-. Journal of Biological Chemistry 2015, 290 (27) , 16989-17003. In contrast to protoheme, heme c is covalently bound to the apocytochromes via one or (more usually) two thioether links between protein-Cys residues and the α-carbon atoms of the ethyl groups at positions 3 and 8 (the positions of the vinyl groups in the precursor protoheme). Three different cytochrome c heme ligating systems have been described (Xie et al., 1998; Nakamoto et al., 2000; Hamel et al., 2003). Nearly all known Tat pathway precursors contain RR in their signal peptides. Cristina Albarrán,, José A. Navarro,, Miguel A. Although CYP76Cs seem specific to Brassicaceae, they share common functions with CYP76s from other plants, such as CYP76B1 from H. tuberosus and CYP76B6 from C. roseus. De la Rosa, and, Manuel Hervás. Explanation of abbreviations are in the text. CES contributes to the biogenesis of all major photosynthetic protein complexes in Chlamydomonas chloroplasts. Identification of the basic residues of cytochrome f responsible for electrostatic docking interactions with plastocyanin in vitro: relevance to the electron transfer reaction in vivo. PS I mutants defective for PsaB expression show no detectable synthesis of the CES subunit PsaA, whether failure to synthesize PsaB results from mutations in the chloroplast psaB gene itself (Girard-Bascou et al., 1987) or in the TAB1 or TAB2 nuclear genes, whose products are specifically required for the translation of psaB mRNA (Stampacchia et al., 1997; Dauvillée et al., 2003). Owing to dense packing of all functional protein complexes in the thylakoid membrane and extremely high rate and efficiency of both excitation energy flow and electric charge flow, the transfer of a single electron from H2O to NADPH is completed in the photosynthetic apparatus of higher plants in a relatively short time of 20 ms (see [1] for a detailed description of the photosynthetic charge transfer processes). Figure 29.1. W.A. An important milestone occurred in 2003, when b6f complex crystal structures were obtained from the thermophilic bacterium Mastigocladus laminosus (Kurisu et al., 2003) and Chlamydomonas (Stroebel et al., 2003). Cytochrome f is an intrinsic membrane component of the cytochrome bf complex, transferring electrons from the Rieske FeS protein to plastocyanin in the thylakoid lumen. The thiol groups of two cysteinyl residues in a Cys–X–Y–Cys–His peptide motif are attached to two vinyl groups of heme through thioether bonds. Indeed, it has been proposed that the mRNA-specific factors mediate this translational control (Choquet and Wollman, 2002). This short investigation was presented by Milikisyants et al. We use cookies to help provide and enhance our service and tailor content and ads. Three of the large subunits (cytochrome b6, cytochrome f, and the Rieske ISP) bind redox-active prosthetic groups. Electron transport via cytochrome b6f is responsible for creating the proton gradient that drives the synthesis of ATP in chloroplasts. Domain structure of targeting peptides for Sec‐ and Tat‐directed thylakoid lumenal proteins. W.I. Also, examination of the electron density map of the Chlamydomonas b6f complex in the heme ci region suggests that the introduced chirality at C-31 is R, rather than the more usual S configuration in other c-type hemes. Its synthesis is regulated by copper availability (Li and Merchant, 1995; Li et al., 1996; see also Chapters 10 and 15Chapter 10Chapter 15 in Volume 2). Cytochrom c 555, auch Cytochrom c 6 oder (meistens) Cytochrom f genannt, findet sich in den Chloroplastenmembranen von Geißeltierchen und in Höheren Pflanzen, wo es mit Cytochrom b 559 beim Elektronentransport in der Photosynthese mitwirkt. Soriano GM, Ponamarev MV, Piskorowski RA, Cramer WA. Organization and structure of the genes for the cytochrome b/c1 complex in purple photosynthetic bacteria. Cytochrome f from the cytochrome b6f complex is a typical CES protein whose rate of synthesis decreases in the absence of cytochrome b6 or subunit IV, to about 10% of that observed in the wild type (Lemaire et al., 1986; Kuras and Wollman, 1994). In eukaryotic organisms, these proteins not only carry out reactions necessary for the metabolism of endogenous compounds, but they are also important in the oxidation of exogenous drugs and other foreign compounds. Among the antenna complexes of PS II is the major one, LHCII, comprising about half of chlorophyll pool on Earth. Restored PsaB synthesis in this suppressor strain is correlated with that of PsaA, suggesting that the presence of PsaB is required for efficient translation of the psaA mRNA (Stampacchia et al., 1997). The figure was generated using PyMol. This system has already been applied several times at early stages of photosynthesis research, at which researchers believed that majority of photosynthetic pigments were not protein-bound but present directly within the lipid phase of the thylakoid membrane (see [7] and [8] for a review). Conversely, its absence does not affect the rate of synthesis of other PS II subunits, even though they are more rapidly degraded in the absence of CP43 (de Vitry et al., 1989). In the M. laminosus structure, PetL is by contrast embedded more deeply in the structure and surrounded by helices A and B from cytochrome b6, helix G from subunit IV and the PetM subunit. Measurements were made of the absorbance change at the 553-nm alpha-band maximum relative to a reference wavelength. According to the current knowledge photosynthetic pigments (both chlorophylls and carotenoids) are bound in vivo to the functional pigment-protein complexes. The cytochrome b 6 f complex provides the electronic connection between the photosystem I and photosystem II reaction centers of oxygenic photosynthesis and generates a transmembrane electrochemical proton gradient for adenosine triphosphate synthesis. Would you like email updates of new search results? One notable feature is that both thylakoid Sec and Tat signal peptides generally possess unusually long N domains that usually contain acidic residues near the N‐terminus. Structure-function studies of the cytochrome b 6 f complex, the central hetero-oligomeric membrane protein complex in the electron transport chain of oxygenic photosynthesis, which formed the basis for a high-resolution (2.5 Å) crystallographic solution of the complex, are described. Each monomeric complex is composed of four large subunits [cytochrome b6, subunit IV, cytochrome f, and the Rieske iron sulfur protein (ISP)] and four small subunits (PetG, PetL, PetM, and PetN).  |  Thus, the expression of cytochrome b6f subunits is seemingly hierarchical with the presence of cytochrome b6 and subunit IV being required for sustained expression of the CES subunit cytochrome f. Studies of Chlamydomonas mutants have uncovered similar hierarchies in the synthesis of chloroplast-encoded subunits from all major photosynthetic proteins, as illustrated in Figure 29.1. The membrane-embedded CF0 portion of the ATP synthase is a selective proton channel that comprises subunits I, II, III, and IV, assembled in a 1:1:(13–14):1 stoichiometry (Seelert et al., 2003; Meyer zu Tittingdorf et al., 2004). One exception is the CP43 core antenna of PS II: its rate of synthesis does not depend on the presence of other PS II subunits, even if it is less stable in strains showing compromised PS II assembly. All of the mutants are deficient in both cytochrome b6f and cytochrome c6, indicating that the same factors are required for synthesizing heme c in both of these cytochromes, although a gene coding specifically for heme c lyase has not been identified. Even if their positions are comparable in the two complexes, the protein assignments for three of them differ depending on the organism. 1988 Feb;20(1):59-83. doi: 10.1007/BF00762138. Inferences from the structure and C2 symmetry of the complex for the pathway of QH2/Q transfer within the complex, problems posed by the presence of lipid in the inter-monomer cavity, and the narrow portal for QH2 passage through the p-side oxidation site proximal to the [2Fe–2S] cluster are discussed. Singlet excited state of P680 promotes an instant electron transfer to pheophytin (Pheo), the first acceptor, followed by further electron transfer to plastoquinone A (QA) and plastoquinone B (QB) along with the direction of a decrease in the oxidation-reduction (redox) potential. It is the last enzyme in the respiratory electron transport chain of cells located in the membrane. The Specificity in the Interaction between Cytochrome f and Plastocyanin from the Cyanobacterium Nostoc sp. In addition to protoheme, chloroplasts contain heme c (Figure 20.2). One exception is the Rieske FeS protein that has a KR in an uncleaved signal peptide [21]. Cytochrome f (cyt f) plays a role analogous to that of cytochrome c1, in spite of their different structures. Virtually all of the lumenal and single‐span membrane proteins are targeted to their respective pathways by cleavable hydrophobic signal peptides with features similar to bacterial signal peptides (Figure 18.2). Biochim Biophys Acta. The function of cytochrome c in the respiratory chain as an electron carrier is well established. Neben dem Photosystem II und dem Photosystem I ist er einer der zentralen Proteinkomplexe der … In both structures, each monomer is composed of 13 transmembrane helices whose overall positions are rather similar (Figure 17.1). It means that the process of enzymatic oxidation of water is a source of electrons that are transferred along the linear electron transfer chain, and protons in the inner space of the thylakoid membranes. Chlamydomonas chloroplasts contain three known c-type hemes: one in cytochrome c6, one located on the cytochrome f moiety of the b6f complex, and a third, named heme ci (or alternatively, heme x), that is also located in the b6f complex near the high-potential heme b on the stromal side of the membrane (Stroebel et al., 2003). Table 17.2. The structures are very similar and feature a dimeric organization of the complex. M, molecular mass of the mature subunit calculated from the sequence without cofactors. The latter system seems to be particularly suited for experimentation on electric charge transfer owing to the fact that redox conditions may be easily manipulated at the opposite sides of a planar lipid membrane. (A) Side view of the Chlamydomonas cytochrome b6f complex in the plane of the membrane is represented with the lumenal side at the bottom. The 251-residue extrinsic functional domain of C. reinhardtii cytochrome f was expressed in Escherichia coli without the 35 C-terminal residues of the intact cytochrome that contain the membrane anchor. Heme ci is unusual in that it is bound to the apoprotein by a single thioether link, the one at the 3 position. Roles of two mRNA-specific translation factors in response to physiological conditions have been proposed. Mutating the twin arginine motif to QQ or even KK prevents translocation as well as several other measurable interactions with Tat pathway machinery [26, 28, 29] (and see later). The measurements were performed at T(1) = T(2) = 7 K, at several TR values ranging from 5 μs to 1 ms. A 3.0 angstrom crystal structure of the dimeric b 6 f complex from the thermophilic cyanobacterium Mastigocladus … For example, the frequent occurrence of basic residues in Tat signal peptide C domains [31] and a somewhat reduced hydrophobicity of Tat signal peptide H domains [2] appear to prevent transport of Tat pathway precursors by the Sec pathway. Another hypothesis proposes that factors localize their target mRNAs within the chloroplast as a prerequisite for translation (see section IX; Zerges and Rochaix, 1998; Zerges, 2000; Dauvillée et al., 2003; Uniacke and Zerges, 2007). (Miller et al. Type I occurs in α- and γ-proteobacteria, Type III occurs in fungal and animal mitochondria, and Type II occurs in chloroplasts and some bacteria including cyanobacteria. This linkage is resistant to heat and hydrolysis, but can be broken with the help of silver or mercury salts. Kenneth Cline, Steven M. Theg, in The Enzymes, 2007. The questions of whether the stoichiometry of the turnover of cytochrome f, and the time-course of its reduction subsequent to a light flash, are consistent with efficient function in noncyclic electron transport have been investigated. In panel (A), the Fe2S2 center is shown as a ball-and-stick model (yellow and orange). The introduced chirality at both C-31 and C-81 is S in all c-type hemes for which the configuration was reported (Timkovich et al., 1998). ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B978012370873100037X, URL: https://www.sciencedirect.com/science/article/pii/B978012370873100006X, URL: https://www.sciencedirect.com/science/article/pii/B9780123708731000253, URL: https://www.sciencedirect.com/science/article/pii/B9780123786302003741, URL: https://www.sciencedirect.com/science/article/pii/S0076687915003912, URL: https://www.sciencedirect.com/science/article/pii/S1874604707250188, URL: https://www.sciencedirect.com/science/article/pii/B9780123786302001584, URL: https://www.sciencedirect.com/science/article/pii/B9780123708731000368, URL: https://www.sciencedirect.com/science/article/pii/S0927519303800591, URL: https://www.sciencedirect.com/science/article/pii/B9780123708731000289, Introduction to Chlamydomonas and its Laboratory Use. In this regard, dual targeting signal peptides have been engineered by fusing a twin arginine N domain to the H and C domains of a Sec pathway signal peptide [26]. Fig. Cytochrome c is primarily known for its function in the mitochondria as a key participant in the life-supporting function of ATP synthesis. Cytochrome f is a subunit of the cytochrome b6f complex that plays a role in photosynthesis by transferring electrons between photosystems II and I in plants, green algae, and cyanobacteria. -, FEBS Lett. In the early literature it was usually identified as cytochrome c-552, and is equivalent to cyanobacterial cytochrome c-553. This distance is in agreement with the X-ray crystallographic distance (14.3 Å to Cβ of Q104) within the uncertainty range related to the conformational flexibility of the nitroxide spin label. Gruszecki, A. Wardak, in Membrane Science and Technology, 2003. Supporting: 3, Disputing: 1, Mentioning: 13 - The questions of whether the stoichiometry of the turnover of cytochrome f, and the time-course of its reduction subsequent to a light flash, are consistent with efficient function in noncyclic electron transport have been investigated. 1988 Aug;85(16):5794-8. doi: 10.1073/pnas.85.16.5794. Genes with names in lower case (psbA, atpB etc.) However, when a cell receives an apoptotic stimulus, cytochrome c is released into the cytosol and triggers programmed cell death through apoptosis. Cytochrom P450-Proteine kommen in allen Organen vor, und sind vor allem in den Leberzellen anzutreffen. Crystal structures were determined for the wild type and three "water chain" mutants (N168F, Q158L, and N153Q) having impaired photosynthetic and electron transfer function. USA.gov. Andreia F. Verissimo, Namita P. Shroff, Fevzi Daldal. Electron transfers amongst cytochrome f, plastocyanin and photosystem I: kinetics and mechanisms. These functions include not only monoterpenol oxidation, but also metabolism and detoxification of herbicides belonging to the class of phenylurea. The questions of whether the stoichiometry of the turnover of cytochrome f, and the time-course of its reduction subsequent to a light flash, are consistent with efficient function in noncyclic electron transport have been investigated. Cytochrome f is the largest subunit of cytochrome b6f complex (plastoquinol—plastocyanin reductase; EC 1.10.99.1). While both structures agree on the position of PetM, the electron densities assigned to PetG, PetL, and PetN in Chlamydomonas (Figure 17.1B) were respectively assigned to PetL, PetN, and PetG in M. laminosus (Figure 17.1D). The nuclear localization of the PETN gene differs from higher plants, where PetN is chloroplast-encoded. A longstanding hypothesis proposes functions of mRNA-specific translation factors in the control of subunit stoichiometry. In this scenario, rather than being translational regulators per se, some are mRNA localizing factors similar to those described in other organisms (reviewed in St. Johnston, 2005). Sie spielen eine wichtige Rolle bei der Hydroxylierung von C-H-Bindungen.. 2 Hintergrund. The planar lipid membrane system is also suitable for precise optical experiments with application of polarized light, primarily owing to well defined plane of the membrane and localization and orientation of photosynthetic pigment-protein complexes strictly determined by physical properties of the lipid matrix. Several light-harvesting pigment-protein complexes serve PS II by supplying the reaction center with excitation energy. Subunits β and α can both be regarded as CES subunits for CF1 biogenesis: mutant strains defective for the expression of the nucleus-encoded subunit γ show reduced synthesis of subunit β (Drapier et al., 2007), the product of the chloroplast atpB gene, while the rate of translation of atpA mRNA, encoding subunit α is strongly reduced in mutant strains defective for subunit β expression (Lemaire and Wollman, 1989b; Drapier et al., 1992). The requirement for the TAB1 factor to translate psaB mRNA can be bypassed in a suppressor strain that carries a chloroplast mutation in the psaB 5′ untranslated region (UTR). The cytochrome b 6 f complex universally functions in oxygenic photosynthesis as plastoquinol-plastocyanin oxidoreductase or as plastoquinol-cytochrome c oxidoreductase between the two light reactions of Photosystem II and Photosystem I. Shown as wireframes are the b-carotenes colored in brown, the chlorophylls colored in green and the hemes colored either in pink (b-type hemes) or in red (c-type hemes). Oxidized P680 is again reduced at the expense of an electron originating from the water splitting enzyme. Das zentrale Eisen(II)-Ion (Fe2+) ist oktaedrisch komplexiert über koordinative Bindungen zu den vier Stickstoff-Atomen (äquatorial) der Pyrrole im Porphyrin und axial zu einem Stickstoff-Atom eines Histidin-R… More recently, ... Cytochrome c entrapped in silica sol–gel materials was studied in detail by Dunn et al. Thylakoid Sec may also integrate multispanning membrane proteins such as D1 (see later). Such studies have been carried out with application of liposomes [2,3], monomolecular layers [4-6] and also planar lipid bilayers (as discussed below). Das Cytochrom c besitzt als prosthetische Gruppe ein Häm c, das über zwei Thioether-Brücken an zwei Cystein-Reste im Protein gebunden ist. In the light dependent reactions of oxygenic photosynthesis, this complex functions as an electronic connection between photosystem II (PSII) and photosystem I (PSI), and in the process serves to generate a proton gradient across the plasma (bacteria) or thylakoid membrane (plants) that will drive ATP Synthase-mediated ATP production. Green arrows indicate epistatic relationships between the subunits of a photosynthetic protein complex and connect an assembly partner whose presence is required for efficient synthesis of a CES subunit, to this CES subunit, emphasized by the arrowhead. This regulation is superimposed on the regulation of petA mRNA abundance by Mca1. Clipboard, Search History, and several other advanced features are temporarily unavailable. 1991 Sep;289(2):244-55 While the mutants can grow photosynthetically in the absence of PetL or PetM (assigned to peripheral locations, see Figure 17.1B), the absence of either PetG or PetN prevents the accumulation of the b6f complex and leads to a non-photosynthetic growth phenotype. Epub 2019 Feb 21. Moreover, analogous classes of translation factors exist in Saccharomyces cerevisiae for the biogenesis of respiratory chain complexes and in maize for photosynthesis complexes (reviewed in Fox, 1996; Stern et al., 2004). Cytochrom f aus Spinat besteht aus 8 Untereinheiten, von denen nur 4 eine Hämgruppe haben. Nagarajan D, Sukumaran S, Deka G, Krishnamurthy K, Atreya HS, Chandra N. J Biol Chem.  |  During transition from state 1 to state 2, a portion of the cytochrome b 6 f complex, together with the mobile LHCII complex, is displaced from the grana to the stromal thylakoid lamellae both in vivo and in vitro ( Vallon et al., 1991 ). The prosthetic group of c-type cytochromes, such as cytochrome c and c1 in mitochondria and cytochrome f in chloroplasts, unlike that of a- and of b-type cytochromes, is covalently linked to the polypeptide moiety. Photosynthetic electron transport is accomplished by integral membrane proteins: Photosystems I and II,and the cytochrome b 6 f complex. 1976 Nov;70(1):191-6 Please enable it to take advantage of the complete set of features! These observations define a “CES cascade,” where PsaB is required for sustained expression of PsaA, which, in turn, is required for significant production of PSAC (Wostrikoff et al., 2004). The globular domain of the turnip protein has recently been crystallised, offering the prospect of a detailed three-dimensional structure. Additional mutants in this genome are listed in Table 7.2, and some nuclear mutations that affect expression of chloroplast genes are given in Table 7.4. The protein is held in the thylakoid membrane by a single transmembrane span located near its C-terminus with a globular hydrophilic domain extending into the lumen. Comparative Analysis of the Complete Chloroplast Genomes of Five Quercus Species. Photosynth Res. CES thus appears to be a major control step during the biogenesis of the photosynthetic apparatus in Chlamydomonas, as almost all major chloroplast-encoded polypeptides – those polypeptides whose rate of synthesis can be accurately measured by pulse-labeling experiments – participate in CES, being either CES subunits or assembly partners required for the expression of CES subunits from the same protein complex, as illustrated in Figure 29.1. Electron exchange among QB pool and cyt b6-f complex is associated with net proton transfer from the outer towards the inner thylakoid space, which is a second source of proton gradient (apart of PS II linked functionally to the water splitting enzyme). Given the prevalence of CES control for each chloroplast photosynthetic complex (see Chapter 29), a role for gene-specific activators should be considered in this context. The cytochrome b 6 f complex functions either in linear electron flow from PS II to PS I or in cyclic electron flow around PS I. All of these mutants still contain mitochondrial c-type cytochromes, which indicates that the two organelles have different heme c ligation factors. 18.2. -, Biochim Biophys Acta. [1] The energy-transducing function of the complex, generation of the proton trans-membrane electrochemical potential gradient, centers on the oxidation/reduction pathways of the plastoquinol/plastoquinone (QH2/Q), the proton donor/acceptor within the complex. This could be a consequence of only sporadic conservation of SD sequences and poor conservation of 5′ UTR sequences overall. The globular domain of the turnip protein has recently been … Schwerpunkt ist die Erforschung der Cytochrome -c- Oxidase, des Schlüsselenzyms, welches im menschlichen Energiestoffwechsel aus dem Luftsauerstoff und der aufgenommenen Nahrung Wasser bildet und die dabei freiwerdende Energie speichert. 2018 Jun 15;293(24):9412-9422. doi: 10.1074/jbc.RA118.001768. Helices for cytochrome b and cytochrome b6 are colored in cyan and labeled from A to D for the latter and from A to H for the former. Yves Choquet, Francis-André Wollman, in The Chlamydomonas Sourcebook, 2009. The light processes of photosynthesis responsible for such a “translation” take place in a highly sophisticated photosynthetic apparatus comprising numerous functional proteins embedded in the lipid membranes called thylakoids. Copyright © 2021 Elsevier B.V. or its licensors or contributors. This electron transfer begins in the Photosystem II (PS II) reaction center in consequence of the absorption of a light quantum by the special pair of chlorophyll a molecules (P680). The numerical simulations took into account the strong g-anisotropy of the heme center (the principal g-values are 0.9, 1.69, and 3.51) and resulted in the mean distance RAB = 18.1 Å and the distance distribution width of ΔRAB = 5.4 Å. However, such mutated precursors do seem to interact with the Tat pathway because they stimulate proton counterflow [30] and can alter the transport behavior of authentic precursor proteins [30]. This article is cited by 81 publications. Cytochrome f is an intrinsic membrane component of the cytochrome bf complex, transferring electrons from the Rieske FeS protein to plastocyanin in the thylakoid lumen. Cytochrome werden nach der Variante des Häms, das sie enthalten, und nach ihrem Licht-Absorptionsspektrum unterschieden. high-potential cytochromes c generally function as isopoten-tial electron carriers between a membrane-bound cytochrome b (or c ]) and the most oxidizing protein electron acceptor of their respective electron- transport chains (Figure 1). H. Tuppy, G. Kreil, in Encyclopedia of Biological Chemistry (Second Edition), 2013. Biochemistry. National Center for Biotechnology Information, Unable to load your collection due to an error, Unable to load your delegates due to an error.  |  Mutant screens in Chlamydomonas have identified at least five nuclear genes and one chloroplast gene that are involved in chloroplast heme c synthesis (Howe and Merchant, 1992; Nakamoto et al., 2000). The tOE17 Tat pathway precursor is N‐terminally truncated to remove the stroma‐targeting peptide and the nonessential A domain. The physiological relevance of A domains is unclear; presumably they regulate access of precursors to the translocases. Cytochrome sind wie andere Hämproteine (beispielsweise Hämoglobin) mit Ausnahme der C-Cytochrome gegen Kohlenmonoxid und HCN empfindlich, die das aktive Zentrum blockieren und damit die Atmungskette zum Erliegen bringen. Life on Earth is driven by energy of the Sun and photosynthesis is a process that “translates” light into forms of energy that can be utilized by living organisms. This proton gradient across the thylakoid membrane drives ATP synthesis in accordance with the chemiosmotic mechanism of activation of ATP-synthase (ATP-ase). Cytochrome P450s (P450s) comprise a superfamily of proteins that catalyze numerous monooxygenase reactions in animals, plants, and bacteria. 2019 Mar 7;123(9):2114-2122. doi: 10.1021/acs.jpcb.8b12157. Biochim Biophys Acta. None of the Chlamydomonas genes required for heme c ligation to cytochromes f and c6 resemble the Type III yeast lyase genes or candidate Type I bacterial lyase genes (Xie et al., 1998). Type II ligating systems consist of a membrane protein with essential Cys residues, that is thought to maintain the ligating Cys residues of apocytochrome c in the reduced state that is necessary for ligation; another membrane protein with a characteristic Trp-rich motif that is thought to convey heme to the apocytochrome c and catalyze the ligation; and other proteins with unknown functions (Thöny-Meyer, 2000). In photosynthesis, the cytochrome b6f complex functions to mediate the transfer of electrons between the two photosynthetic reaction center complexes, from Photosystem II to Photosystem I, while transferring protons from the chloroplast stroma across the thylakoid membrane into the lumen. Tca1 is a limiting factor for petA translation and the levels of this factor and the petA product, cytochrome f, were found to vary coordinately with the availability of nitrogen (Raynaud et al., 2007). Design of a heme-binding peptide motif adopting a β-hairpin conformation.