On the function of two vitamin K1 molecules in the PS I electron acceptor complex

The Phytol Phosphorylation Pathway Is Essential for the Biosynthesis of Phylloquinone, which Is Required for Photosystem I Stability in Arabidopsis

Phytyl-diphosphate, which provides phytyl moieties as a common substrate in both tocopherol and phylloquinone biosynthesis, derives from de novo isoprenoid biosynthesis or a salvage pathway via phytol phosphorylation. However, very little is known about the role and origin of the phytyl moiety for phylloquinone biosynthesis. Since VTE6, a phytyl-phosphate kinase, is a key enzyme for phytol phosphorylation, we characterized Arabidopsis vte6 mutants to gain insight into the roles of phytyl moieties in phylloquinone biosynthesis and of phylloquinone in photosystem I (PSI) biogenesis. The VTE6 knockout mutants vte6-1 and vte6-2 lacked detectable phylloquinone, whereas the phylloquinone content in the VTE6 knockdown mutant vte6-3 was 90% lower than that in wild-type. In vte6 mutants, PSI function was impaired and accumulation of the PSI complex was defective. The PSI core subunits PsaA/B were efficiently synthesized and assembled into the PSI complex in vte6-3. However, the degradation rate of PSI subunits in the assembled PSI complex was more rapid in vte6-3 than in wild-type. In vte6-3, PSI was more susceptible to high-light damage than in wild-type. Our results provide the first genetic evidence that the phytol phosphorylation pathway is essential for phylloquinone biosynthesis, and that phylloquinone is required for PSI complex stability.

The AAE14 gene encodes the Arabidopsis o-succinylbenzoyl-CoA ligase that is essential for phylloquinone synthesis and photosystem-I function

Phylloquinone is the one-electron carrier at the A(1) site of photosystem I, and is essential for photosynthesis. Arabidopsis mutants deficient in early steps of phylloquinone synthesis do not become autotrophic and are seedling lethals, even when grown on sucrose-supplemented media. Here, we identify acyl-activating enzyme 14 (AAE14, At1g30520) as the o-succinylbenzoyl-coenzyme A (OSB-CoA) ligase acting in phylloquinone synthesis. Three aae14 mutant alleles, identified by reverse genetics, were found to be seedling lethal, to contain no detectable phylloquinone (< 0.1 pmol mg(-1) fresh weight) compared with 10 pmol mg(-1) fresh weight in wild-type leaves, and to accumulate OSB. AAE14 was able to restore menaquinone biosynthesis when expressed in an Escherichia coli mutant disrupted in the menE gene that encodes the bacterial OSB-CoA ligase. Weak expression of an AAE14 transgene in mutant plants (controlled by the uninduced XVE promoter) resulted in chlorotic, slow-growing plants that accumulated an average of 4.7 pmol mg(-1) fresh weight of phylloquinone. Inducing the XVE promoter in these plants, or expressing an AAE14 transgene under the control of the CaMV 35S promoter, led to full complementation of the mutant phenotype. aae14-mutant plants were also able to synthesize phylloquinone when provided with 1,4-dihydroxy-2-naphthoate, an intermediate in phylloquinone synthesis downstream of the OSB-CoA ligase reaction. Expression of an AAE14:GFP reporter construct indicated that the protein accumulated in discrete foci within the chloroplasts. This and other evidence suggests that the enzymes of phylloquinone synthesis from isochorismate may form a complex in the chloroplast stroma to facilitate the efficient channeling of intermediates through the pathway.

Multiple Rieske proteins in prokaryotes: where and why?

Many microbial genomes have been sequenced in the recent years. Multiple genes encoding Rieske iron-sulfur proteins, which are subunits of cytochrome bc-type complexes or oxygenases, have been detected in many pro- and eukaryotic genomes. The diversity of substrates, co-substrates and reactions offers obvious explanations for the diversity of the low potential Rieske proteins associated with oxygenases, but the physiological significance of the multiple genes encoding high potential Rieske proteins associated with the cytochrome bc-type complexes remains elusive. For some organisms, investigations into the function of the later group of genes have been initiated. Here, we summarize recent finding on the characteristics and physiological functions of multiple high potential Rieske proteins in prokaryotes. We suggest that the existence of multiple high potential Rieske proteins in prokaryotes could be one way of allowing an organism to adapt their electron transfer chains to changing environmental conditions.

Inactivation and deficiency of core proteins of photosystems I and II caused by genetical phylloquinone and plastoquinone deficiency but retained lamellar structure in a T-DNA mutant of Arabidopsis

Phylloquinone, a substituted 1,4-naphthoquinone with an 18-carbon-saturated phytyl tail, functions as a bound one-electron carrier cofactor at the A1 site of photosystem I (PSI). A Feldmann tag line mutant, no. 2755 (designated as abc4 hereafter), showed pale-green young leaves and white old leaves. The mutated nuclear gene encoded 1,4-dihydroxy-2-naphtoic acid phytyltransferase, an enzyme of phylloquinone biosynthesis, and high-performance liquid chromatography analysis revealed that the abc4 mutant contained no phylloquinone, and only about 3% plastoquinone. Photooxidation of P700 of PSI in the abc4 mutant was not observed, and reduced-versus-oxidized difference spectroscopy indicated that the abc4 mutant had no P700. The maximum quantum yield of photosystem II (PSII) in the abc4 mutant was much decreased, and the electron transfer from PSII to PSI in the abc4 mutant did not occur. For the pale-green leaves of the abc4 mutant plant, the ultrastructure of the chloroplasts was almost the same as that of the wild-type plant. However, the chloroplasts in the albino leaves of the mutant were smaller and had a lot of grana thylakoids and few stroma thylakoids. The amounts of PSI and PSII core subunits in the abc4 mutant were significantly decreased compared with those in the wild type. These results suggested that a deficiency of phylloquinone in PSI caused the abolishment of PSI and a partial defect of PSII due to a significant decrease of plastoquinone, but did not influence the ultrastructure of the chloroplasts in young leaves.

Quenching of excited states of chlorophyll molecules in submembrane fractions of Photosystem I by exogenous quinones

The ability of three substituted quinones, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), 2,6-dichloro-p-benzoquinone (DCBQ), and tetramethyl-p-benzoquinone (duriquinone) to quench the excited states of chlorophyll (Chl) molecules in Photosystem I (PSI) was studied. Chl fluorescence emission measured with isolated PSI submembrane fractions was reduced following the addition of exogenous quinones. This quenching progressively increased with rising concentrations of the exogenous quinones according to the Stern-Volmer law. The values of Stern-Volmer quenching coefficients were found to be 3.28 x 10(5) M(-1) (DBMIB), 1.31 x 10(4) M(-1) (DCBQ), and 3.7 x 10(3) M(-1) (duroquinone). The relative quenching capacities of the various exogenous quinones in PSI thus strictly coincided to those found for the quenching of Fo level of Chl fluorescence in isolated thylakoids, which is emitted largely by Photosystem II (PSII) [Biochim. Biophys. Acta (2003) 1604, 115-123]. Quenching of Chl excited states in PSI submembrane fractions by exogenous quinones slowed down the rate of P700, primary electron donor of PSI, photooxidation measured at limiting actinic light irradiances thus revealing a reduced photochemical capacity of absorbed quanta. The possible involvement of non-photochemical quenching of excited Chl states by oxidized phylloquinones, electron acceptors of PSI, and oxidized plastoquinones, mobile electron carriers between PSII and the cytochrome b(6)/f complex, into the control of photochemical activity of PSI is discussed.

Electron transfer in cyanobacterial photosystem I: II. Determination of forward electron transfer rates of site-directed mutants in a putative electron transfer pathway from A0 through A1 to FX

The directionality of electron transfer in Photosystem I (PS I) is investigated using site-directed mutations in the phylloquinone (QK) and FX binding regions of Synnechocystis sp. PCC 6803. The kinetics of forward electron transfer from the secondary acceptor A1 (phylloquinone) were measured in mutants using time-resolved optical difference spectroscopy and transient EPR spectroscopy. In whole cells and PS I complexes of the wild-type both techniques reveal a major, slow kinetic component of tau approximately 300 ns while optical data resolve an additional minor kinetic component of tau approximately 10 ns. Whole cells and PS I complexes from the W697FPsaA and S692CPsaA mutants show a significant slowing of the slow kinetic component, whereas the W677FPsaB and S672CPsaB mutants show a less significant slowing of the fast kinetic component. Transient EPR measurements at 260 K show that the slow phase is approximately 3 times slower than at room temperature. Simulations of the early time behavior of the spin polarization pattern of P700+A1-, in which the decay rate of the pattern is assumed to be negligibly small, reproduce the observed EPR spectra at 260 K during the first 100 ns following laser excitation. Thus any spin polarization from P700+FX- in this time window is very weak. From this it is concluded that the relative amplitude of the fast phase is negligible at 260 K or its rate is much less temperature-dependent than that of the slow component. Together, the results demonstrate that the slow kinetic phase results from electron transfer from QK-A to FX and that this accounts for at least 70% of the electrons. Although the assignment of the fast kinetic phase remains uncertain, it is not strongly temperature dependent and it represents a minor fraction of the electrons being transferred. All of the results point toward asymmetry in electron transfer, and indicate that forward transfer in cyanobacterial PS I is predominantly along the PsaA branch.

Reversed-phase HPLC determination of chlorophyll a' and naphthoquinones in photosystem I of red algae: existence of two menaquinone-4 molecules in photosystem I of Cyanidium caldarium

Chlorophyll (Chl) a', the C13(2)-epimer of Chl a, is one of the two Chl molecules constituting the primary electron donor (P700) of photosystem (PS) I of a thermophilic cyanobacterium Synechococcus elongatus. To examine whether PS I of other oxygenic photosynthetic organisms in general contain one Chl a' molecule in P700, the pigment composition of thylakoid membranes and PS I preparations isolated from red algae Porphyridium purpureum and Cyanidium caldarium was examined by reversed-phase HPLC with particular attention to Chl a' and phylloquinone (PhQ), the secondary electron acceptor of PS I. The two red algae contained one Chl a' molecule at the core part of PS I. In PS I of C. caldarium, two menaquinone-4 (MQ-4) molecules were detected in place of PhQ used by higher plants and cyanobacteria. The 1:2:1 stoichiometry among Chl a', PhQ (MQ-4) and P700 in PS I of the red algae indicates that one Chl a' molecule universally exists in PS I of oxygenic photosynthetic organisms, and two MQ-4 molecules are associated with PS I of C. caldarium.

Reversed-phase HPLC determination of chlorophyll a' and phylloquinone in Photosystem I of oxygenic photosynthetic organisms. Universal existence of one chlorophyll a' molecule in Photosystem I

Chlorophyll (Chl) a', the C132-epimer of Chl a, is a constituent of the primary electron donor (P700) of Photosystem (PS) I of a thermophilic cyanobacterium Synechococcus (Thermosynechococcus) elongatus, as was recently demonstrated by X-ray crystallography. To determine whether PS I of oxygenic photosynthetic organisms universally contains one molecule of Chl a', pigment compositions of thylakoid membranes and PS I complexes isolated from the cyanobacteria T. elongatus and Synechocystis sp. PCC 6803, the green alga Chlamydomonas reinhardtii, and the green plant spinach, were examined by simultaneous detection of phylloquinone (the secondary electron acceptor of PS I) and Chl a' by reversed-phase HPLC. The results were compared with the Chl a/P700 ratio determined spectrophotometrically. The Chl a'/PS I ratios of thylakoid membranes and PS I were about 1 for all the organisms examined, and one Chl a' molecule was found in PS I even after most of the peripheral subunits were removed. Chl a' showed a characteristic extraction behaviour significantly different from the bulk Chl a in acetone/methanol extraction upon varying the mixing ratio. These findings confirm that a single Chl a' molecule in P700 is the universal feature of PS I of the Chl a-based oxygenic photosynthetic organisms.

The menD and menE homologs code for 2-succinyl-6-hydroxyl-2,4-cyclohexadiene-1-carboxylate synthase and O-succinylbenzoic acid-CoA synthase in the phylloquinone biosynthetic pathway of Synechocystis sp. PCC 6803

The genome of the cyanobacterium Synechocystis sp. PCC 6803 contains genes identified as menD and menE, homologs of Escherichia coli genes that code for 2-succinyl-6-hydroxyl-2,4-cyclohexadiene-1-carboxylate (SHCHC) synthase and O-succinylbenzoic acid-CoA ligase in the menaquinone biosynthetic pathway. In cyanobacteria, the product of this pathway is 2-methyl-3-phytyl-1,4-naphthoquinone (phylloquinone), a molecule used exclusively as an electron transfer cofactor in Photosystem (PS) I. The menD(-) and menE(-) strains were generated, and both were found to lack phylloquinone. Hence, no alternative pathways exist in cyanobacteria to produce O-succinylbenzoyl-CoA. Q-band EPR studies of photoaccumulated quinone anion radical and optical kinetic studies of the P700(+) [F(A)/F(B)](-) backreaction indicate that in the mutant strains, plastoquinone-9 functions as the electron transfer cofactor in the A(1) site of PS I. At a light intensity of 40 microE m(-2) s(-1), the menD(-) and menE(-) mutant strains grew photoautotrophically and photoheterotrophically, but with doubling times slower than the wild type. Both of which are sensitive to high light intensities. Low-temperature fluorescence studies show that in the menD(-) and menE(-) mutants, the ratio of PS I to PS II is reduced relative to the wild type. Whole-chain electron transfer rates in the menD(-) and menE(-) mutant cells are correspondingly higher on a chlorophyll basis. The slower growth rate and high-light sensitivity of the menD(-) and menE(-) mutants are therefore attributed to a lower content of PS I per cell.

Modification of photosystem I reaction center by the extraction and exchange of chlorophylls and quinones

The photosystem (PS) I photosynthetic reaction center was modified thorough the selective extraction and exchange of chlorophylls and quinones. Extraction of lyophilized photosystem I complex with diethyl ether depleted more than 90% chlorophyll (Chl) molecules bound to the complex, preserving the photochemical electron transfer activity from the primary electron donor P700 to the acceptor chlorophyll A(0). The treatment extracted all the carotenoids and the secondary acceptor phylloquinone (A(1)), and produced a PS I reaction center that contains nine molecules of Chls including P700 and A(0), and three Fe-S clusters (F(X), F(A) and F(B)). The ether-extracted PS I complex showed fast electron transfer from P700 to A(0) as it is, and to FeS clusters if phylloquinone or an appropriate artificial quinone was reconstituted as A(1). The ether-extracted PS I enabled accurate detection of the primary photoreactions with little disturbance from the absorbance changes of the bulk pigments. The quinone reconstitution created the new reactions between the artificial cofactors and the intrinsic components with altered energy gaps. We review the studies done in the ether-extracted PS I complex including chlorophyll forms of the core moiety of PS I, fluorescence of P700, reaction rate between A(0) and reconstituted A(1), and the fast electron transfer from P700 to A(0). Natural exchange of chlorophyll a to 710-740 nm absorbing chlorophyll d in PS I of the newly found cyanobacteria-like organism Acaryochloris marina was also reviewed. Based on the results of exchange studies in different systems, designs of photosynthetic reaction centers are discussed.

Electron transfer in photosystem I

This mini-review focuses on recent experimental results and questions, which came up since the last more comprehensive reviews on the subject. We include a brief discussion of the different techniques used for time-resolved studies of electron transfer in photosystem I (PS I) and relate the kinetic results to new structural data of the PS I reaction centre.

Structure of photosystem I

In plants and cyanobacteria, the primary step in oxygenic photosynthesis, the light induced charge separation, is driven by two large membrane intrinsic protein complexes, the photosystems I and II. Photosystem I catalyses the light driven electron transfer from plastocyanin/cytochrome c(6) on the lumenal side of the membrane to ferredoxin/flavodoxin at the stromal side by a chain of electron carriers. Photosystem I of Synechococcus elongatus consists of 12 protein subunits, 96 chlorophyll a molecules, 22 carotenoids, three [4Fe4S] clusters and two phylloquinones. Furthermore, it has been discovered that four lipids are intrinsic components of photosystem I. Photosystem I exists as a trimer in the native membrane with a molecular mass of 1068 kDa for the whole complex. The X-ray structure of photosystem I at a resolution of 2.5 A shows the location of the individual subunits and cofactors and provides new information on the protein-cofactor interactions. [P. Jordan, P. Fromme, H.T. Witt, O. Klukas, W. Saenger, N. Krauss, Nature 411 (2001) 909-917]. In this review, biochemical data and results of biophysical investigations are discussed with respect to the X-ray crystallographic structure in order to give an overview of the structure and function of this large membrane protein.

Recruitment of a foreign quinone into the A1 site of photosystem I. In vivo replacement of plastoquinone-9 by media-supplemented naphthoquinones in phylloquinone biosynthetic pathway mutants of Synechocystis sp. PCC 6803

Interruption of the phylloquinone (PhQ) biosynthetic pathway by interposon mutagenesis of the menA and menB genes in Synechocystis sp. PCC 6803 results in plastoquinone-9 (PQ-9) occupying the A(1) site and functioning in electron transfer from A(0) to the FeS clusters in photosystem (PS) I (Johnson, T. W., Shen, G., Zybailov, B., Kolling, D., Reategui, R., Beauparlant, S., Vassiliev, I. R., Bryant, D. A., Jones, A. D., Golbeck, J. H., and Chitnis, P. R. (2000) J. Biol. Chem. 275, 8523-8530. We report here the isolation of menB26, a strain of the menB mutant that grows in high light by virtue of a higher PS I to PS II ratio. PhQ can be reincorporated into the A(1) site of the menB26 mutant strain by supplementing the growth medium with authentic PhQ. The reincorporation of PhQ also occurs in cells that have been treated with protein synthesis inhibitors, consistent with a displacement of PQ-9 from the A(1) site by mass action. The doubling time of the menB26 mutant cells, but not the menA mutant cells, approaches the wild type when the growth medium is supplemented with naphthoquinone (NQ) derivatives such as 2-CO(2)H-1,4-NQ and 2-CH(3)-1,4-NQ. Since PhQ replaces PQ-9 in the supplemented menB26 mutant cells, but not in the menA mutant cells, the phytyl tail accompanies the incorporation of these quinones into the A(1) site. Studies with menB26 mutant cells and perdeuterated 2-CH(3)-1,4-NQ shows that phytylation occurs at position 3 of the NQ ring because the deuterated 2-methyl group remains intact. Therefore, the specificity of the phytyltransferase enzyme is selective with respect to the group present at ring positions 2 and 3. Supplementing the growth medium of menB26 mutant cells with 1,4-NQ also leads to its incorporation into the A(1) site, but typically without either the phytyl tail or the methyl group. These findings open the possibility of biologically incorporating novel quinones into the A(1) site by supplementing the growth medium of menB26 mutant cells.

Selective extraction of antenna chlorophylls, carotenoids and quinones from photosystem I reaction center

By the ether treatment of lyophilized PSI pigment-protein complexes, all the carotenoids and the secondary acceptor phylloquinone (A1), and more than 90% of the Chl were removed to yield the PSI complex with 9-11 molecules of Chl per reaction-center unit. The complexes retained the primary electron donor and acceptor (P700 and A0), in addition to three FeS clusters (F(X), F(A) and F(B)), and showed an activity of highly efficient electron transfer when phylloquinone was reconstituted. The methods for the preparation and the characterization of the ether-extracted PSI complexes are reviewed in this article. We also review the studies done with this PSI preparation on (1) the identification of the absorption and fluorescence spectra of P700, (2) the nano- and picosecond reaction of A0 and A1, (3) the energy-gap dependency of the reaction rate between A0 and the artificial quinones reconstituted at the A1 site, (4) the direct excitation of P700 followed by the ultra-fast electron transfer from P700 to A0, and (5) the de- and re-stabilization of the PSI structure by the removal and reconstitution, respectively, of antenna Chl in the presence of certain lipids.

Recruitment of a foreign quinone into the A(1) site of photosystem I. I. Genetic and physiological characterization of phylloquinone biosynthetic pathway mutants in Synechocystis sp. pcc 6803

Genes encoding enzymes of the biosynthetic pathway leading to phylloquinone, the secondary electron acceptor of photosystem (PS) I, were identified in Synechocystis sp. PCC 6803 by comparison with genes encoding enzymes of the menaquinone biosynthetic pathway in Escherichia coli. Targeted inactivation of the menA and menB genes, which code for phytyl transferase and 1,4-dihydroxy-2-naphthoate synthase, respectively, prevented the synthesis of phylloquinone, thereby confirming the participation of these two gene products in the biosynthetic pathway. The menA and menB mutants grow photoautotrophically under low light conditions (20 microE m(-2) s(-1)), with doubling times twice that of the wild type, but they are unable to grow under high light conditions (120 microE m(-2) s(-1)). The menA and menB mutants grow photoheterotrophically on media supplemented with glucose under low light conditions, with doubling times similar to that of the wild type, but they are unable to grow under high light conditions unless atrazine is present to inhibit PS II activity. The level of active PS II per cell in the menA and menB mutant strains is identical to that of the wild type, but the level of active PS I is about 50-60% that of the wild type as assayed by low temperature fluorescence, P700 photoactivity, and electron transfer rates. PS I complexes isolated from the menA and menB mutant strains contain the full complement of polypeptides, show photoreduction of F(A) and F(B) at 15 K, and support 82-84% of the wild type rate of electron transfer from cytochrome c(6) to flavodoxin. HPLC analyses show high levels of plastoquinone-9 in PS I complexes from the menA and menB mutants but not from the wild type. We propose that in the absence of phylloquinone, PS I recruits plastoquinone-9 into the A(1) site, where it functions as an efficient cofactor in electron transfer from A(0) to the iron-sulfur clusters.

A common ancestor for oxygenic and anoxygenic photosynthetic systems: a comparison based on the structural model of photosystem I

The 4 A structural model of photosystem I (PSI) has elucidated essential features of this protein complex. Inter alia, it demonstrates that the core proteins of PSI, PsaA and PsaB each consist of an N-terminal antenna-binding domain, and a C-terminal reaction center (RC)-domain. A comparison of the RC-domain of PSI and the photosynthetic RC of purple bacteria (PbRC), reveals significantly analogous structures. This provides the structural support for the hypothesis that the two RC-types (I and II) share a common evolutionary origin. Apart from a similar set of constituent cofactors of the electron transfer system, the analogous features include a comparable cofactor arrangement and a corresponding secondary structure motif of the RC-cores. Despite these analogies, significant differences are evident, particularly as regards the distances between and the orientation of individual cofactors, and the length and orientation of alpha-helices. Inferred roles of conserved amino acids are discussed for PSI, photosystem II (PSII), photosystem C (PSC, green sulfur bacteria) and photosystem H (PSH, heliobacteria). Significant sequence homology between the N-terminal, antenna-binding domains of the core proteins of type-I RCs, PsaA, PsaB, PscA and PshA (of PSI, PSC and PSH respectively) with the antenna-binding subunits CP43 and CP47 of PSII indicate that PSII has a modular structure comparable to that of PSI.

Photosystem I of Synechococcus elongatus at 4 A resolution: comprehensive structure analysis

An improved structural model of the photosystem I complex from the thermophilic cyanobacterium Synechococcus elongatus is described at 4 A resolution. This represents the most complete model of a photosystem presently available, uniting both a photosynthetic reaction centre domain and a core antenna system. Most constituent elements of the electron transfer system have been located and their relative centre-to-centre distances determined at an accuracy of approximately 1 A. These include three pseudosymmetric pairs of Chla and three iron-sulphur centres, FX, FA and FB. The first pair, a Chla dimer, has been assigned to the primary electron donor P700. One or both Chla of the second pair, eC2 and eC'2, presumably functionally link P700 to the corresponding Chla of the third pair, eC3 and eC'3, which is assumed to constitute the spectroscopically-identified primary electron acceptor(s), A0, of PSI. A likely location of the subsequent phylloquinone electron acceptor, QK, in relation to the properties of the spectroscopically identified electron acceptor A1 is discussed. The positions of a total of 89 Chla, 83 of which constitute the core antenna system, are presented. The maximal centre-to-centre distance between antenna Chla is < or = 16 A; 81 Chla are grouped into four clusters comprising 21, 23, 17 and 20 Chla, respectively. Two "connecting" Chla are positioned to structurally (and possibly functionally) link the Chla of the core antenna to those of the electron transfer system. Thus the second and third Chla pairs of the electron transfer system may have a dual function both in energy transfer and electron transport. A total of 34 transmembrane and nine surface alpha-helices have been identified and assigned to the 11 subunits of the PSI complex. The connectivity of the nine C-terminal (seven transmembrane, two "surface") alpha-helices of each of the large core subunits PsaA and PsaB is described. The assignment of the amino acid sequence to the transmembrane alpha-helices is proposed and likely residues involved in co-ordinating the Chla of the electron transfer system discussed.

Normal-phase HPLC quantitation of chlorophyll a' and phylloquinone in Photosystem I particles

Fractionated Photosystem (PS) I particles consisting of six, five or two core proteins were analyzed by HPLC for chlorophyll (Chl) a' and phylloquinone (PhQ). Each particle had a Chl a/P700 molar ratio of 50-55 and contained ca. 2 molecules of Chl a' per P700. Deliberate control of eluent composition led to isolated elution of PhQ and β-carotene in the normal-phase chromatogram. Based on these a simple HPLC procedure has been established to determine the PhQ/P700 molar ratio, which was ca. 2 for the larger two PS I particles and ca. 1 for the smallest particle, in line with previous reports.

Contribution of vitamin K1 to the electron spin polarization in spinach photosystem I

The electron spin polarized (ESP) electron paramagnetic resonance (EPR) signal observed in spinach photosystem I (PSI) particles was examined in preparations depleted of vitamin K1 by solvent extraction and following biological reconstitution by the quinone. The ESP EPR signal was not detected in the solvent-extracted PSI sample but was restored upon reconstitution with either protonated or deuterated vitamin K1 under conditions that also restored electron transfer to the terminal PSI acceptors. Reconstitution using deuterated vitamin K1 resulted in a line narrowing of the ESP EPR signal, supporting the conclusion that the ESP EPR signals in the reconstituted samples arise from a radical pair consisting of the oxidized PSI primary donor, P700+, and reduced vitamin K1.

Photosystem I complex

Photosystem I is an integral component of the thylakoid membrane which catalyzes the photoreduction of ferredoxin using plastocyanin or cytochrome c as electron donor. In higher plants, the photosystem I complex is composed of eight protein subunits, chlorophyll a, carotenoids, phylloquinone and bound iron sulfur clusters. The molecular biology and biochemistry of the complex are discussed in relation to the structure and function of the individual components. The mechanisms involved in the assembly of the components into a functional complex are also discussed.

Light-induced charge separation in Photosystem I at low temperature is not influenced by vitamin K-1

The photoreduction of iron-sulfur centers was studied at low temperature in Photosystem I particles from spinach and the cyanobacterium Synechocystis 6803, which contain various amounts of vitamin K-1 (recently tentatively identified as the acceptor A1). The irreversible charge separation that was progressively induced at low temperature between P-700 and FA (or FB) by successive laser flashes was studied at 15 K. Its maximum amount after a large number of flashes was shown to be fairly independent of the number (0, 1 or 2) of vitamins K-1 per reaction center. Moreover, the first flash yield of this charge separation was diminished by only about 50% when vitamin K-1 was completely absent from the particles by comparison with particles containing one or two vitamin K-1 per reaction center. When FA and FB were prereduced, the iron-sulfur center FX was also reversibly photoreduced at 9 K in the absence of vitamin K-1. The implications of these results for the electron pathways of Photosystem I are discussed and it is proposed that a direct electron transfer from A0- to the iron-sulfur centers is highly efficient at low temperature.

Molecular cloning and nucleotide sequence of the psaA and psaB genes of the cyanobacterium Synechococcus sp. PCC 7002

The psaA and psaB genes, which encode the P700 chlorophyll a apoproteins of the Photosystem I complex, have been cloned from the unicellular, transformable cyanobacterium Synechococcus sp. PCC 7002. The nucleotide sequence of these genes and of their flanking sequences have been determined by the chain termination method. As found in the chloroplast genomes of higher plants, the psaA gene lies 5' to the psaB gene; however, the cyanobacterial genes are separated by a greater distance (173 vs. 25-26 bp). The psaA gene is predicted to encode a polypeptide of 739 amino acid residues (81.7 kDa), and the psaB gene is predicted to encode a polypeptide of 733 residues (81.4 kDa). The cyanobacterial psa gene products are 76% to 81% identical to their higher plant homologues; moreover, because of conservative amino acid replacements, the cyanobacterial sequences are more than 95% homologous to those determined for higher plants. These results provide the basis for a genetic analysis of Photosystem I, and are discussed in relationship to structural and functional aspects of the Photosystem I complexes of both cyanobacteria and higher plants.

Is phylloquinone an obligate electron carrier in photosystem I?

Comparative quantitative analysis of phylloquinone content and photochemically competent P-700 has been performed on photosystem I particles subjected to photolysis with ultraviolet irradiation. Nonirradiated control particles exhibit a phylloquinone/P-700 stoichiometry of 1.9 +/- 0.2. Photolysis of the photosystem I particles induces a progressive depletion of phylloquinone, however, photochemistry as assayed at room temperature by the photooxidation of P-700 is unaffected. These data are not consistent with the assignment of phylloquinone as a functional intermediate at room temperature between P-700 and the iron-sulfur clusters, center A and center B.