But, in the process, its ancestral genome was drastically downsized, as many genes were lost or transferred to the host's nuclear genome. The petD mRNA has been established as a useful model transcript with which to study RNA–protein interactions and processing at the 3′ end of RNA in vitro. By sequencing chloroplast transformants, we revealed 149 mutations in the 300-bp target petD sequence that resulted in 92 amino acid substitutions in the 100-residue target subunit IV sequence. Adapted from Ghirardi ML and Mohanty P (2010) Oxygenic hydrogen photoproduction – current status of the technology. Apocytochrome f is a mutant version of cytochrome f with a 6 His tag at the C-terminus. Examination by SDS-PAGE of the, f complex, which was kept in different detergents at room temperature for 7-14 days, showed proteolysis (3). Cytochrome b is a protein found in the mitochondria of eukaryotic cells. Oxygenic photosynthesis originated more than three billion years ago when primordial cyanobacteria evolved an apparatus capable of utilizing sunlight for the extraction of electrons from water (Crowe et al., 2013). Concomitantly, protons (H+) are carried from the stromal to the lumenal side of the thylakoids, creating a proton gradient across the thylakoid membrane, which is utilized by the enzyme adenosine triphosphate (ATP) synthase to generate the ATP that is also required for CO2 fixation. Murthy, M.L. Two possibilities can be proposed for the continued operation of the Tat pathway in thylakoids without a ΔpH. The purified b, f complex from Tyr112Phe or Phe133Leu mutants was characterized by a loss of bound Chl and heme b, a shift in the absorbance peak and increase in bandwidth, and relatively small time-resolved absorbance anisotropy values of the Chl Q, band (7). In this study, the Q104C mutant cytochrome f protein was spin labeled by a nitroxide radical at position 104, and the dipole interaction between the spin label and the intrinsic low-spin ferric heme center was studied. The three-dimensional structures of the p-side, n-side, and transmembrane domains facilitate their roles in enzyme activity. Background. The correct processing at the 3′ end of the petD mRNA can be reproduced in vitro with a petD 3′-UTR RNA probe synthesized by transcription from a plasmid carrying the petD 3′ UTR.6 The synthetic RNA contains 70 nucleotides of the coding region and extends 58 nucleotides 3′ proximal to the stem–loop, which itself is 46 nucleotides long. Journal of Photochemistry and Photobiology B: Biology. This short investigation was presented by Milikisyants et al. The coupling of the oxidation-reduction and protonation-deprotonation is central to the mechanism of proton translocation in the complex (2). The results of DALI (Z=26.4) and protein BLAST (E=7e-135) searches show that the cyt b6f of C. reinhardtii has greater primary and tertiary similarities with the cyt b6f of M. laminosus than were found with cytochrome bc1. The simplest n-side proton pathway extends from the aqueous phase via Asp20 and Arg207 (cytochrome b6 subunit) to quinone bound axially to heme c (n). This interpretation invokes an entropic role for the ΔpH, consistent with indications that a ΔpH is required for the assembly of the Tat translocase in isolated thylakoids (Mori and Cline, 2002). On a timescale of femto- to picoseconds, excited xanthophylls in the light-harvesting complexes transfer energy to chlorophylls, probably from both the S2- and the S1-excited states (Walla et al., 2002), but they also protect the photosynthetic apparatus by quenching triplet chlorophyll in the range of nanoseconds (Peterman et al., 1995, 1997). Another possible consequence of the presence of an electrochemical proton gradient in the dark is the ability to assemble the photosynthetic apparatus in the absence of any photosynthetic light reactions. Photosynthesis in chloroplasts is a major source of ROS in green plant tissues (Dietz et al., 2010). The function of the chlorophyll a is unknown, but it may fill structure gaps, similarly to bound lipids in membrane proteins. The reduced iron-sulfur protein transfers an electron through cytochrome f to plastocyanin. Reduction of a, to a plastosemiquinone takes place at the Q, site of the n-side central cavity. These complexes are involved in electron transport, the pumping of protons to create a proton-motive force . The sample is electrophoresed for 30 min at 200 V, after which the wet gel is covered with Saran Wrap and exposed to X-ray film. This whole process is dependent on two membrane-embedded multisubunit complexes: Photosystem II (PSII) oxidizes water to molecular oxygen and 4 H+, while photosystem I (PSI) provides the negative redox potential required for CO2 fixation. The full-length transcript (300 nucleotides) is excised and eluted from the gel matrix in an Eppendorf tube by adding 180 µl of diethyl pyrocarbonate (DEPC)-treated water, 20 µl of DEPC-treated 3 M sodium acetate (pH 5.5), 1 µl of DEPC-treated 0.25 M EDTA, and 1 µl of 20% (w/v) sodium dodecyl sulphate (SDS). Reduction of a plastoquinone to a plastosemiquinone takes place at the Qn site of the n-side central cavity. CES also contributes to the biogenesis of both CF0 and CF1 moieties of the ATP synthase complex. The Q, active site is the location for plastoquinol binding, resulting in oxidation and electron transfer. Trace amounts of protease could be destructive to the integrity of the dimer complex, necessitating a mixture of protease inhibitors in all solutions used for purification. Four major protein complexes are located in the thylakoid membrane: Photosystem II (PSII), Cytochrome b6f complex, Photosystem I (PSI), and ATP synthase that works together in carrying out the light reaction in plants. Instead, the latter is downregulated in the absence of ATP production (see section H2 Production: Issues and Challenges). Schematic representation of a plant respiratory burst oxidase homolog (RBOH) protein. The reduced iron-sulfur protein transfers an electron through cytochrome f to plastocyanin. Electrons are shuttled between the two photosystems via the cytochrome b6f (Cytb6f) complex by two mobile, low-molecular-weight carriers–plastoquinone and plastocyanin–while a proton gradient is generated across the membrane to drive an ATP synthase (see Fig. In plant chloroplasts and cyanobacteria, there is an analogous protein, cytochrome b6, a component of the plastoquinone-plastocyanin reductase (EC, also known as the b6f complex. Three of the polypeptide redox-active subunits in the b 6 fcomplex: cytochrome f, cytochrome b and the Rieske-type 2Fe-2S protein contain prosthetic groups. To understand the biogenesis of the plastid cytochrome b 6 f complex and to identify the underlying auxiliary factors, we have characterized the nuclear mutant hcf164 of Arabidopsis and isolated the affected gene. Interestingly, the activity of RBOHD might be under redox regulation during the plant immune response (Yun et al., 2011) and similar findings have also been reported for these cognate NADPH oxidases in mammals (Qian et al., 2012). Heme x does not appear to be required for Q cycle function because the other elements of the Q cycle (hemes b, complexes, have identical interheme distances, and have similar hydrophobic environments between hemes (1). The roles of the PetG, PetL, PetM, and PetN subunits are not known, but stabilizing activity is a possibility. In contrast, protein translocation via the Tat pathway has been reported to be insensitive to NTP hydrolysis, being strictly dependent on a ΔpH (Mould and Robinson, 1991). Differences between the b (6)f and bc (1) complexes are emphasized. A key role of the thylakoid membranes in chloroplasts is the generation of a proton electrochemical gradient. The cytochrome b, subunit IV, PetG, PetL, PetM, and PetN subunits are common to both b, f cytochromes, but the Rieske iron-sulfur protein and cytochrome f have been replaced in, . Created by Jordan Kramer Cytochrome b6f (1VF5) from Mastigocladus laminosus is an integral membrane protein that mediates electron transfer between the photosystem II and photosystem I reaction centers by oxidizing lipophilic plastoquinol and reducing plastocyanin. Furthermore, these proteins have an N-terminal EF hand domain, which presumably functions as a calcium-binding domain (Keller et al., 1998). Each monomer consists of four large subunits, including cytochrome f, cytochrome b6, the Rieske iron-sulfur protein (ISP), and subunit IV; as well as four small hydrophobic subunits, PetG, PetL, PetM, and PetN. CF1, the soluble subcomplex which catalyzes ATP synthesis, is composed of five subunits α,β,γ,δ, and ε, assembled in the stoichiometry α3β3γ1δ1ε1 (see Chapter 18). The cytochrome bc (1) and b (6)f dimeric complexes diverge in structure from a … Invariant Tyr33, Gly38, Phe203, and Ile206 of cytochrome b6 and Phe40 and Ile44 of subunit IV contact heme x (2). In this view, both CP47 and D1 are CES subunits in PS II biogenesis. CES contributes to the biogenesis of all major photosynthetic protein complexes in Chlamydomonas chloroplasts. The second mode of gradient formation involves only the cytochrome b6f complex and photosystem I in the process known as cyclic electron transfer. The positive surface potential of cytochrome b, on the stromal side would facilitate docking of anionic ferredoxin to the n side of the complex near heme x. Heme c, acts a redox cofactor on cytochrome f and provides a binding site for tridecyl-stigmatellin, a p-side quinone analog inhibitor. Aspects of the crystal structures of the hetero-oligomeric cytochrome bc (1) and b (6)f ("bc") complexes relevant to their electron/proton transfer function and the associated redox reactions of the lipophilic quinones are discussed. 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. As expected, removal of CO2 from the headspace enhances light-driven H2-evolution, indicating competition for electrons between the CO2-fixation and the H2-evolution processes. The Qn site in cytochrome b6f is more accessible because it is not an enclosed pocket. The binding of a second plastoquinol causes one electron to reduce oxidized plastocyanin in the high-potential electron transport chain. In the stroma, electrons are transferred through PSI out of the thylakoid and form H2O2 with molecular oxygen (Asada, 1999), which then interacts with the reaction center of chlorophyll P680 in PSII to form singlet oxygen (1O2). The two-electron oxidation of plastoquinol (PQH 2 ) occur via a split pathway; one electron donated to the high potential [ 2Fe-2S ] protein which is the electron donor to cytochrome f , the other to the pair of lower potential … 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. Over the course of evolution, the endosymbiont was transformed into the chloroplast, an organelle which still possesses its own genome and its own gene-expression machinery. It is important to point out that CO2 reduction requires ATP, whereas H2 photoproduction does not. The study, led by the University of Sheffield and published in the journal Nature, reveals the structure of cytochrome b6f — the protein complex that significantly influences plant growth via photosynthesis. A heme is a prosthetic group that contains an iron atom in the center of a heterocyclic organic ring. The purified cytochrome b6f complex contains stoichiometrically bound chlorophyll a and beta-carotene at a ratio of one per cytochrome f, and bound … As a result, chloroplasts are dependent on the import of structural and regulatory proteins from the cytosol to maintain their function, assemble and repair their photosynthetic complexes, and coordinate gene expression and protein synthesis between the nucleus and chloroplast. Cytochrome b6f. In addition to photosynthesis, unicellular green algae are able to perform many biotechnologically interesting metabolic reactions, such as fermentation and hydrogen photoproduction. Reduced complex stability was also confirmed by in vitro treatments of isolated thylakoids with chaotropic reagents. As a proof of concept, we targeted a 300-bp sequence of the petD gene that encodes subunit IV of the thylakoid membrane-bound cytochrome b6f complex. Onle one of the two units in the dimer is shown. Proteolysis of cytochrome b, occurs slowly enough that crystals can be obtained when the crystallization process is accelerated through addition of lipid to the purified complex. 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/S0076687915003912, URL: https://www.sciencedirect.com/science/article/pii/S0076687901425626, URL: https://www.sciencedirect.com/science/article/pii/B9780123749208008018, URL: https://www.sciencedirect.com/science/article/pii/B9780128008546000245, URL: https://www.sciencedirect.com/science/article/pii/B9780123749208001193, URL: https://www.sciencedirect.com/science/article/pii/B9780123708731000290, URL: https://www.sciencedirect.com/science/article/pii/S193764481600006X, URL: https://www.sciencedirect.com/science/article/pii/B9780123708731000265, URL: https://www.sciencedirect.com/science/article/pii/B9780123786302004461, Electron Paramagnetic Resonance Investigations of Biological Systems by Using Spin Labels, Spin Probes, and Intrinsic Metal Ions, Part A, Redox-Regulated Plant Transcription Factors, Alboresi et al., 2011; Kim et al., 2012; Ramel et al., 2012, Biophysical Techniques for Structural Characterization of Macromolecules, International Review of Cell and Molecular Biology, Giovanni Finazzi, ... Fabrice Rappaport, in, Encyclopedia of Biological Chemistry (Second Edition). To produce the RNA substrate, the plasmid encoding the transcript is linearized by digestionwith XbaI and purified by phenol-chloroform extraction. The binding of a second plastoquinol causes one electron to reduce oxidized plastocyanin in the high-potential electron transport chain. For this purpose, the precipitated RNA is resuspended in formamide dye [deionized formamide, 0.1%(w/v) xylene cyanol, 0.1 %(w/v) bromphenol blue], heated to 85° for 2 min, and loaded onto a denaturing acrylamide gel (minigel) [5%(w/v) acrylamide (19:1), 8 M urea in 0.5× TBE], Gels are preelectrophoresed for 1 hr at 200  V in 0.5× TBE before sample loading. Schematic representation of the major photosynthetic proteins. The major source of ROS under such circumstances is catalyzed by respiratory burst oxidase homologs (RBOHs; Figure 24.2), analogous to the enzymes found in mammalian phagocytes (Grant and Loake, 2000; Lambeth, 2004). Seven prosthetic groups per monomer of cytochrome b6f have been identified. The study was unable to determine the role of chlorophyll a, but it was concluded that the aromatic residues of subunit IV are important in maintaining the short lifetime of the singlet excited state which would normally undergo quenching from β-carotene. Two iron-sulfur subunits are used in place of the Rieske iron-sulfur protein. The Rieske iron-sulfur protein oxidizes the plastoquinol to a semiquinone and two protons are released to the thylakoid lumen. A newly discovered feature of b-heme orientation in b, about the normal to the membrane plane relative to the heme orientation in Nostoc and Chlamydomonas reinhar, prevents obstruction of electron transfer from the anionic semiquinone. Transfer of the second electron from plastoquinol across the complex through two b hemes, or as anionic plastosemiquinone, and the resulting proton uptake from the electronegative side generate a proton electrochemical gradient across the membrane (1). treatments of isolated thylakoids with chaotropic reagents. A.-V. Bohne, ... J. Nickelsen, in International Review of Cell and Molecular Biology, 2016. Two additional natural lipids, MGDG, for a total of three lipids per monomer are required for crystallization of the, complex (6). Multi-subunit complex of cytb6/fis a crucial component for the photosynthetic electron transport chain of higher plants, green algae and cyanobacteria. Hemes b, , in the core of the complex that is structurally conserved between bc, f complexes, bridge the second and fourth transmembrane helices of the cytochrome b polypeptide (3). Afterward, the electron acceptor of PSI may redirect electrons to ferredoxin–thioredoxin reductase (FTR) to form reduced thioredoxin, to ferredoxin–NADPH reductase (FNR) to generate NADPH, or to molecular oxygen to form hydrogen peroxide (H2O2). This explanation is supported by experiments in which the Δψ was inferred to contribute to powering the Tat pathway in vitro under select experimental conditions (Theg et al., 2005). The xanthophylls lutein and violaxanthin are bound to the light-harvesting antennae of both photosystems, while loroxanthin and neoxanthin appear to be confined to the light-harvesting complexes of PS II (Pineau et al., 2001). The role of the single molecule of β-carotene in the cytochrome b6f complex is still under investigation (Kim et al., 2005; Shao et al., 2006). These proteins are generally membrane-bound and are known as respiratory pigments because they are involved in various electron transport systems in oxidative phosphorylation. Addition of DOPC or DOPG lipid to the complex immediately after the last step of the purification process at a stoichiometry of 10:1, lipid:cytochrome f, resulted in formation of hexagonal crystals (3). Although neither of the photosystems directly moves protons across the membrane they are important examples of proteins which principally contribute to the generation of the proton electrochemical gradient as a result of moving electrons across the membrane. We show that Stt7 is associated with photosynthetic complexes including LHCII, photosystem I, and the cytochrome b6f complex. Photosynthesis extracts electron from H2O and via the electron transport chain delivers them to the [FeFe]-hydrogenase where H2 is generated. On the positive side, the heme-proximal Glu78 (subunit IV), which accepts protons from plastosemiquinone, defines a route for H (+) transfer to the aqueous phase. Cytochrome bc1 is a dimeric protein. Cytochrome f from the cytochrome b6f complex is a typical CES protein whose rate of... Ribonucleases - Part B. Sacha Baginsky, Wilhelm Gruissem, in Methods in Enzymology, 2001 The petD mRNA has been... Bioenergetics. The total molecular weight is 217 kDa. A protein complex that includes CYTOCHROME B6 and CYTOCHROME F. It is found in the THYLAKOID MEMBRANE and plays an important role in process of PHOTOSYNTHESIS by transferring electrons from PLASTOQUINONE to PLASTOCYANIN or CYTOCHROME C6. Three prosthetic groups in the C. reinhardtii complex, eicosane, 1,2-distearoyl-monoglactosyl-diglyceride, and sulfoquinovosyldiacylglycerol, are not found in the M. laminosus complex. In Chlamydomonas zygospores, the chloroplasts are reduced in size and the cytosol is orange-colored (Cavalier-Smith, 1976). Initially heterotrophic eukaryotes first acquired the ability to perform oxygenic photosynthesis by entering into an endosymbiotic relationship with an ancestral cyanobacterium (Gray and Archibald, 2012), and modern algae and plants descended from such organisms. Photosynthetic electron transport pathways in the green alga, Chlamydomonas reinhardtii. Figure 29.1. The two iron-sulfur subunits contain a total of 176 amino acids while the Rieske iron-sulfur protein is comprised of 179, indicating a 3 residue difference that does not alter function. These transmembrane proteins reduce apoplastic oxygen to the superoxide radical, O2–. The plant genome contains 10 RBOH genes (RBOHA–RBOHJ; Torres and Dangl, 2005). The two complexes have different Qn sites due to the additional Thr188 between the two histidine ligands in the D helix of cytochrome b6f and the introduction of heme x where the ubiquinone and antimycin A binding site would be in cytochrome bc1. (2009) as a demonstration of application of their just introduced five-pulse RIDME sequence. It has been shown that nonspecific adsorption of lipids is about 100 to 200 times lower than the specific adsorption of detergent. Thus, NR might function as a source of NO in plants. The process carried out by cytochrome b, f is called the Q cycle because of the activity of quinone variants. An obstacle in the crystal structure studies has been proteolysis and resulting monomerization of the complex, which degrades the complex from its active dimer form. The probability of singlet oxygen formation decreases upon interaction with the cytochrome b. In linear electron transfer, an electron extracted from water in the thylakoid lumen is transferred through photosystem II (PSII), plastoquinone (PQ), cytochrome b6f (Cyt), plastocyanin (PC), and photosystem I (PSI) into the chloroplast stroma. The intercofactor distances and the organization of 8 of the 13 transmembrane helices (A to D in cytochrome b6, E and F in subunit IV, ISP, and cytochrome f) are similar in the b6f and bc, sites within the cavity are different. The Sec and SRP pathways require hydrolysis of nucleotide triphosphates – ATP and GTP respectively – although a proton motive force may also play a role (Ernst et al., 1994). A mutant strain lacking synthesis of CF0 subunit III because it lacks stable accumulation of the atpH mRNA (Majeran et al., 2001) shows a pleiotropic loss of all ATP synthase subunits, but a specific reduction in the rate of synthesis of subunit IV (Lemaire and Wollman, 1989b). The transfer of electrons is coupled to the transport of … It functions as part of the electron transport chain and is the main subunit of transmembrane cytochrome bc1 and b6f … The mixture is incubated overnight at 4°, after which the elution buffer is removed and transferred to a new Eppendorf tube, and the RNA is recovered by ethanol precipitation. It has been reported that NR can convert NO2– to NO under low oxygen tensions and high nitrate concentrations with very low efficiency in vitro (Rockel et al., 2002). The results of DALI (Z=19.5) and protein BLAST (E=3e-46) searches show that cytochrome bc1 has both primary and tertiary similarity to cytochrome b6f. Lack of activity in the Q cycle by heme x indicates a potential function as ferrodoxin-plastoquinone reductase, a reagent necessary for cyclic electron transfer. These three pathways differ not only in their protein specificities but also from an energetic point of view. Oxygenic organisms harness solar energy to extract electrons from H2O, which are required for CO2 fixation. Proteolysis inhibitors at moderate concentration could retard but not inhibit proteolysis over a period of 1 week. Phe, side of the E helix, is parallel to heme x and near (6 to 9 Å) plastoquinone in the cavity (1). Proteolysis of cytochrome b6f in Mastigocladus laminosus occurs slowly enough that crystals can be obtained when the crystallization process is accelerated through addition of lipid to the purified complex. A molecule of, occurs near the center of the transmembrane region between the helices of PetL and PetM. In vivo protein … 1). f composition is complicated by six of the eight subunits of the complex being plastid genome-encoded. The roof is formed by cd1 and cd2 p-side peripheral helices connecting helices C and D of cytochrome b6 and the C terminus of the ISP transmembrane helix. The red circles denote the major barriers limiting technological use of algae for photobiological H2 production. 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. John S. Rieske and co-workers first discovered and isolated the proteins in 1964. In the model plant Arabidopsis, NIA1 and NIA2 are genes that encode nitrate reductase (NR), with NIA2 being responsible for most of the NR activity (Wilkinson and Crawford, 1991). Invariant Tyr, Val26, Pro27, Pro28, His29, Asn31, Arg207, and Gln209, f has two primary functions: the oxidation of plastoquinol to a semiquinone and the reduction of plastocyanin. 1). The existence of currently unknown factors (likely chaperones) is reasonable given that the Tat thylakoid pathway is adapted for the transport of folded proteins (reviewed in Robinson and Bolhuis, 2001), which are rarely used as in vitro substrates. Cyclooxygenase 2, an enzyme involved in inflammation, is a cytochrome b protein. The starting point for all biological solar-driven H2-production methods is the process of photosynthesis (see Figure 1). Figure 1. By analyzing the single- and double-mutant of RBOH in Arabidopsis, RBOHD and RBOHF were found to be the main sources generating ROS after recognition of pathogen-associated molecular patterns (PAMPS) and avirulent pathogens (Torres et al., 2002). The 4 large subunits of the cytochrome b6-f complex are cytochrome b6, subunit IV (17 kDa polypeptide, PetD), cytochrome f and the Rieske protein, while the 4 small subunits are PetG, PetL, PetM and PetN. The Q, pocket is bounded by the [2Fe-2S] cluster, heme b, , and the “ef loop” connecting helices E and F of subunit IV (1). 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. ROS production is also an important early event during plant immune responses (Grant and Loake, 2000; Skelly and Loake, 2013). The cytochrome b 6 f complex is a dimer, with each monomer composed of eight subunits. A small portal in the wall of each cavity is formed by helices C, cd1, and F, and leads to the Q, pocket on each side of the membrane. The cytochrome b, subunit IV, PetG, PetL, PetM, and PetN subunits are common to both b6f cytochromes, but the Rieske iron-sulfur protein and cytochrome f have been replaced in C. reinhardtii. Alternatively the overall Δμ˜H+, rather than ΔpH alone, may be able to power the Tat pathway. For further explanation, see Section 4. Cys35 on the n side of the A helix makes a single covalent thioether bond with heme x, placing the prosthetic group in close proximity to the center of the complex. The cavity floor is formed by the N-terminal 25 residues of cytochrome b, and by lipid head groups that fill the cavity in situ. The intercofactor distances and the organization of 8 of the 13 transmembrane helices (A to D in cytochrome b6, E and F in subunit IV, ISP, and cytochrome f) are similar in the b6f and bc1 complexes, but access to the Qp and Qn sites within the cavity are different. As in vascular plants, in high light-stressed cells of Chlamydomonas the formation of zeaxanthin due to increased VDE activity correlates with an enhanced thermal dissipation of singlet chlorophyll in the light-harvesting antennae (Niyogi et al., 1997b), and zeaxanthin has also been implicated in the scavenging of reactive oxygen species (Baroli et al., 2003). Previous research has linked singlet oxygen to light-induced oxidative damage and photoinhibition of PSII (Hideg et al., 1998). 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). However, a NOS similar to those found in mammals has not been found in higher plants. By continuing you agree to the use of cookies. U.M.N. The two monomers form a protein-free central cavity on each side of the thylakoid membrane for the p-side and n-side domains. A heme is a prosthetic group that contains an iron atom in the center of a heterocyclic organic ring. Cytochrome b. carry out similar electron transport functions despite being located in the chloroplast and mitochondria respectively. Electron transfer through Cytb6f is coupled with photon pumping into the thylakoid to produce a transthylakoid proton motive force to drive the synthesis of ATP (Sierla et al., 2013; Figure 24.1). The major functions that have been suggested are that it may operate as a sunscreen to shield the photosynthetic pigments from excessive light (Yong and Lee, 1991; Hagen et al., 1994; Wang et al., 2003) or that it might prevent oxidative damage caused by reactive oxygen species.