Cloning and correct expression in E. coli of the petJ gene encoding cytochrome c6 from Synechocystis 6803

Designing a modified clostridial 2[4Fe-4S] ferredoxin as a redox coupler to directly link photosystem I with a Pt nanoparticle

A methodology previously developed in our laboratory utilized an aliphatic hydrocarbon terminated by thiol groups to tether two redox proteins, i.e., the [4Fe-4S] cluster F_B of photosystem I (PS I) and the distal [4Fe-4S] cluster of a [FeFe]-hydrogenase, to create a biohybrid dihydrogen-generating complex. These studies guided the design of a modified 2[4Fe-4S] cluster ferredoxin from Clostridium pasteurianum (CpFd) containing two externally facing cysteine residues in close proximity to each [4Fe-4S] cluster that replaces the aliphatic hydrocarbon dithiol tether. The advantage of using a protein is the potential to create a coupled dihydrogen-generating system in vivo. The wild-type CpFd_WT and variants CpFd_S11C/D40C, CpFd_P20C/P49C, CpFd_D7S/D36S, CpFd_{S11C/D40C/D7S/D36S} and CpFd_{P20C/P49C/D7S/D36S} were expressed in Escherichia coli and found to contain ~ 8 Fe and ~ 8 S atoms. The absorption spectra of the wild-type and CpFd variants displayed a peak centered at ~ 390 nm characteristic of a S → Fe charge transfer band that diminishes upon reduction with Na-dithionite. Low-temperature X-band EPR studies of the Na-dithionite-reduced wild-type and CpFd variants showed a complex spectrum indicative of two magnetically coupled [4Fe-4S]¹⁺ clusters. EPR-monitored redox titrations of CpFd_WT, CpFd_D7S/D36S, CpFd_S11C/D40C, CpFd_P20C/P49C, CpFd_{S11C/D40C/D7S/D36S} and CpFd_{P20C/P49C/D7S/D36S} revealed redox potentials of - 412 ± 8 mV, - 395 ± 4 mV, - 408 ± 7 mV, - 426 ± 11 mV, - 384 ± 4 mV and - 423 ± 4 mV, respectively. The in vitro PS I-CpFd_{S11C/D40C/D7S/D36S}-Pt nanoparticle complex was the highest performer, generating dihydrogen at a rate of 3.25 μmol H₂ mg Chl^-1 h^-1 or 278.8 mol H₂ mol PS I^-1 h^-1 under continuous illumination.

Plasmon-induced absorption of blind chlorophylls in photosynthetic proteins assembled on silver nanowires

We demonstrate that controlled assembly of eukaryotic photosystem I with its associated light harvesting antenna complex (PSI-LHCI) on plasmonically active silver nanowires (AgNWs) substantially improves the optical functionality of such a novel biohybrid nanostructure. By comparing fluorescence intensities measured for PSI-LHCI complex randomly oriented on AgNWs and the results obtained for the PSI-LHCI/cytochrome c₅₅₃ (cyt c₅₅₃) bioconjugate with AgNWs we conclude that the specific binding of photosynthetic complexes with defined uniform orientation yields selective excitation of a pool of chlorophyll (Chl) molecules that are otherwise almost non-absorbing. This is remarkable, as this study shows for the first time that plasmonic excitations in metallic nanostructures can not only be used to enhance native absorption of photosynthetic pigments, but also - by employing cyt c₅₅₃ as the conjugation cofactor - to activate the specific Chl pools as the absorbing sites only when the uniform and well-defined orientation of PSI-LHCI with respect to plasmonic nanostructures is achieved. As absorption of PSI alone is comparatively low, our approach lends itself as an innovative approach to outperform the reported-to-date biohybrid devices with respect to solar energy conversion.

Biofunctionalisation of p-doped silicon with cytochrome c553minimises charge recombination and enhances photovoltaic performance of the all-solid-state photosystem I-based biophotoelectrode

Passivation of p-doped silicon substrate was achieved by its biofunctionalisation with hexahistidine-tagged cytochrome c₅₅₃, a soluble electroactive photosynthetic protein responsible for electron donation to photooxidised photosystem I.

Quantum yield measurements of light-induced H₂ generation in a photosystem I-[FeFe]-H₂ase nanoconstruct

The quantum yield for light-induced H2 generation was measured for a previously optimized bio-hybrid cytochrome c 6-crosslinked PSI(C13G)-1,8-octanedithiol-[FeFe]-H2ase(C97G) (PSI-H2ase) nanoconstruct. The theoretical quantum yield for the PSI-H2ase nanoconstruct is 0.50 molecules of H2 per photon absorbed, which equates to a requirement of two photons per H2 generated. Illumination of the PSI-H2ase nanoconstruct with visible light between 400 and 700 nm resulted in an average quantum yield of 0.10-0.15 molecules of H2 per photon absorbed, which equates to a requirement of 6.7-10 photons per H2 generated. A possible reason for the difference between the theoretical and experimental quantum yield is the occurrence of non-productive PSI(C13G)-1,8-octanedithiol-PSIC13G (PSI-PSI) conjugates, which would absorb light without generating H2. Assuming the thiol-Fe coupling is equally efficient at producing PSI-PSI conjugates as well as in producing PSI-H2ase nanoconstructs, the theoretical quantum yield would decrease to 0.167 molecules of H2 per photon absorbed, which equates to 6 photons per H2 generated. This value is close to the range of measured values in the current study. A strategy that purifies the PSI-H2ase nanoconstructs from the unproductive PSI-PSI conjugates or that incorporates different chemistries on the PSI and [FeFe]-H2ase enzyme sites could potentially allow the PSI-H2ase nanoconstruct to approach the expected theoretical quantum yield for light-induced H2 generation.

Respiratory complexes III and IV can each bind two molecules of cytochrome c at low ionic strength

The transient interactions of respiratory cytochrome c with complexes III and IV is herein investigated by using heterologous proteins, namely human cytochrome c, the soluble domain of plant cytochrome c1 and bovine cytochrome c oxidase. The binding molecular mechanisms of the resulting cross-complexes have been analyzed by Nuclear Magnetic Resonance and Isothermal Titration Calorimetry. Our data reveal that the two cytochrome c-involving adducts possess a 2:1 stoichiometry - that is, two cytochrome c molecules per adduct - at low ionic strength. We conclude that such extra binding sites at the surfaces of complexes III and IV can facilitate the turnover and sliding of cytochrome c molecules and, therefore, the electron transfer within respiratory supercomplexes.

The evolution of photosystem I in light of phage-encoded reaction centres

Recent structural determinations and metagenomic studies shed light on the evolution of photosystem I (PSI) from the homodimeric reaction centre of primitive bacteria to plant PSI at the top of the evolutionary development. The evolutionary scenario of over 3.5 billion years reveals an increase in the complexity of PSI. This phenomenon of ever-increasing complexity is common to all evolutionary processes that in their advanced stages are highly dependent on fine-tuning of regulatory processes. On the other hand, the recently discovered virus-encoded PSI complexes contain a minimal number of subunits. This may reflect the unique selection scenarios associated with viral replication. It may be beneficial for future engineering of productive processes to utilize 'primitive' complexes that disregard the cellular regulatory processes and to avoid those regulatory constraints when our goal is to divert the process from its original route. In this article, we discuss the evolutionary forces that act on viral reaction centres and the role of the virus-carried photosynthetic genes in the evolution of photosynthesis.

Solar hydrogen-producing bionanodevice outperforms natural photosynthesis

Although a number of solar biohydrogen systems employing photosystem I (PSI) have been developed, few attain the electron transfer throughput of oxygenic photosynthesis. We have optimized a biological/organic nanoconstruct that directly tethers F(B), the terminal [4Fe-4S] cluster of PSI from Synechococcus sp. PCC 7002, to the distal [4Fe-4S] cluster of the [FeFe]-hydrogenase (H(2)ase) from Clostridium acetobutylicum. On illumination, the PSI-[FeFe]-H(2)ase nanoconstruct evolves H(2) at a rate of 2,200 ± 460 μmol mg chlorophyll(-1) h(-1), which is equivalent to 105 ± 22 e(-)PSI(-1) s(-1). Cyanobacteria evolve O(2) at a rate of approximately 400 μmol mg chlorophyll(-1) h(-1), which is equivalent to 47 e(-)PSI(-1) s(-1), given a PSI to photosystem II ratio of 1.8. The greater than twofold electron throughput by this hybrid biological/organic nanoconstruct over in vivo oxygenic photosynthesis validates the concept of tethering proteins through their redox cofactors to overcome diffusion-based rate limitations on electron transfer.

Understanding of the binding interface between PsaC and the PsaA/PsaB heterodimer in photosystem I

The PsaC subunit of Photosystem I (PS I) is tightly bound to the PsaA/PsaB heterodimer via an extensive network of ionic and hydrogen bonds. To improve our understanding of the design of the PsaC-PsaA/PsaB binding interface, variants of PsaC were generated, each lacking a key binding contact with the PsaA/PsaB heterodimer. The characteristics of the reconstituted, variant PS I complexes were monitored by time-resolved optical spectroscopy, low-temperature EPR spectroscopy, and electron transfer throughput measurements. In the absence of the ionic bond forming contacts R52(C) or R65(C), a markedly slower charge recombination occurs between P(700)(+) and [F(A)/F(B)](-). The addition of PsaD leads to the restoration of native recombination kinetics in a fraction of the PS I complexes reconstituted with R52A(C), but not with R65A(C). Contrary to expectation, the absence of Y80(C), which forms two symmetry-breaking H-bonds with PsaB, does not significantly affect the binding of PsaC as judged by the rate of charge recombination between P(700)(+) and [F(A)/F(B)](-). However, the removal of the entire C-terminus results in a dramatic decrease in the rate of charge recombination. Low-temperature EPR spectra of the variant PS I complexes indicate that the magnetic environments of F(A) and F(B) are altered when compared to that of native PS I. The slowing of the rate of charge recombination in the variant PS I complexes could be due to an increase in the distance between F(X) and F(A)/F(B) as the result of non-native binding or to an altered reduction potential of the iron-sulfur clusters, which would result in a different rate of thermalization up the electron acceptor chain. The most significant finding is that the variant PS I complexes support lower rates of light-induced flavodoxin reduction and that the rates deteriorate rapidly on exposure to dioxygen due to the degradation of F(A) and F(B). We suggest that the extensive set of ionic bonds and H-bonds between PsaC and the PsaA/PsaB heterodimer has evolved to ensure an exceedingly tight binding interface, thereby rendering the [4Fe-4S] clusters in PsaC inaccessible to dioxygen at the onset of oxygenic photosynthesis.

Kinetics of interprotein electron transfer between cytochromec6and the soluble CuAdomain of cyanobacterial cytochromecoxidase

Cytochrome c6 is a soluble metalloprotein located in the periplasmic space and the thylakoid lumen of many cyanobacteria and is known to carry electrons from cytochrome b6f to photosystem I. The CuA domain of cytochrome c oxidase, the terminal enzyme which catalyzes the four-electron reduction of molecular oxygen in the respiratory chains of mitochondria and many bacteria, also has a periplasmic location. In order to test whether cytochrome c6 could also function as a donor for cytochrome c oxidase, we investigated the kinetics of the electron transfer between recombinant cytochrome c6 (produced in high yield in Escherichia coli by coexpressing the maturation proteins encoded by the ccmA-H gene cluster) and the recombinant soluble CuA domain (i.e., the donor binding and electron entry site) of subunit II of cytochrome c oxidase from Synechocystis PCC 6803. The forward and the reverse electron transfer reactions were studied by the stopped-flow technique and yielded apparent bimolecular rate constants of (3.3 +/- 0.3) x 10(5) M(-1) s(-1) and (3.9 +/- 0.1) x 10(6) M(-1) s(-1), respectively, in 5 mM potassium phosphate buffer, pH 7, containing 20 mM potassium chloride and 25 degrees C. This corresponds to an equilibrium constant Keq of 0.085 in the physiological direction (DeltarG'0 = 6.1 kJ/mol). The reduction of the CuA fragment by cytochrome c6 is almost independent on ionic strength, which is in contrast to the reaction of the CuA domain with horse heart cytochrome c, which decreases with increasing ionic strength. The findings are discussed with respect to the potential role of cytochrome c6 as mobile electron carrier in both cyanobacterial electron transport pathways.

Production and preliminary characterization of a recombinant triheme cytochrome c(7) from Geobacter sulfurreducens in Escherichia coli

Multiheme cytochromes c have been found in a number of sulfate- and metal ion-reducing bacteria. Geobacter sulfurreducens is one of a family of microorganisms that oxidize organic compounds, with Fe(III) oxide as the terminal electron acceptor. A triheme 9.6 kDa cytochrome c(7) from G. sulfurreducens is a part of the metal ion reduction pathway. We cloned the gene for cytochrome c(7) and expressed it in Escherichia coli together with the cytochrome c maturation gene cluster, ccmABCDEFGH, on a separate plasmid. We designed two constructs, with and without an N-terminal His-tag. The untagged version provided a good yield (up to 6 mg/l of aerobic culture) of the fully matured protein, with all three hemes attached, while the N-terminal His-tag appeared to be detrimental for proper heme incorporation. The recombinant protein (untagged) is properly folded, it has the same molecular weight and displays the same absorption spectra, both in reduced and in oxidized forms, as the protein isolated from G. sulfurreducens and it is capable of reducing metal ions in vitro. The shape parameters for the recombinant cytochrome c(7) determined by small angle X-ray scattering are in good agreement with the ones calculated from a homologous cytochrome c(7) of known structure.

A comparative structural and functional analysis of cytochrome cM cytochrome c6 and plastocyanin from the cyanobacterium Synechocystis sp. PCC 6803

Cytochrome cM is a new c-class photosynthetic haem protein whose physiological role is still unknown. It has been proposed previously that cytochrome cM can replace cytochrome c6 and plastocyanin in transferring electrons between the two membrane complexes cytochrome b6-f and photosystem I in organisms growing under stress conditions. The experimental evidence herein provided allows us to discard such a hypothesis. We report a procedure to overexpress cytochrome cM from the cyanobacterium Synechocystis sp. PCC 6803 in Escherichia coli cells in mg quantities. This has allowed us to perform a comparative laser flash-induced kinetic analysis of photosystem I reduction by the three metalloproteins from Synechocystis. The bimolecular rate constant for the overall reaction is up to 100 times lower with cytochrome cM than with cytochrome c6 or plastocyanin. In addition, the redox potential value and surface electrostatic potential distribution of cytochrome cM are quite different from those of cytochrome c6 and plastocyanin. These findings strongly indicate that cytochrome cM cannot be recognised by and interact with the same redox partners as the other two metalloproteins.

Rapid electron transfer to photosystem I and unusual spectral features of cytochrome c(6) in Synechococcus sp. PCC 7002 in vivo

Cytochrome c(6) donates electrons to photosystem I (PS I) in Synechococcus sp. PCC 7002. In this work, we provide evidence for rapid electron transfer (t(1/2) = 3 micros) from cytochrome c(6) to PS I in this cyanobacterium in vivo, indicating prefixation of the reduced donor protein to the photosystem. We have investigated the cytochrome c(6)-PS I interaction by laser flash-induced spectroscopy of intact and broken cells and by redox titrations of membrane and supernatant fractions. Redox studies revealed the expected membrane-bound cytochrome f, b(6), and b(559) species and two soluble cytochromes with alpha-band absorption peaks of 551 and 553 nm and midpoint potentials of -100 and 370 mV, respectively. The characteristics and the symmetrical alpha-band spectrum of the latter correspond to typical cyanobacterial cytochrome c(6) proteins. Rapid oxidation of cytochrome c(6) by PS I in vivo results in a unique, asymmetric oxidation spectrum, which differs significantly from the spectra obtained for cytochrome c(6) in solution. The basis for the unusual cytochrome c(6) spectrum and possible mechanisms of cytochrome c(6) fixation to PS I are discussed. The occurrence of rapid electron transfer to PS I in cyanobacteria suggests that this mechanism evolved before the endosymbiotic origin of chloroplasts. Its selective advantage may lie in protection against photo-oxidative damage as shown for Chlamydomonas.

High yield of Methylophilus methylotrophus cytochrome c by coexpression with cytochrome c maturation gene cluster from Escherichia coli

Heterologous expression of c-type cytochromes in the periplasm of Escherichia coli often results in low soluble product yield, apoprotein formation, or protein degradation. We have expressed cytochrome c from Methylophilus methylotrophus in E. coli by coexpression of the gene encoding the cytochrome (cycA) with the host-specific cytochrome c maturation elements, within the ccmA-H gene cluster. Aerobic cultures produced up to 10 mg holoprotein per liter after induction with IPTG. In the absence of the maturation factors E. coli failed to produce a stable haem protein. Cytochrome c" isolated from the natural host was compared with the recombinant protein. No structural differences were detected using SDS-PAGE, UV-Visible spectroscopy, differential scanning calorimetry, and (1)H-NMR spectroscopy. The success in expressing the mature cytochrome c in E. coli allows the engineering of the cycA gene by site-directed mutagenesis thereby providing an ideal method for producing mutant protein for studying the structure/function relationship.

Ferredoxin-dependent iron-sulfur flavoprotein glutamate synthase (GlsF) from the Cyanobacterium synechocystis sp. PCC 6803: expression and assembly in Escherichia coli

The unicellular cyanobacterium Synechocystis sp. PCC 6803 contains two different glutamate synthases whose genes, gltB and glsF (previously known as gltS), have been cloned (F. Navarro et al., 1995, Plant Mol. Biol. 27, 753-767). The glsF gene has been expressed in the glutamate auxotrophic Escherichia coli strain CLR207 RecA, but the corresponding protein does not complement the auxotrophy. The transformed strain showed ferredoxin-dependent glutamate synthase (Fd-GOGAT) activity, demonstrating the capability of E. coli for providing and correctly assembling both the iron-sulfur center and the flavin cofactor of the enzyme. Fd-GOGAT (GlsF) is correctly cleaved at Cys37 to form the mature enzyme in E. coli, as occurs with the large subunit of its own NADPH-GOGAT. The recombinant Fd-GOGAT has been purified to electrophoretic homogeneity, using as the main purification step a ferredoxin-affinity chromatography. The pure enzyme, with a molecular mass of about 180 kDa, shows an absorption spectrum characteristic of iron-sulfur flavoproteins. The analyses of the prosthetic groups indicate that Fd-GOGAT contains only one FMN, but no FAD, and one [3Fe-4S](+,0) cluster per molecule. Oxidation-reduction titration, using absorbance changes of the FMN group in the visible region, gave a midpoint redox potential of -200 +/- 25 mV at pH 7.5. The recombinant enzyme is strictly ferredoxin-dependent and shows apparent K(M) values similar to those of the native Synechocystis protein: 4.5 vs 3.5 microM, 2.2 vs 2.5 mM, and 0.6 vs 0.5 mM for ferredoxin, glutamine, and 2-oxoglutarate, respectively. The addition of the reductant dithionite to the enzyme resulted in the loss of the absorption peak at 436 nm, characteristic of oxidized flavins, which was restored by the anaerobic addition of 2-oxoglutarate, in the presence of glutamine.

Expression and characterization of recombinant Rhodocyclus tenuis high potential iron-sulfur protein

The high potential iron-sulfur protein (HiPIP) from Rhodocyclus tenuis strain 2761 has been overproduced in Escherichia coli from its structural gene, purified to apparent homogeneity, and then characterized by an array of methods. UV-visible spectra of the reduced and oxidized recombinant protein were similar to those of the native protein. EPR of the oxidized protein shows g values of 2. 11, 2.03, and 2.03. ESI-MS gave a mass difference of 350 Da between the holoprotein and acid-treated protein, consistent with incorporation of a [Fe(4)S(4)] cluster in the holoprotein. The observed mass of the apoprotein was 6296.6 Da compared to the expected average molecular mass of 6297.2 Da of the apoprotein. The reduction potential was determined using cyclic and square-wave voltammetry to be 321 and 314 mV versus NHE, respectively. All the observed properties of the recombinant protein parallel those of the native protein or those of native HiPIPs in general, indicating correct folding and incorporation of the iron-sulfur cluster.

Accumulation of pre-apocytochrome f in a Synechocystis sp. PCC 6803 mutant impaired in cytochrome c maturation

Cytochrome c maturation involves heme transport and covalent attachment of heme to the apoprotein. The 5' end of the ccsB gene, which is involved in the maturation process and resembles the ccs1 gene from Chlamydomonas reinhardtii, was replaced by a chloramphenicol resistance cartridge in the cyanobacterium Synechocystis sp. PCC 6803. The resulting Delta(M1-A24) mutant lacking the first 24 ccsB codons grew only under anaerobic conditions. The mutant retained about 20% of the wild-type amount of processed cytochrome f with heme attached, apparently assembled in a functional cytochrome b(6)f complex. Moreover, the mutant accumulated unprocessed apocytochrome f in its membrane fraction. A pseudorevertant was isolated that regained the ability to grow under aerobic conditions. The locus of the second-site mutation was mapped to ccsB, and the mutation resulted in the formation of a new potential start codon in the intergenic region, between the chloramphenicol resistance marker and ccsB, in frame with the remaining part of ccsB. In this pseudorevertant the amount of holocyt f increased, whereas that of unprocessed apocytochrome f decreased. We suggest that the original deletion mutant Delta(M1-A24) expresses an N-terminally truncated version of the protein. The stable accumulation of unprocessed apocytochrome f in membranes of the Delta(M1-A24) mutant may be explained by its association with truncated and only partially functional CcsB protein resulting in protection from degradation. Our attempt to delete the first 244 codons of ccsB in Synechocystis sp. PCC 6803 was not successful, suggesting that this would lead to a lack of functional cytochrome b(6)f complex. The results suggest that the CcsB protein is an apocytochrome chaperone, which together with CcsA may constitute part of cytochrome c lyase.

Extinction coefficients and midpoint potentials of cytochrome c(6) from the cyanobacteria Arthrospira maxima, Microcystis aeruginosa, and Synechocystis 6803

Cytochrome c(6) is a soluble heme protein that serves as a photosynthetic electron transport component in cyanobacteria and algae, carrying electrons from the cytochrome bf complex to photosystem I. The rapid accumulation of cytochrome c(6) sequence data from a wide range of species, combined with significant advances in determining high resolution three-dimensional structures, provides a powerful database for investigating the relationship between structure and function. The fact that the gene encoding cytochrome c(6) can be readily modified in a number of species adds to the usefulness of cytochrome c(6) as a tool for comparative analysis. Efforts to relate cytochrome c(6) sequence information to structure, and structural information to function depend on knowledge of the physical and thermodynamic properties of the cytochrome from different species. To this end we have determined the optical extinction coefficient, the oxidation/reduction midpoint potential, and the pH dependence of the midpoint potential of cytochrome c(6) isolated from three cyanobacteria, Arthrospira maxima, Microcystis aeruginosa, and Synechocystis 6803.

Expression of plastocyanin and cytochrome f of the cyanobacterium Phormidium laminosum in Escherichia coli and Paracoccus denitrificans and the role of leader peptides

The gene for plastocyanin from the cyanobacterium Phormidium laminosum was successfully expressed in Escherichia coli. Expression of the gene for cytochrome f resulted in the production of holocytochrome f in the periplasmic space of E. coli, but the yield was low. Expression in Paracoccus denitrificans yielded no holoprotein. When the region encoding the cytochrome f leader sequence was replaced with more typical bacterial leader sequences (those from the P. laminosum plastocyanin gene and the Paracoccus versutus cytochrome c-550 gene), much higher yields were consistently obtained in both species. Overexpressed proteins were compared to those isolated from P. laminosum and found to be identical in mass, isoelectric point, redox midpoint potential and (for plastocyanin) 1H-NMR spectrum.

PHOTOSYNTHETIC CYTOCHROMES c IN CYANOBACTERIA, ALGAE, AND PLANTS

The cytochromes that function in photosynthesis in cyanobacteria, algae, and higher plants have, like the other photosynthetic catalysts, been largely conserved in their structure and function during evolution. Cyanobacteria and algae contain cytochrome c6, which is not found in higher plants and which may enhance survival in their planktonic mode of life. Cyanobacteria and algae contain another cytochrome, low-potential c549, which is not found in higher plants. This cytochrome has a structural role in PSII and may contribute to anaerobic survival. There is a third unique cytochrome, cytochrome M, in the planktonic photosynthesizers, and its function is unknown. New evidence is appearing to indicate evolution of cytochrome interaction mechanisms during the evolution of photosynthesis. The ease of cytochrome gene manipulation in cyanobacteria and in Chlamydomonas reinhardtii now provides great advantages in understanding of photosynthesis. The solution of tertiary and quaternary structures of cytochromes and cytochrome complexes will provide structural and functional detail at atomic resolution.

Bacterial expression of a mitochondrial cytochrome c. Trimethylation of lys72 in yeast iso-1-cytochrome c and the alkaline conformational transition

Saccharomyces cerevisiae iso-1-cytochrome c has been expressed in Escherichia coli by coexpression of the genes encoding the cytochrome (CYC1) and yeast cytochrome c heme lyase (CYC3). Construction of this expression system involved cloning the two genes in parallel into the vector pUC18 to give the plasmid pBPCYC1(wt)/3. Transcription was directed by two promoters, Lac and Trc, that were located upstream from CYC1. Both proteins were expressed in the cytoplasm of E. coli cells harboring the plasmid. Semianaerobic cultures grown in a fermentor produced 15 mg of recombinant iso-1-cytochrome c per liter of culture. Attempts to increase production by addition of IPTG suppressed the number of copies of the CYC1 gene within the population. Wild-type iso-1-cytochrome c expressed with pBPCYC1(wt)/3 in E. coli was compared to the same protein expressed in yeast. At neutral pH, the two proteins exhibit indistinguishable spectroscopic and physical (Tm, Em') characteristics. However, electrospray mass spectrometry revealed that the lysyl residue at position 72 is not trimethylated by E. coli as it is by S. cerevisiae. Interestingly, the pKa of the alkaline transition of the protein expressed in E. coli is approximately 0.6 pKa unit lower than that observed for the cytochrome expressed in yeast (8.5-8.7). 1H NMR spectroscopy of the bacterially expressed cytochrome collected at high pH revealed the presence of a third alkaline conformer that is not observed in the corresponding spectrum of the cytochrome expressed in yeast. These observations suggest that Lys72 can serve as an axial ligand to the heme iron of alkaline iso-1-ferricytochrome c if it is not modified posttranscriptionally to trimethyllysine.

Cloning and correct expression in Escherichia coli of the petE and petJ genes respectively encoding plastocyanin and cytochrome c6 from the cyanobacterium Anabaena sp. PCC 7119

The genes coding for plastocyanin (petE) and cytochrome c6 (petJ) from Anabaena sp. PCC 7119 have been cloned and properly expressed in Escherichia coli. The recombinant proteins are identical to those purified from the cyanobacterial cells. The products of both the petE and petJ genes are correctly processed in E. coli, as deduced from their identical N-terminal amino acid sequences as compared with those of the metalloproteins isolated from the cyanobacterium. Physicochemical and functional properties of the native and recombinant protein preparations are also identical, thereby confirming that expression of petE and petJ genes in E. coli is an adequate tool to address the study of the structure/function relationships in plastocyanin and cytochrome c6 from Anabaena by site-directed mutagenesis.

Genetic analysis of chloroplast c-type cytochrome assembly in Chlamydomonas reinhardtii: One chloroplast locus and at least four nuclear loci are required for heme attachment

Chloroplasts contain up to two c-type cytochromes, membrane-anchored cytochrome f and soluble cytochrome c6. To elucidate the post-translational events required for their assembly, acetate-requiring mutants of Chlamydomonas reinhardtii that have combined deficiencies in both plastid-encoded cytochrome f and nucleus-encoded cytochrome c6 have been identified and analyzed. For strains ct34 and ct59, where the phenotype displays uniparental inheritance, the mutations were localized to the chloroplast ccsA gene, which was shown previously to be required for heme attachment to chloroplast apocytochromes. The mutations in another eight strains were localized to the nuclear genome. Complementation tests of these strains plus three previously identified strains of the same phenotype (ac206, F18, and F2D8) indicate that the 11 ccs strains define four nuclear loci, CCS1-CCS4. We conclude that the products of the CCS1-CCS4 loci are not required for translocation or processing of the preproteins but, like CcsA, they are required for the heme attachment step during assembly of both holocytochrome f and holocytochrome c6. The ccsA gene is transcribed in each of the nuclear mutants, but its protein product is absent in ccs1 mutants, and it appears to be degradation susceptible in ccs3 and ccs4 strains. We suggest that Ccsl may be associated with CcsA in a multisubunit "holocytochrome c assembly complex," and we hypothesize that the products of the other CCS loci may correspond to other subunits.

The 2.15 A crystal structure of a triple mutant plastocyanin from the cyanobacterium Synechocystis sp. PCC 6803

The crystal structure of the triple mutant A42D/D47P/A63L plastocyanin from the cyanobacterium Synechocystis sp. PCC 6803 has been determined by Patterson search methods using the known structure of the poplar protein. Crystals of the triple mutant A42D/D47P/A63L, which are stable for days in its oxidized form, were grown from ammonium sulfate, with the cell constants a = b = 34.3 A and c = 111.8 A belonging to space group P3(2)21. The structure was refined using restrained crystallographic refinement to an R-factor of 16.7% for 4070 independent reflections between 8.0 and 2.15 A with intensities greater than 2 sigma (I), with root mean square deviations of 0.013 A and 1.63 degrees from ideal bond lengths and bond angles, respectively. The final model comprises 727 non-hydrogen protein atoms within 98 residues, 75 water molecules and a single copper ion. The overall tertiary fold of Synechocystis plastocyanin consists of a compact ellipsoidal beta-sandwich structure made up of two beta-sheets embracing a hydrophobic core. Each sheet contains parallel and antiparallel beta-strands. In addition to the beta-sheets, the structure contains an alpha-helix from Pro47 to Lys54 that follows beta-strand 4. The three-dimensional structure of Synechocystis plastocyanin is thus similar to those reported for the copper protein isolated from eukaryotic organisms and, in particular, from the cyanobacterium Anabaena variabilis, the only cyanobacterial plastocyanin structure available so far. The molecule holds an hydrophobic region surrounding His87, as do other plastocyanins, but the lack of negatively charged residues at the putative distant remote site surrounding Tyr83 could explain why the Synechocystis protein exhibits a collisional reaction mechanism for electron transfer to photosystem I (PSI), which involves no formation of the transient plastocyanin-PSI complex kinetically observed in green algae and higher plants.

A comparative thermodynamic analysis by laser-flash absorption spectroscopy of photosystem I reduction by plastocyanin and cytochrome c6 in Anabaena PCC 7119, Synechocystis PCC 6803 and Spinach

A comparative thermodynamic analysis of photosystem I (PSI) reduction by plastocyanin (Pc) and cytochrome c6 (Cyt) has been carried out by laser-flash absorption spectroscopy in the cyanobacteria Anabaena PCC 7119 and Synechocystis PCC 6803 as well as in spinach. These three organisms have been reported to exhibit different reaction mechanisms [Hervas, M., Navarro, J. A.. Díaz, A., Bottin, H., & De la Rosa, M. A. (1995) Biochemistry, 34, 11321-11326]. Whereas the activation free energy for the overall reaction is mainly enthalpic in nature, long-range electrostatic interactions appear to be attractive in Anabaena, but repulsive in Synechocystis and spinach. The net interaction between PSI and its two donor proteins in Anabaena is similarly affected by ionic strength (the rate constant decreases with increasing salt concentration), but the activation parameters delta H+/+ delta S+/+ show different dependencies on ionic strength. A compensation effect between entropy and enthalpy at varying ionic strength is found in all these Pc/PSI and Cyt/PSI systems, except with Cyt and PSI from Anabaena. Such a compensation effect is proposed to be mainly due to stabilization of the intermediate electrostatic complex by hydrophobic forces. The electron transfer step seems to be well optimized in the Anabaena Cyt/PSI couple, which exhibits a temperature-independent fast kinetic phase and, therefore, a low activation energy barrier. Short-distance forces appear to have gained relevancy in the reaction mechanism of PSI reduction by Cyt and Pc throughout evolution, whereas long-range interactions are prevalent in less evolved organisms.

Requirements for maturation of Bradyrhizobium japonicum cytochrome c550 in Escherichia coli

Various forms of Bradyrhizobium japonicum cytochrome c550 (the cycA gene product) were overexpressed in Escherichia coli cells grown under different conditions. Antibodies directed against a synthetic cytochrome c550 peptide were used as tools to detect both, apoprotein and holoprotein. Complete maturation of the apoprotein into its holo form with haem covalently bound to the polypeptide was observed only under anaerobic growth conditions and in E. coli K12 derivatives, whereas haem binding did not occur in the E. coli BL21 host. When maturation was complete, holocytochrome c550 was found exclusively in the periplasmic fraction. A cycA-expressing plasmid construct lacking the genetic information for the signal sequence produced apoprotein that was rapidly degraded without further maturation. Mutations in the haem-binding site resulted in products that were translocated through the cytoplasmic membrane, but apparently became degraded. Our results support the view that attachment of haem to the apoprotein is not a prerequisite for cleavage of the signal sequence and occurs on the periplasmic side of the membrane, subsequent to translocation of the apoprotein precursor.