Evidence for a hetero-oligomeric structure of the chloroplast cytochrome b -559

The N-terminal sequence of the extrinsic PsbP protein modulates the redox potential of Cyt b559 in photosystem II

The PsbP protein, an extrinsic subunit of photosystem II (PSII) in green plants, is known to induce a conformational change around the catalytic Mn₄CaO₅ cluster securing the binding of Ca²⁺ and Cl^– in PSII. PsbP has multiple interactions with the membrane subunits of PSII, but how these affect the structure and function of PSII requires clarification. Here, we focus on the interactions between the N-terminal residues of PsbP and the α subunit of Cytochrome (Cyt) b₅₅₉ (PsbE). A key observation was that a peptide fragment formed of the first N-terminal 15 residues of PsbP, ‘pN15’, was able to convert Cyt b₅₅₉ into its HP form. Interestingly, addition of pN15 to NaCl-washed PSII membranes decreased PSII’s oxygen-evolving activity, even in the presence of saturating Ca²⁺ and Cl^– ions. In fact, pN15 reversibly inhibited the S₁ to S₂ transition of the OEC in PSII. These data suggest that pN15 can modulate the redox property of Cyt b₅₅₉ involved in the side-electron pathway in PSII. This potential change of Cyt b₅₅₉, in the absence of the C-terminal domain of PsbP, however, would interfere with any electron donation from the Mn₄CaO₅ cluster, leading to the possibility that multiple interactions of PsbP, binding to PSII, have distinct roles in regulating electron transfer within PSII.

Light-induced quinone reduction in photosystem II

The photosystem II core complex is the water:plastoquinone oxidoreductase of oxygenic photosynthesis situated in the thylakoid membrane of cyanobacteria, algae and plants. It catalyzes the light-induced transfer of electrons from water to plastoquinone accompanied by the net transport of protons from the cytoplasm (stroma) to the lumen, the production of molecular oxygen and the release of plastoquinol into the membrane phase. In this review, we outline our present knowledge about the "acceptor side" of the photosystem II core complex covering the reaction center with focus on the primary (Q(A)) and secondary (Q(B)) quinones situated around the non-heme iron with bound (bi)carbonate and a comparison with the reaction center of purple bacteria. Related topics addressed are quinone diffusion channels for plastoquinone/plastoquinol exchange, the newly discovered third quinone Q(C), the relevance of lipids, the interactions of quinones with the still enigmatic cytochrome b559 and the role of Q(A) in photoinhibition and photoprotection mechanisms. This article is part of a Special Issue entitled: Photosystem II.

Evidence for a Novel Quinone-Binding Site in the Photosystem II (PS II) Complex That Regulates the Redox Potential of Cytochrome b559

The present study provides a thorough analysis of effects on the redox properties of cytochrome (Cyt) b559 induced by two photosystem II (PS II) herbicides [3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,4-dinitro-6-sec-butylphenol (dinoseb)], an acceleration of the deactivation reactions of system Y (ADRY) agent carbonylcyanide-m-chlorophenylhydrazone (CCCP), and the lipophilic PS II electron-donor tetraphenylboron (TPB) in PS II membrane fragments from higher plants. The obtained results revealed that (1) all four compounds selectively affected the midpoint potential (E(m)) of the high potential (HP) form of Cyt b559 without any measurable changes of the E(m) values of the intermediate potential (IP) and low potential (LP) forms; (2) the control values from +390 to +400 mV for HP Cyt b559 gradually decreased with increasing concentrations of DCMU, dinoseb, CCCP, and TPB; (3) in the presence of high TPB concentrations, a saturation of the E(m) decrease was obtained at a level of about +240 mV, whereas no saturation was observed for the other compounds at the highest concentrations used in this study; (4) the effect of the phenolic herbicide dinoseb on the E(m) is independent of the occupancy of the Q(B)-binding site by DCMU; (5) at high concentrations of TPB or dinoseb, an additional slow and irreversible transformation of HP Cyt b559 into IP Cyt b559 or a mixture of the IP and LP Cyt b559 is observed; and (6) the compounds stimulate autoxidation of HP Cyt b559 under aerobic conditions. These findings lead to the conclusion that a binding site Q(C) exists for the studied substances that is close to Cyt b559 and different from the Q(B) site. On the basis of the results of the present study and former experiments on the effect of PQ extraction and reconstitution on HP Cyt b559 [Cox, R. P., and Bendall, D. S. (1974) The functions of plastoquinone and beta-carotene in photosystem II of chloroplasts, Biochim. Biophys. Acta 347, 49-59], it is postulated that the binding of a plastoquinone (PQ) molecule to Q(C) is crucial for establishing the HP form of Cyt b559. On the other hand, the binding of plastoquinol (PQH2) to Q(C) is assumed to cause a marked decrease of E(m), thus, giving rise to a PQH2 oxidase function of Cyt b559. The possible physiological role of the Q(C) site as a regulator of the reactivity of Cyt b559 is discussed.

Effect of dehydration on light-induced reactions in photosystem II: photoreactions of cytochrome b559

The effect of dehydration on the reaction pattern of photosystem II (PS II) has been studied by measuring and analyzing spectral changes induced by continuous wavelength illumination in films of untreated and hydroxylamine-washed PS II membrane fragments dehydrated to different levels. The obtained data revealed (i) the extent of light-induced formation of about one Q(A)(-*)per 230 chlorophylls (Chl) remains virtually invariant to dehydration down to the lowest values of relative humidity (6-8% RH); (ii) a decrease of the RH to 30% leads to severe blockage of the electron transfer from Q(A)(-*) to Q(B) and the progressive replacement of water oxidation by photooxidation of high potential (HP) cytochrome (Cyt) b559 in untreated PS II samples or accessory Chl and carotenoid (Car) molecules in samples with preoxidized Cyt b559; (iii) photooxidation of Cyt b559 is followed by its photoreduction, concomitant with photooxidation of Chl and Car; (iv) in dry samples with preoxidized Cyt b559, not more than a half of total Cyt b559 can be photochemically reduced, independent of the extent of Cyt b559 in the HP form; (v) at low RH values, Cyt b559 photoreduction in samples with preoxidized heme groups and photoaccumulation of Q(A)(-*) take place with biphasic kinetics with similar rate constants for both processes; (vi) Cyt b559 photoreduction in dry samples is DCMU insensitive, while the dark rereduction of photooxidized Cyt b559 is inhibited by DCMU; (vii) fast and slow kinetic phases of Cyt b559 photoreduction dramatically differ in their dependencies on the intensity of CW illumination and are associated with electron donation to Cyt b559 from Q(A)(-*) and pheophytin(-*), respectively. The pathways of light-induced electron transfer in PS II involving Cyt b559 are discussed.

Histidine residue 252 of the Photosystem II D1 polypeptide is involved in a light-induced cross-linking of the polypeptide with the alpha subunit of cytochrome b-559: study of a site-directed mutant of Synechocystis PCC 6803

Properties of the Photosystem II (PSII) complex were examined in the wild-type (control) strain of the cyanobacterium Synechocystis PCC 6803 and its site-directed mutant D1-His252Leu in which the histidine residue 252 of the D1 polypeptide was replaced by leucine. This mutation caused a severe blockage of electron transfer between the PSII electron acceptors Q(A) and Q(B) and largely inhibited PSII oxygen evolving activity. Strong illumination induced formation of a D1-cytochrome b-559 adduct in isolated, detergent-solubilized thylakoid membranes from the control but not the mutant strain. The light-induced generation of the adduct was suppressed after prior modification of thylakoid proteins either with the histidine modifier platinum-terpyridine-chloride or with primary amino group modifiers. Anaerobic conditions and the presence of radical scavengers also inhibited the appearance of the adduct. The data suggest that the D1-cytochrome adduct is the product of a reaction between the oxidized residue His(252) of the D1 polypeptide and the N-terminal amino group of the cytochrome alpha subunit. As the rate of the D1 degradation in the control and mutant strains is similar, formation of the adduct does not seem to represent a required intermediary step in the D1 degradation pathway.

Factors determining the special redox properties of photosynthetic cytochrome b559

Factors controlling the redox properties of the two conventional forms of cytochrome b559, i.e. the unstable high-potential form and the stable low-potential form, have been further investigated using PSII-enriched membranes from pea and spinach chloroplasts. The redox potential of the stable form of cytochrome b559 is pH independent both above pH 7.5 (E'm approximately +110 mV) and below pH 6.0 (E'm approximately +203 mV), but it changes with a slope of 58 mV per pH unit between these two pH values. Thus, cytochrome b559 seems to have a single ionizing group influencing its redox potential, with a higher affinity for protons in the reduced form (pK(red) = 7.5) and a lower affinity in the oxidized form (pK(ox) = 6.0); consequently, one unprotonated low-potential form (LP) and one protonated intermediate-potential form (IP). The redox potential of the high-potential form (HP) is pH-independent between pH 5.0 and 8.0, but its relative content (compared to the total amount of protein) decreases progressively above pH 7.0. This conversion to the stable LP form is interpreted as corresponding to the loss of a proton by one ionizing group, the protonation of which is essential for maintaining the unstable HP state. According to chemical modification experiments with diethylpyrocarbonate, one of the two histidine ligands of the heme seems to be the ionizing group responsible for the existence of both the protonated IP and HP forms. It is proposed that the difference between the IP and HP forms is due to the formation of an additional hydrogen bond between the protonated histidine and the protein in the HP state that stabilizes a special hydrophobic heme environment responsible for its high redox potential.

Assembly of chloroplast cytochromes b and c

The synthesis of holocytochromes in plastids is a catalyzed process. Several proteins, including plastid CcsA, Ccs1, possibly CcdA and a thioredoxin, plus at least two additional Ccs factors, are required in sub-stoichiometric amounts for the conversion of apocytochromes f and c(6) to their respective holoforms. CcsA, proposed to be a heme delivery factor, and Ccs1, an apoprotein chaperone, are speculated to interact physically in vivo. The formation of holocytochrome b(6) is a multi-step pathway in which at least four, as yet unidentified, Ccb factors are required for association of the b(H) heme. The specific requirement of reduced heme for in vitro synthesis of a cytochrome b(559)-derived holo-beta(2) suggests that cytochrome b synthesis in PSII might also be catalyzed in vivo.

Interconversion of low- and high-potential forms of cytochrome b(559) in Tris-washed photosystem II membranes under aerobic and anaerobic conditions

In this study, the reversible conversion between the high- (HP) and low-potential (LP) forms of Cytb(559) has been analyzed in Tris-washed photosystem II (PSII) enriched membranes. These samples are deprived of the Mn cluster of the water-oxidizing complex (WOC) and the extrinsic regulatory proteins. The results obtained by application of optical and EPR spectroscopy reveal that (i) under aerobic conditions, the vast majority of Cytb(559) exhibits a low midpoint potential, (ii) after removal of O(2) in the dark, a fraction of Cytb(559) is converted to the high-potential form which reaches level of about 25% of the total Cytb(559), (iii) a similar dark transformation of LP --> HP Cytb(559) occurs under reducing conditions (8 mM hydroquinone), (iv) under anaerobic conditions and in the presence of 8 mM hydroquinone, about 60% of the Cytb(559) attains the HP form, (v) the interconversion is reversible with the re-establishment of aerobic conditions, and (vi) aerobic and oxidizing conditions (2 mM ferricyanide or 0.5 mM potassium iridate) induce a decrease of the amount of the HP form, also showing that the conversion is reversible. This reversible interconversion between LP and HP Cytb(559) is not observed in PSII membrane fragments with an intact WOC. On the basis of these findings, the possibility is discussed that the O(2)-dependent conversion of Cytb(559) in PSII complexes lacking a functionally competent WOC is related to a protective role of Cytb(559) in photoinhibition and/or that it is involved in the regulation of the assembly of a competent water-oxidizing complex in PSII.

Restoration of the high-potential form of cytochrome b559 of photosystem II occurs via a two-step mechanism under illumination in the presence of manganese ions

Spinach photosystem II membranes that had been depleted of the Mn cluster contained four forms of cytochrome (Cyt) b559, namely, high-potential (HP), HP', intermediate-potential (IP) and low-potential (LP) forms that exhibited the redox potentials of +400, +310, +170 and +35 mV, respectively, in potentiometric titration. When the membranes were illuminated with flashing light in the presence of 0.1 mM Mn2+, the IP form was converted to the HP' form by two flashes and then the HP' form was converted to the HP form by an additional flash. The quantum efficiency of the first conversion appeared to be quite high since the conversion was almost complete after two flashes. By contrast, the second conversion proceeded with low quantum efficiency and 40 flashes were required for completion. The effects of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) suggested that the first conversion did not require electron transfer from QA to QB while the second conversion had an absolute requirement for it. It was also suggested that the first conversion involved the reduction of the heme of Cyt b559, probably by QA-, and we propose that direct reduction by QA- induces a shift in the redox potential of the heme. The second conversion was also accompanied by the reduction of heme but it appeared that this conversion did not necessarily involve the reduction. The effects of DCMU on the reduction of heme suggested that the heme became reducible by QB- after the first conversion had been completed. This observation implies that the efficiency of electron transfer from QA to QB increased upon the conversion of the IP form to the HP' form, and we propose that restoration of the high-potential forms of Cyt b559 itself acts to make the acceptor side of photosystem II functional.

In vitro assembly of a beta2 cytochrome b559-like complex from the chemically synthesised beta-subunit encoded by the Synechocystis sp. 6803 psbF gene

The alpha- and beta-subunits of cytochrome b559 encoded by the psbE and psbF gene, respectively, are essential components of photosystem II. The exact structure of this cytochrome is not yet known. The beta-subunit of the Synechocystis sp. 6803 cytochrome b559 complex was synthesised by means of solid-phase peptide synthesis. Under reducing conditions, two beta-peptide molecules could be assembled specifically with one haem to form a beta2 cytochrome b559-like complex. The spectral properties and the midpoint redox potential (48+/-5 mV) of the in vitro assembled beta2 cytochrome are nearly identical to those of the low potential form of the native cytochrome b559.

Structural model of cytochrome b559 in photosystem II based on a mutant with genetically fused subunits

Photosystem II is a reaction center protein complex located in photosynthetic membranes of plants, algae, and cyanobacteria. Using light energy, photosystem II catalyzes the oxidation of water and the reduction of plastoquinone, resulting in the release of molecular oxygen. A key component of photosystem II is cytochrome b559, a membrane-embedded heme protein with an unknown function. The cytochrome is unusual in that a heme links two separate polypeptide subunits, alpha and beta, either as a heterodimer (alphabeta) or as two homodimers (alpha2 and beta2). To determine the structural organization of cytochrome b559 in the membrane, we used site-directed mutagenesis to fuse the coding regions of the two respective genes in the cyanobacterium Synechocystis sp. PCC 6803. In this construction, the C terminus of the alpha subunit (9 kDa) is attached to the N terminus of the beta subunit (5 kDa) to form a 14-kDa alphabeta fusion protein that is predicted to have two membrane-spanning alpha-helices with antiparallel orientations. Cells containing the alphabeta fusion protein grow photoautotrophically and assemble functional photosystem II complexes. Optical spectroscopy shows that the alphabeta fusion protein binds heme and is incorporated into photosystem II. These data support a structural model of cytochrome b559 in which one heme is coordinated to an alpha2 homodimer and a second heme is coordinated to a beta2 homodimer. In this model, each photosystem II complex contains two cytochrome b559 hemes, with the alpha2 heme located near the stromal side of the membrane and the beta2 heme located near the lumenal side.

Pigment quantitation and analysis by HPLC reverse phase chromatography: a characterization of antenna size in oxygen-evolving photosystem II preparations from cyanobacteria and plants

Photosystem II, the photosynthetic water-oxidizing complex, can be isolated from both plants and cyanobacteria. A variety of methods have been developed for purification of this enzyme, which can be isolated in several functional and structural forms. Knowledge of the pigment content of photosystem II preparations is important for precise spectroscopic, biochemical, and functional analysis. We have determined pigment stoichiometries in oxygen-evolving photosystem II preparations from plants and cyanobacteria. We have employed a solvent system for the isocratic elution of a reverse phase HPLC column in which we have determined the extinction coefficients of the relevant pigments. Pigments were extracted from four photosystem II preparations. These preparations included spinach photosystem II membranes [Berthold, D. A., Babcock, G. T., & Yocum, C. F. (1981) FEBS Lett. 134, 231-234], spinach photosystem II reaction center complexes [Ghanotakis, D. F., & Yocum, C. F. (1986) FEBS Lett. 197, 244-248], spinach photosystem II complexes [MacDonald, G. M., & Barry, B. A. (1992) Biochemistry 31, 9848-9856], and photosystem II particles isolated from the cyanobacterium, Synechocystis sp. PCC 6803 [Noren, G. H., Boerner, R. J., & Barry, B. A. (1991) Biochemistry 30, 3943-3950]. Pigment stoichiometries were determined using two different methods of data analysis and were based on the assumption that there are two pheophytin a molecules per photosystem II reaction center. The pigment stoichiometries obtained were comparable for the two methods of data analysis and agreed with previous biophysical and biochemical characterizations of the preparations. The average pigment stoichiometries (chlorophyll:plastoquinone-9 per 2 pheophytin a) determined using the two data analysis methods were as follows: photosystem II membranes, 274:3.2; photosystem II reaction center complexes, 78:2.5; Synechocystis PS II particles, 55:2.4; photosystem II complexes, 121:2.0.

Composition and function of cytochrome b559 in reaction centers of photosystem II of green plants

A review of a recent study of the spectral and thermodynamic properties of cytochrome b559 as well as of the electron transfer between b559 and photosystem II reaction center cofactors in isolated D1/D2/cytochrome b559 complex RC-2 is presented. Attention is paid to the existence of intermediary-potential (IP, +150 mV) and extra-low-potential (XLP, -45 mV) hemes located close to the acceptor (quinone) and donor (P680) sides of the reaction center cofactors, respectively. These hemes found in isolated RC-2 probably correspond to the high-potential and low-potential hemes in chloroplasts, respectively. The above location of the hemes is believed to allow the photoreduction of the XLP heme and photooxidation of the IP heme. The electron transfer between the two hemes is discussed in terms of the cyclic electron flow and possible involvement in water splitting.

Topography of the heme prosthetic group of cytochrome b-559 in the photosystem II reaction center

The topography of the heme prosthetic group of cytochrome b-559 of the photosystem II reaction center was determined from measurement of the orientation of its alpha- and beta-polypeptides in thylakoid membranes of spinach chloroplasts and in osmotically disrupted cells of the cyanobacterium Synechocystis sp. PCC 6803. The accessibility to trypsin proteolysis of an epitope located near the solvent-exposed N-terminus of the beta-subunit was compared to that of the alpha-subunit, whose N- and C-termini had previously been localized from the trypsinolysis pattern to the stromal and lumenal sides of spinach thylakoid membranes, respectively [Tae et al. (1988) Biochemistry 27, 9075-9080; Vallon et al. (1989) Biochim. Biophys. Acta 975, 132-141]. The N-terminal epitope of the cyanobacterial beta-subunit was modified by introducing a tridecapeptide epitope, previously found to be immunoreactive, from the C-terminal region of the spinach chloroplast alpha-subunit. This epitope had no homology with the cyanobacterial alpha-subunit. The cells with the hybrid beta-subunit retained full photosynthetic activity. The intactness of membranes from osmotically shocked cyanobacteria was tested by trypsin inaccessibility to (a) the alpha-subunit C-terminus and (b) the manganese-stabilizing protein (MSP) of the oxygen-evolving complex that is on the lumenal side of the membrane. The loss after trypsinolysis of most of the beta-subunit immunoreactivity, under conditions where (i) the alpha-subunit was cleaved near the N-terminus in both spinach thylakoids and osmotically shocked cyanobacterial membranes and (ii) the MSP protein in cyanobacteria was not disrupted, implied that the orientation of the beta-subunit was parallel to that of the alpha-subunit in both kinds of membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of cytochrome b-559 content in photosystem II complexes from spinach and Synechocystis species PCC 6803

Cytochrome b-559 is an integral component of photosystem II complexes from both plants and cyanobacteria. However, the number of cytochrome b-559 associated with the photosystem II reaction center has been the subject of controversy. Some studies have concluded that there is one heme equivalent of cytochrome b-559 per reaction center, some studies have found two, and some studies have reported intermediate values. Most of the previous experiments have used only one method to quantitate the antenna size of the preparation. In this study, we compare the cytochrome b-559 content in a cyanobacterial and a plant photosystem II preparation. The plant preparation is derived from spinach, and previous work has shown that it has an antenna size of approximately 100 chlorophylls [MacDonald, G. M., & Barry, B. A. (1992) Biochemistry 31, 9848-9856]. The cyanobacterial preparation is from Synechocystis sp. PCC 6803, and previous work has shown that it has an antenna size of approximately 60 chlorophylls [Noren, G. H., Boerner, R. J., & Barry, B. A. (1991) Biochemistry 30, 3943-3950]. Both preparations are isolated through the use of ion-exchange chromatography, and both preparations are monodisperse in the same nonionic detergent. In our comparative study, we quantitate antenna size by three different methods. Our work shows that, depending on the method used to estimate antenna size, the oxygen-evolving spinach photosystem II preparation contains 0.82-1.0 cytochrome b-559 per reaction center, while the oxygen-evolving cyanobacterial preparation contains 1.5-2.1 cytochrome b-559 per reaction center.

Spectral and thermodynamic properties of the two hemes of the D1D2cytochrome b-559 complex of spinach

In agreement with previous work [Shuvalov, Heber and Schreiber (1988) FEBS Lett. 258, 27-31] two hemes (low potential (LP) and extra low potential (XLP)) per two pheophytins were found in isolated D1D2Cyt b-559 complexes. Reductive and oxidative redox titrations demonstrate that the Em of the LP form is at about +150 mV. It is independent of pH between pH 7.2 and 9.4. The XLP heme is autoxidizable at pH 7.2 and displays, at this pH, an Em of -45 mV. Both the LP and XLP hemes show absorption peaks at 559 nm. They are proposed to have bis-histidine ligation of the heme iron. At pH 9.4, the XLP heme splits into two forms. One of them has an Em of +40 mV, and absorption peaks at 559 nm showing the bis-histidine ligation. The other displays an Em of -220 mV and the peak is shifted to 562 nm. This last form is proposed to be due to the incorporation of OH- which occupies the 6th coordination position of the heme Fe(III) at high pH. The pK value for the conversion of the XLP heme is close to 7.7. In a structure simulation of the alpha-helices of alpha- and beta-polypeptide, the beta-polypeptide, but not the alpha-polypeptide, reveals a distance between the histidine N and the heme Fe which permits stable N-Fe coordination. In the alpha-polypeptide, OH- can be incorporated between N and Fe. The functional role of the two hemes of cyt b-559 is briefly discussed with respect to water oxidation and cyclic electron transfer.

The enigmatic cytochrome b-559 of oxygenic photosynthesis

The ubiquitous and obligatory association of cytochrome b-559 with the photosystem II reaction center of oxygenic photosynthesis is a conundrum since it seems not to have a function in the primary electron transport pathway of oxygen evolution. A model for the cytochrome structure that satisfies the cis-positive rule for membrane protein assembly consists of two short, non-identical hydrophobic membrane-spanning polypeptides (α and β), each containing a single histidine residue, as ligands for the bridging heme prosthetic group that is on the side of the membrane opposite to the water splitting apparatus. The ability of the heterodimer, but not the single α-subunit, to satisfy the cis-positive rule implies that the cytochrome inserts into the membrane as a heterodimer, with some evidence implicating it as the first membrane inserted unit of the assembling reaction center. The very positive redox potential of the cytochrome can be explained by a position for the heme in a hydrophobic niche near the stromal aqueous interface where it is also influenced by the large positive dipole potential of the parallel α-helices of the cytochrome. The requirement for the cytochrome in oxygenic photosynthesis may be a consequence of the presence of the strongly oxidizing reaction center needed for H₂ O-splitting. This may lead to the need, under conditions of stress or plastid development, for an alternate source of electrons when the H₂ O-splitting system is not operative as a source of reductant for the reaction center.

The 'positive-inside rule' applies to thylakoid membrane proteins

In integral membrane proteins, regions that span the lipid bilayer alternate with regions that are exposed on either side of the membrane. For proteins from the plasma membrane of both prokaryotic and eukaryotic cells it has been shown that the exposed parts follow a 'positive-inside rule': on average, segments that are translocated across the membrane have a 2-4-fold lower frequency of positively charged residues than non-translocated segments. We now present an analysis of proteins from the thylakoid membrane of chloroplasts. It is shown that these proteins have the same charge asymmetry as has been reported for proteins from other membrane systems, with their more highly charged regions facing the stromal compartment.

Enhanced susceptibility of the oxidized and unprotonated forms of high potential cytochrome b-559 toward DCMU

The nature of interaction of cytochrome b-559 high potential (HP) with electron transport on the reducing side of photosystem II was investigated by measuring the susceptibility of cytochrome b-559HP to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) under different conditions. Submicromolar DCMU concentrations decreased the rate of absorbance change corresponding to cytochrome b-559HP photoreduction while the amplitude was lowered at higher concentrations (up to 10 μM). Appreciable extents of cytochrome b-559HP photoreduction were observed at DCMU concentrations which completely abolished the electron transport from water to methyl viologen under the same experimental conditions. However, the susceptibility of cytochrome b-559HP to DCMU increased with the degree of cytochrome b-559HP oxidation, induced either by ferricyanide or by illumination of low intensity (2 W/m(2)) of red light in the presence of 2 μM carbonyl cyanide-m-chlorophenylhydrazone. Also, the DCMU inhibition was more severe when the pH increased from 6.5 to 8.5, indicating that the unprotonated form of cytochrome b-559HP is more susceptible to DCMU. These results demonstrate that cytochrome b-559HP can accept electrons prior to the QB site, probably via QA although both QA and QB can be involved to various extents in this reaction. We suggest that the redox state and the degree of protonation of cytochrome b-559HP alter its interaction with the reducing side of photosystem II.

Prediction and comparison of the haem-binding sites in membrane haemoproteins

This article contains a comparative review of the structural properties of membrane haemoproteins, with particular emphasis on the possible similarities of the haem-binding peptides. A procedure is suggested for identifying the peptides which may bind membrane-buried haems on the basis of the primary sequences of the proteins. The integration of this procedure with the information deduced by refined hydropathy analysis indicates that the basic structural model for the haemoproteins which interact with quinones may be a transmembrane helical bundle containing the haem(s) at its centre. Structural similarities exist in the sequence of hydrophobic segments that are predicted to bind the membrane-buried haems of b-cytochromes which interact with quinones. The predicted haem-binding sites show similarities also with the peptides that bind the non-haem iron in the bacterial reaction centres, and this may be correlated to the common function of interacting with quinones and their intermediates. The analysis of the amino-acid composition of the proposed ligand peptides in the membrane haemoproteins examined has provided a molecular rationale for explaining the highly anisotropic low-spin EPR signal which is characteristic of many membrane-bound b-cytochromes.

N-terminal sequencing of low-molecular-mass components in cyanobacterial photosystem II core complex. Two components correspond to unidentified open reading frames of plant chloroplast DNA

We recently reported the presence of several low-molecular-mass protein components in the PS II O2-evolving core complex from the thermophilic cyanobacterium, Synechococcus vulcanus [(1989) FEBS Lett. 244, 391-396]. Here we have characterized the three components (4.1, 4.7, 5 kDa) of the same cyanobacterial core complex by N-terminal sequencing. There were two components in the 4.7 kDa region, both having a blocked N-terminus. One has a sequence highly homologous to open reading frame 34 of plant chloroplast DNA (tentatively designated psbM), while the other has a sequence partially homologous to open reading frame 43 of chloroplast DNA (designated psbN), although neither of the two gene products has yet been confirmed in chloroplasts. The cyanobacterial 4.1 kDa protein partially corresponds to the 4.1 kDa nuclear-encoded core component of higher plant PS II. The cyanobacterial 5 kDa component, however, shows a sequence that is unrelated to any other known proteins.

Visualization of antibody binding to the photosynthetic membrane: the transmembrane orientation of cytochrome b-559

We have used immuno-gold labeling and electron microscopy to study the topography of thylakoid membrane polypeptides. Thylakoid vesicles formed by passage through a French press were adsorbed onto a plastic film supported by an electron microscope grid and processed for single or double immuno-gold labeling. After shadowing with platinum, the inside-out and right-side-out vesicles were identified by their distinctive morphologies. Right-side-out vesicles were labeled by a monoclonal antibody recognizing an epitope located in the trypsin-cleaved, N-terminal portion of the LHC II apoprotein, and by an antibody to CF1. A monoclonal antibody to the alpha-subunit of cytochrome b-559 reacted with a synthetic tridecapeptide corresponding to the C-terminal portion of the polypeptide. Both this antibody and a polyclonal antibody to the synthetic peptide labeled inside-out vesicles exclusively, indicating that the polypeptide C-terminus was exposed on the lumenal (exoplasmic) surface of the membrane.

Two small open reading frames are co-transcribed with the pea chloroplast genes for the polypeptides of cytochrome b-559

The genes encoding the 9 kDa and 4 kDa polypeptides of cytochrome b-559 have been located in pea chloroplast DNA by coupled transcription-translation of cloned restriction fragments of chloroplast DNA in a cell-free extract of Escherichia coli and by nucleotide sequence analysis. The genes (psbE and psbF) are located approximately 1.0 kbp downstream of the gene for cytochrome f and are transcribed in the opposite direction, similar to the arrangement in the chloroplast genomes of other higher plants. Nucleotide sequence analysis of this region revealed four open reading frames encoding hydrophobic proteins of 83 (psbE), 39 (psbF), 38 and 40 amino acid residues, which are co-transcribed as a single major RNA of 1.1 kb. The 5' and 3' ends of this RNA have been located by primer extension and S1 nuclease mapping. The 5' end of the RNA is located 140 bp upstream of the initiating ATG codon of psbE and is preceded by typical chloroplast promoter sequences. The 3' end of the RNA is located approximately 515 bp downstream of the TAA stop codon of psbF close to a stable stem-loop structure.

Amino acid sequence of the bacterioferritin (cytochrome b1) of Escherichia coli-K12

The complete amino acid sequence of bacterioferritin (cytochrome b1) from Escherichia coli-K12 has been derived from the nucleotide sequence of the cloned gene. It comprises 158 amino acid residues giving an Mr of 18,495. The identity of the gene product was confirmed by an 87 residue N-terminal sequence obtained from the purified protein, but it differs significantly from much of the previously published partial amino acid sequence (1). Secondary structure prediction indicates a high alpha-helical content consistent with a 4-helix-bundle conformation. The fully assembled bacterioferritin molecule comprising 24 identical subunits and 12 haem moieties is a tetracosamer with an Mr of approximately 452,000.

Molecular size and symmetry of the bacterioferritin of Escherichia coli. X-ray crystallographic characterization of four crystal forms

X-ray crystallographic data from four crystal forms of Escherichia coli bacterioferritin show that the molecule has a diameter in the range 119 to 128 A. Molecules are composed of 24 subunits arranged in 432 symmetry. In both size and symmetry the molecule resembles ferritin from eukaryotes. The four crystal forms are monoclinic, space group P2(1) with unit cell dimensions a = 118.7 A, b = 211.6 A, c = 123.3 A and beta = 119.1 degrees; orthorhombic, C222(1), a = 128.7 A, b = 197.1 A, c = 202.8 A; tetragonal, P4(2)2(1)2, a = b = 210.6 A, c = 145.0 A and cubic, I432, a = 146.9 A.

N-terminal sequencing of photosystem II low-molecular-mass proteins. 5 and 4.1 kDa components of the O2-evolving core complex from higher plants

High resolution gel electrophoresis in the low-molecular-mass region combined with electroblotting using polyvinylidene difluoride membranes enabled us to sequence the low-molecular-mass proteins of photosystem II membrane fragments from spinach and wheat. The determined N-terminal sequences, all showing considerable homology between the two plants, involved two newly determined sequences for the 4.1 kDa protein and one for the 5 kDa proteins. The sequence of the 4.1 kDa protein did not match any part of the chloroplast DNA sequence from tobacco or liverwort, suggesting that it is encoded by the nuclear genome. In contrast, the sequence of the 5 kDa protein matched ORF38, which is located just downstream of psbE and psbF in the chloroplast DNA and is assumed to be co-transcribed with them. These two components were associated with the O2-evolving core complex. Sequences of other low-molecular-mass proteins confirmed the previous identification as photosystem II components.

Thylakoid membrane protein topography: transmembrane orientation of the chloroplast cytochrome b-559 psbE gene product

Protease accessibility and antibody to a COOH-terminal peptide were used as probes for the in situ topography of the Mr 10,000 psbE gene product (alpha subunit) of the chloroplast cytochrome b-559. Exposure of thylakoid membranes to trypsin or Staphylococcus aureus V8 protease cleaved the alpha subunit to a slightly smaller polypeptide (delta Mr approximately -1000) as detected on Western blots, without loss of reactivity to COOH-terminal antibody. The disappearance of the parent Mr 10,000 polypeptide from thylakoids in the presence of trypsin correlated with the appearance of the smaller polypeptide with delta Mr = -750, the conversion having a half-time of approximately 15 min. Exposure of inside-out vesicles to trypsin resulted in almost complete loss of reactivity to the antibody, showing that the COOH terminus is exposed on the lumenal side of the membrane. Removal of the extrinsic polypeptides of the oxygen-evolving complex resulted in an increase of the accessibility of the alpha subunit to trypsin. These data establish that the alpha subunit of cytochrome b-559 crosses the membrane once, as predicted from its single, 26-residue, hydrophobic domain. The NH2 terminus of the alpha polypeptide is on the stromal side of the membrane, where it is accessible, most likely at Arg-7 or Glu-6/Asp-11, to trypsin or V8 protease, respectively. As a consequence of this orientation, the single histidine residue in the alpha subunit is located on the stromal side of the hydrophobic domain.(ABSTRACT TRUNCATED AT 250 WORDS)

A new photosystem II reaction center component (4.8 kDa protein) encoded by chloroplast genome

The photosystem II reaction center complex, so-called D1-D2-cytochrome b-559 complex, isolated from higher plants contains a new component of about 4.8 kDa [(1988) Plant Cell Physiol. 29, 1233-1239]. The partial amino acid sequence of this component from spinach was determined after release of N-terminal blockage. The determined sequence matched an open reading frame (ORF36) of the chloroplast genome from tobacco and liverwort, which is located downstream from the psbK gene and forms an operon with psbK. The predicted product consists of 36 amino acid residues and has a single membrane-spanning segment. High homology between the tobacco and liverwort genes, and its presence in the reaction center complex suggest an important role for this component in the photosystem II complex. Since this gene corresponds to a part of the formerly designated psbI gene, we propose to revise the definition of psbI as the gene encoding the 4.8 kDa reaction center component.

Nicotiana chloroplast genes for components of the photosynthetic apparatus

In order to understand more fully chloroplast genetic systems, we have determined the complete nucleotide sequence (155, 844 bp) of tobacco (Nicotiana tabacum var. Bright Yellow 4) chloroplast DNA. It contains two copies of an identical 25,339 bp inverted repeat, which are separated by 86, 684 bp and 18,482 bp single-copy regions. The genes for 4 different rRNAs, 30 different tRNAs, 44 different proteins and 9 other predicted protein-coding genes have been located. Fifteen different genes contain introns.Twenty-two genes for components of the photosynthetic apparatus have so far been identified. Most of the genes (except the gene for the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase) code for thylakoid membrane proteins. Twenty of them are located in the large single-copy region and one gene for a 9-kd polypeptide of photosystem I is located in the small single-copy region. The gene for the 32-kd protein of photosystem II as well as the gene for the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase have strong promoters and are transcribed monocistronically while the other genes are transcribed polycistronically. We have found that the predicted amino acid sequences of six DNA sequences resemble those of components of the respiratory-chain NADH dehydrogenase from human mitochondria. As these six sequences are highly transcribed in tobacco chloroplasts, they are probably genes for components of a chloroplast NADH dehydrogenase. These observations suggest the existence of a respiratory-chain in the chloroplast of higher plants.