron transfer between the cytochrome heme and plastocyanin copper, which are approximately isopotential. The unprecedented axial heme iron ligand also provides information on the sequence of events (i.e. cleavage of signal peptide and ligation of heme) associated with translocation of the cytochrome across the membrane and its subsequent folding.

Amino Acid Sequence↗
PubMedDate not supplied

Membrane association of the Rieske iron-sulfur protein.

The mode of membrane attachment of the Rieske iron-sulfur proteins from cytochrome b6f complex of pea thylakoids and from cytochrome bc1 complex of yeast mitochondria has been studied using biochemical approaches. The relative sensitivity of the Rieske protein to trypsin in the thylakoid membrane shows that all trypsin sites of the Rieske protein are on the lumen side of the thylakoid membrane. In contrast to cytochrome f the chloroplast Rieske protein was extracted from thylakoids using chaotropic agents (NaSCN, urea), an alkaline pH and relatively low concentrations of Trinon X-100. The cytochrome bc1 complex Rieske protein from mitochondrial membranes of yeast was also released by NaSCN and alkaline treatment. The results presented here led us to the conclusion that the mitochondrial and chloroplast Rieske proteins are extrinsic and that their association with the rest of the complex involves hydrophobic interactions.

Antibodies↗
PubMed1997

Ccs1, a nuclear gene required for the post-translational assembly of chloroplast c-type cytochromes.

Nuclear genes play important regulatory roles in the biogenesis of the photosynthetic apparatus of eukaryotic cells by encoding factors that control steps ranging from chloroplast gene transcription to post-translational processes. However, the identities of these genes and the mechanisms by which they govern these processes are largely unknown. By using glass bead-mediated transformation to generate insertional mutations in the nuclear genome of Chlamydomonas reinhardtii, we have generated four mutants that are defective in the accumulation of the cytochrome b6f complex. One of them, strain abf3, also fails to accumulate holocytochrome c6. We have isolated a gene, Ccs1, from a C. reinhardtii genomic library that complements both the cytochrome b6f and cytochrome c6 deficiencies in abf3. The predicted protein product displays significant identity with Ycf44 from the brown alga Odontella sinensis, the red alga Porphyra purpurea, and the cyanobacterium Synechocystis strain PCC 6803 (25-33% identity). In addition, we note limited sequence similarity with ResB of Bacillus subtilis and an open reading frame in a homologous operon in Mycobacterium leprae (11-12% identity). On the basis of the pleiotropic c-type cytochrome deficiency in the ccs1 mutant, the predicted plastid localization of the protein, and its relationship to candidate cytochrome biosynthesis proteins in Gram-positive bacteria, we conclude that Ccs1 encodes a protein that is required for chloroplast c-type holocytochrome formation.

Amino Acid Sequence↗
PubMed1999

Mutants of Chlamydomonas: tools to study thylakoid membrane structure, function and biogenesis.

The unicellular green alga Chlamydomonas reinhardtii is a model system for the study of photosynthesis and chloroplast biogenesis. C. reinhardtii has a photosynthesis apparatus similar to that of higher plants and it grows at rapid rate (generation time about 8 h). It is a facultative phototroph, which allows the isolation of mutants unable to perform photosynthesis and its sexual cycle allows a variety of genetic studies. Transformation of the nucleus and chloroplast genomes is easily performed. Gene transformation occurs mainly by homologous recombination in the chloroplast and heterologous recombination in the nucleus. Mutants are precious tools for studies of thylakoid membrane structure, photosynthetic function and assembly. Photosynthesis mutants affected in the biogenesis of a subunit of a protein complex usually lack the entire complex; this pleiotropic effect has been used in the identification of the other subunits, in the attribution of spectroscopic signals and also as a 'genetic cleaning' process which facilitates both protein complex purification, absorption spectroscopy studies or freeze-fracture analysis. The cytochrome b6f complex is not required for the growth of C. reinhardtii, unlike the case of photosynthetic prokaryotes in which the cytochrome complex is also part of the respiratory chain, and can be uniquely studied in Chlamydomonas by genetic approaches. We describe in greater detail the use of Chlamydomonas mutants in the study of this complex.

Animals↗
PubMed2001

Kinetic modeling of the photosynthetic electron transport chain.

A simulation model of the photosynthetic electron transport chain operating under steady state conditions is presented. The model enables the calculation of (1) the rates of electron transport and transmembrane proton translocation, (2) the proton/electron stoichiometry, (3) the number of electrons stored in the different redox centers and (4) the stationary transmembrane pH difference. Light intensity and proton permeability of the thylakoid membrane are varied in order to compare the predictions of the model with experimental data. The routes of electron transport and proton translocation are simulated by two coupled arithmetic loops. The first one represents the sequence of reaction steps making up the linear electron transport chain and the Q-cycle. This loop yields the electron flow rate and the proton/electron ratio. The second loop balances the H+ fluxes and yields the internal H+ concentration. The bifurcation of the electron transport pathways at the stage of plastoquinol oxidation is obligatory. The first electron enters always the linear branch and is transferred to photosystem I. The electron of the remaining semiquinone can enter the Q-cycle or, alternatively, the semiquinone can be lost from the cytochrome b6f complex. The competition between these two reactions explains the experimentally observed variability of the proton/electron ratio. We also investigated additional model variants, where the variation of the proton/electron stoichiometry is attributed to other loss reactions within the cytochrome b6f complex. However, the semiquinone detachment seems to be the best candidate for a satisfactory description of the experimental data. Additional calculations were done in order to assess the effects of the movement of the Rieske protein on linear electron transport; it was found that this conformational change does not limit the electron transport rate, if it occurs with a time constant of at least 1000 s(-1).

Computer Simulation↗
PubMed2004

Cyclic electron flow around photosystem I is essential for photosynthesis.

Photosynthesis provides at least two routes through which light energy can be used to generate a proton gradient across the thylakoid membrane of chloroplasts, which is subsequently used to synthesize ATP. In the first route, electrons released from water in photosystem II (PSII) are eventually transferred to NADP+ by way of photosystem I (PSI). This linear electron flow is driven by two photochemical reactions that function in series. The cytochrome b6f complex mediates electron transport between the two photosystems and generates the proton gradient (DeltapH). In the second route, driven solely by PSI, electrons can be recycled from either reduced ferredoxin or NADPH to plastoquinone, and subsequently to the cytochrome b6f complex. Such cyclic flow generates DeltapH and thus ATP without the accumulation of reduced species. Whereas linear flow from water to NADP+ is commonly used to explain the function of the light-dependent reactions of photosynthesis, the role of cyclic flow is less clear. In higher plants cyclic flow consists of two partially redundant pathways. Here we have constructed mutants in Arabidopsis thaliana in which both PSI cyclic pathways are impaired, and present evidence that cyclic flow is essential for efficient photosynthesis.

Adenosine Triphosphate↗
PubMed1999

Amino acid sequence, crystallization and structure determination of reduced and oxidized cytochrome c6 from the green alga Scenedesmus obliquus.

Cytochrome c6from the unicellular green alga Scenedesmus obliquus was sequenced, crystallized in its reduced and oxidized state and the three-dimensional structure of the protein in both redox states was determined by X-ray crystallography. Reduced cytochrome c6crystallized as a monomer in the space group P 21212, whereas the oxidized protein crystallized as a dimer in the space group P 3121. The structures were solved by molecular replacement and refined to 1. 9 and 2.0 A, respectively. Comparison of the structures of both redox states revealed only slight differences on the protein surface, whereas a distortion along the axis between the heme iron and its coordinating Met61 residue was observed. No redox-dependent movement of internal water molecules could be detected. The high degree of similarity of the surfaces and charge distributions of both redox states, as well as the dimerization of cytochrome c6as observed in the oxidized crystal, is discussed with respect to its biological relevance and its implications for the reaction mechanisms between cytochrome c6and its redox partners. The dimer of oxidized cytochrome c6may represent a molecular structure occurring in a binary complex with cytochrome b6f. This assembly might be required for the correct orientation of cytochrome c6with respect to its redox partner cytochrome b6f, facilitating the electron transfer within the complex. If the dimerization is not redox-dependent in vivo, the almost identical surfaces of both redox states do not support a long range differentiation between reduced and oxidized cyt c6, i.e. a random collision model for the formation of an electron transfer complex must be assumed.

Amino Acid Sequence↗
PubMed1987

Sulfide-dependent photosynthetic electron flow coupled to proton translocation in thylakoids of the cyanobacterium Oscillatoria limnetica.

Light-induced proton translocation coupled to sulfide-dependent electron transport has been studied in isolated thylakoids of the cyanobacterium Oscillatoria limnetica. The thylakoids are obtained by osmotic shock of washed spheroplasts, prepared with glycine-betaine as the osmotic stabilizer. 13C NMR studies suggests that betaine is the major osmoregulator in O. limnetica. Thylakoid preparations obtained from both sulfide-induced anoxygenic cells and noninduced oxygenic cells are capable of proton pumping coupled to phenazinemethosulfate-mediated cyclic electron flow. However, only in the induced thylakoids can sulfide-dependent proton gradient (delta pH) formation be measured, using either NADP or methyl viologen as the terminal acceptor. Sulfide-dependent delta pH formation correlates with a high-affinity electron donation site (apparent Km 44 microM at pH 7.9). This site is not lost upon washing of the thylakoids. In addition, both sulfide-dependent electron transport and delta pH formation are sensitive to inhibitors of the cytochrome b6f complex such as 2-n-nonyl-4-hydroxyquinoline-N-oxide, 2,4-dinitrophenyl ether of 2-iodo-4-nitrothymol, or stigmatellin. Sulfide-dependent NADP photoreduction of low affinity (which does not saturate by as much as 7 mM sulfide) is detected in both induced and noninduced thylakoids, but this activity is insensitive to the inhibitors and is not coupled to proton transport. It is suggested that the adaptation of O. limnetica to anoxygenic photosynthesis involves the induction of a thylakoid factor(s) which creates a high-affinity site for sulfide, and the transfer of its electrons via the cytochrome b6f complex, coupled to proton translocation.

Cyanobacteria↗