Site-directed mutagenesis of PsaA residue W693 affects phylloquinone binding and function in the photosystem I reaction center of Chlamydomonas reinhardtii

Accelerating Chloroplast Engineering: A New System for Rapid Generation of Marker-Free Transplastomic Lines of Chlamydomonas reinhardtii

'Marker-free' strategies for creating transgenic microorganisms avoid the issue of potential transmission of antibiotic resistance genes to other microorganisms. An already-established strategy for engineering the chloroplast genome (=plastome) of the green microalga Chlamydomonas reinhardtii involves the restoration of photosynthetic function using a recipient strain carrying a plastome mutation in a key photosynthesis gene. Selection for transformant colonies is carried out on minimal media, such that only those cells in which the mutated gene has been replaced with a wild-type copy carried on the transgenic DNA are capable of phototrophic growth. However, this approach can suffer from issues of efficiency due to the slow growth of C. reinhardtii on minimal media and the slow die-back of the untransformed lawn of cells when using mutant strains with a limited photosensitivity phenotype. Furthermore, such phototrophic rescue has tended to rely on existing mutants that are not necessarily ideal for transformation and targeted transgene insertion: Mutants carrying point mutations can easily revert, and those with deletions that do not extend to the intended transgene insertion site can give rise to a sub-population of rescued lines that lack the transgene. In order to improve and accelerate the transformation pipeline for C. reinhardtii, we have created a novel recipient line, HNT6, carrying an engineered deletion in exon 3 of psaA, which encodes one of the core subunits of photosystem I (PSI). Such PSI mutants are highly light-sensitive allowing faster recovery of transformant colonies by selecting for light-tolerance on acetate-containing media, rather than phototrophic growth on minimal media. The deletion extends to a site upstream of psaA-3 that serves as a neutral locus for transgene insertion, thereby ensuring that all of the recovered colonies are transformants containing the transgene. We demonstrate the application of HNT6 using a luciferase reporter.

Kinetics and Energetics of Phylloquinone Reduction in Photosystem I: Insight From Modeling of the Site Directed Mutants

Two phylloquinone molecules (A ₁), one being predominantly coordinated by PsaA subunit residues (A _1A) the other by those of PsaB (A _1B), act as intermediates in the two parallel electron transfer chains of Photosystem I. The oxidation kinetics of the two phyllosemiquinones by the iron-sulfur cluster F_X differ by approximately one order of magnitude, with A 1 A - being oxidized in about 200 ns and A 1 B - in about 20 ns. These differences are generally explained in terms of asymmetries in the driving force for F_X reduction on the two electron transfer chains. Site directed mutations of conserved amino acids composing the A ₁ binding site have been engineered on both reaction center subunits, and proved to affect selectively the oxidation lifetime of either A 1 A - , for PsaA mutants, or A 1 B - , for PsaB mutants. The mutation effects are here critically reviewed, also by novel modeling simulations employing the tunneling formalism to estimate the electron transfer rates. Three main classes of mutation effects are in particular addressed: (i) those leading to an acceleration, (ii) those leading to a moderated slowing (~5-folds), and (iii) those leading to a severe slowing (>20-folds) of the kinetics. The effect of specific amino acid perturbations contributing to the poising of the phylloquinones redox potential and, in turn, to PSI functionality, is discussed.

Modelling electron transfer in photosystem I: limits and perspectives

Uncovering the parameters underlying the electron transfer (ET) in photosynthetic reaction centres is of importance for understanding the molecular mechanisms underpinning their functionality. The reductive nature of most cofactors involved in photosynthetic ET makes the direct estimation of their properties difficult. Photosystem I (PSI) operates in a highly reducing regime, making the assessment of cofactor properties even more difficult. Kinetic modelling coupled to a non-adiabatic description of ET is a useful approach in overcoming this hindrance. Here we review the theory and modelling approaches that have been used in assessing parameters associated with ET reactions in PSI, with particular attention to ET reactions involving the phylloquinones and the iron-sulphur clusters. In most modelling studies, the goal is to estimate the driving force of ET, which is usually associated with the cofactor midpoint potentials. The driving force is sensitive to many factors, which define the ET rate, i.e. the reorganisation energy, the coupling with nuclear modes and the electronic matrix elements, which are explored and discussed here. The importance of an inclusive modelling of both forward and reverse ET processes is discussed and highlighted. It is shown that although estimates are indeed sensitive to the exact parameter sets employed in the modelling, a general consensus is still attained, pointing to a scenario where Δ G A 1 A → F X 0 / Δ G A 1 B → F X 0 is weakly endergonic/exergonic, respectively. It is emphasised that to further refine those estimates, it will require a joint effort between computational modelling and more wide-ranging experimental studies.

Codon reassignment to facilitate genetic engineering and biocontainment in the chloroplast of Chlamydomonas reinhardtii

There is a growing interest in the use of microalgae as low-cost hosts for the synthesis of recombinant products such as therapeutic proteins and bioactive metabolites. In particular, the chloroplast, with its small, genetically tractable genome (plastome) and elaborate metabolism, represents an attractive platform for genetic engineering. In Chlamydomonas reinhardtii, none of the 69 protein-coding genes in the plastome uses the stop codon UGA, therefore this spare codon can be exploited as a useful synthetic biology tool. Here, we report the assignment of the codon to one for tryptophan and show that this can be used as an effective strategy for addressing a key problem in chloroplast engineering: namely, the assembly of expression cassettes in Escherichia coli when the gene product is toxic to the bacterium. This problem arises because the prokaryotic nature of chloroplast promoters and ribosome-binding sites used in such cassettes often results in transgene expression in E. coli, and is a potential issue when cloning genes for metabolic enzymes, antibacterial proteins and integral membrane proteins. We show that replacement of tryptophan codons with the spare codon (UGG→UGA) within a transgene prevents functional expression in E. coli and in the chloroplast, and that co-introduction of a plastidial trnW gene carrying a modified anticodon restores function only in the latter by allowing UGA readthrough. We demonstrate the utility of this system by expressing two genes known to be highly toxic to E. coli and discuss its value in providing an enhanced level of biocontainment for transplastomic microalgae.

Modeling Photosystem I with the alternative reaction center protein PsaB2 in the nitrogen fixing cyanobacterium Nostoc punctiforme

Five nitrogen fixing cyanobacterial strains have been found to contain PsaB2, an additional and divergent gene copy for the Photosystem I reaction center protein PsaB. In all five species the divergent gene, psaB2, is located separately from the normal psaAB operon in the genome. The protein, PsaB2, was recently identified in heterocysts of Nostoc punctiforme sp. strain PCC 73102. 12 conserved amino acid replacements and one insertion, were identified by a multiple sequence alignment of several PsaB2 and PsaB1 sequences. Several, including an inserted glutamine, are located close to the iron-sulfur cluster F(X) in the electron transfer chain. By homology modeling, using the Photosystem I crystal structure as template, we have found that the amino acid composition in PsaB2 will introduce changes in critical parts of the Photosystem I protein structure. The changes are close to F(X) and the phylloquinone (PhQ) in the B-branch, indicating that the electron transfer properties most likely will be affected. We suggest that the divergent PsaB2 protein produces an alternative Photosystem I reaction center with different structural and electron transfer properties. Some interesting physiologcial consequences that this can have for the function of Photosystem I in heterocysts, are discussed.

Bidirectional electron transfer in the reaction centre of photosystem I

In the past decade light-induced electron transfer reactions in photosystem I have been the subject of intensive investigations that have led to the elucidation of some unique characteristics, the most striking of which is the existence of two parallel, functional, redox active cofactors chains. This process is generally referred to as bidirectional electron transfer. Here we present a review of the principal evidences that have led to the uncovering of bidirectionality in the reaction centre of photosystem I. A special focus is dedicated to the results obtained combining time-resolved spectroscopic techniques, either difference absorption or electron paramagnetic resonance, with molecular genetics, which allows, through modification of the binding of redox active cofactors with the reaction centre subunits, an effect on their physical-chemical properties.

Investigation of the Stationary and Transient A(1) Radical in Trp --> Phe Mutants of Photosystem I

Photosystem I (PS I) contains two symmetric branches of electron transfer cofactors. In both the A- and B-branches, the phylloquinone in the A(1) site is pi-stacked with a tryptophan residue and is H-bonded to the backbone nitrogen of a leucine residue. In this work, we use optical and electron paramagnetic resonance (EPR) spectroscopies to investigate cyanobacterial PS I complexes, where these tryptophan residues are changed to phenylalanine. The time-resolved optical data show that backward electron transfer from the terminal electron acceptors to P(700) (.+) is affected in the A- and B-branch mutants, both at ambient and cryogenic temperatures. These results suggest that the quinones in both branches take part in electron transport at all temperatures. The electron-nuclear double resonance (ENDOR) spectra of the spin-correlated radical pair P(700) (.+)A(1) (.-) and the photoaccumulated radical anion A(1) (.-), recorded at cryogenic temperature, allowed the identification of characteristic resonances belonging to protons of the methyl group, some of the ring protons and the proton hydrogen-bonded to phylloquinone in the wild type and both mutants. Significant changes in PS I isolated from the A-branch mutant are detected, while PS I isolated from the B-branch mutant shows the spectral characteristics of wild-type PS I. A possible short-lived B-branch radical pair cannot be detected by EPR due to the available time resolution; therefore, only the A-branch quinone is observed under conditions typically employed for EPR and ENDOR spectroscopies.

Transient EPR: using spin polarization in sequential radical pairs to study electron transfer in photosynthesis

The light induced electron transfer in photosynthesis generates a series of sequential spin polarized radical pairs, and transient electron paramagnetic resonance (TREPR) is ideally suited to study the lifetimes and physical and electronic structures of these radical pairs. In this article, the basic principles of TREPR are outlined with emphasis on the electron spin polarization (ESP) that develops during the electron transfer process. Examples of the analysis of TREPR data are given to illustrate the information that can be obtained. Recent applications of the technique to study the functionality of reaction centers, light-induced structural changes, and protein-cofactor interactions are reviewed.

Replacement of the methionine axial ligand to the primary electron acceptor A0 slows the A0− reoxidation dynamics in Photosystem I

The recent crystal structure of photosystem I (PSI) from Thermosynechococcus elongatus shows two nearly symmetric branches of electron transfer cofactors including the primary electron donor, P(700), and a sequence of electron acceptors, A, A(0) and A(1), bound to the PsaA and PsaB heterodimer. The central magnesium atoms of each of the putative primary electron acceptor chlorophylls, A(0), are unusually coordinated by the sulfur atom of methionine 688 of PsaA and 668 of PsaB, respectively. We [Ramesh et al. (2004a) Biochemistry 43:1369-1375] have shown that the replacement of either methionine with histidine in the PSI of the unicellular green alga Chlamydomonas reinhardtii resulted in accumulation of A(0)(-) (in 300-ps time scale), suggesting that both the PsaA and PsaB branches are active. This is in contrast to cyanobacterial PSI where studies with methionine-to-leucine mutants show that electron transfer occurs predominantly along the PsaA branch. In this contribution we report that the change of methionine to either leucine or serine leads to a similar accumulation of A(0)(-) on both the PsaA and the PsaB branch of PSI from C. reinhardtii, as we reported earlier for histidine mutants. More importantly, we further demonstrate that for all the mutants under study, accumulation of A(0)(-) is transient, and that reoxidation of A(0)(-) occurs within 1-2 ns, two orders of magnitude slower than in wild type PSI, most likely via slow electron transfer to A(1). This illustrates an indispensable role of methionine as an axial ligand to the primary acceptor A(0) in optimizing the rate of charge stabilization in PSI. A simple energetic model for this reaction is proposed. Our findings support the model of equivalent electron transfer along both cofactor branches in Photosystem I.

Bidirectional electron transfer in photosystem I: Replacement of the symmetry-breaking tryptophan close to the PsaB-bound phylloquinone (A1B) with a glycine residue alters the redox properties of A1B and blocks forward electron transfer at cryogenic temperatures

A conserved tryptophan residue located between the A(1B) and F(X) redox centres on the PsaB side of the Photosystem I reaction centre has been mutated to a glycine in Chlamydomonas reinhardtii, thereby matching the conserved residue found in the equivalent position on the PsaA side. This mutant (PsaB:W669G) was studied using EPR spectroscopy with a view to understanding the molecular basis of the reported kinetic differences in forward electron transfer from the A(1A) and the A(1B) phyllo(semi)quinones. The kinetics of A(1)(-) reoxidation due to forward electron transfer or charge recombination were measured by electron spin echo spectroscopy at 265 K and 100 K, respectively. At 265 K, the reoxidation kinetics are considerably lengthened in the mutant in comparison to the wild-type. Under conditions in which F(X) is initially oxidised the kinetics of charge recombination at 100 K are found to be biphasic in the mutant while they are substantially monophasic in the wild-type. Pre-reduction of F(X) leads to biphasic kinetics in the wild-type, but does not alter the already biphasic kinetic properties of the PsaB:W669G mutant. Reduction of the [4Fe-4S] clusters F(A) and F(B) by illumination at 15 K is suppressed in the mutant. The results provide further support for the bi-directional model of electron transfer in Photosystem I of C. reinhardtii, and indicate that the replacement of the tryptophan residue with glycine mainly affects the redox properties of the PsaB bound phylloquinone A(1B).

Assignment of a kinetic component to electron transfer between iron–sulfur clusters FX and FA/B of Photosystem I

We studied the kinetics of reoxidation of the phylloquinones in Chlamydomonas reinhardtii Photosystem I using site-directed mutations in the PhQ(A)-binding site and of the residues serving as the axial ligand to ec3(A) and ec3(B) chlorophylls. In wild type PS I, these kinetics are biphasic, and mutations in the binding region of PhQ(A) induced a specific slowing down of the slow component. This slowing allowed detection of a previously unobserved 180-ns phase having spectral characteristics that differ from electron transfer between phylloquinones and F(X). The new kinetic phase thus reflects a different reaction that we ascribe to oxidation of F(X)(-) by the F(A/B) FeS clusters. These absorption changes partly account for the differences between the spectra associated with the two kinetic components assigned to phylloquinone reoxidation. In the mutant in which the axial ligand to ec3(A) (PsaA-Met688) was targeted, about 25% of charge separations ended in P(700)(+)A(0)(-) charge recombination; no such recombination was detected in the B-side symmetric mutant. Despite significant changes in the amplitude of the components ascribed to phylloquinone reoxidation in the two mutants, the overall nanosecond absorption changes were similar to the wild type. This suggests that these absorption changes are similar for the two different phylloquinones and that part of the differences between the decay-associated spectra of the two components reflect a contribution from different electron acceptors, i.e. from an inter-FeS cluster electron transfer.

Solvent-induced changes in photochemical activity and conformation of photosystem I particles by glycerol

It has been shown that a large number of water molecules coordinate with the pigments and subunits of photosystem I (PSI); however, the function of these water molecules remains to be clarified. In this study, the photosynthetic properties of PSI from spinach were investigated using different spectroscopic and activity measurements under conditions of decreasing water content caused by increasing concentrations of glycerol. The results show that glycerol addition caused pronounced changes in the photochemical activity of PSI particles. At low concentrations (<60%, v/v), glycerol stimulated the rate of oxygen uptake in PSI particles, while higher concentrations of glycerol cause inhibition of PSI activity. The capacity of P700 photooxidation also increased with glycerol concentrations lower than 60%. In contrast, this capacity decreased at higher glycerol concentrations. On the other hand, glycerol addition considerably affected the distribution of the bulk and red antenna chlorophyll (Chl) forms or states, with the population of red-shifted Chl forms augmented with increasing glycerol. In addition, glycerol-treated PSI particles showed a blue shift of the tryptophan fluorescence emission maximum and an increase in their capacity to bind the hydrophobic probe 1-anilino-8-naphthalene sulfonate, indicating a more non-polar environment for tryptophan residues and increased exposure of hydrophobic surfaces.

Structure and function of photosystems I and II

Oxygenic photosynthesis, the principal converter of sunlight into chemical energy on earth, is catalyzed by four multi-subunit membrane-protein complexes: photosystem I (PSI), photosystem II (PSII), the cytochrome b(6)f complex, and F-ATPase. PSI generates the most negative redox potential in nature and largely determines the global amount of enthalpy in living systems. PSII generates an oxidant whose redox potential is high enough to enable it to oxidize H(2)O, a substrate so abundant that it assures a practically unlimited electron source for life on earth. During the last century, the sophisticated techniques of spectroscopy, molecular genetics, and biochemistry were used to reveal the structure and function of the two photosystems. The new structures of PSI and PSII from cyanobacteria, algae, and plants has shed light not only on the architecture and mechanism of action of these intricate membrane complexes, but also on the evolutionary forces that shaped oxygenic photosynthesis.

Electrostatic influence of PsaC protein binding to the PsaA/PsaB heterodimer in photosystem I

The absence of the PsaC subunit in the photosystem I (PSI) complex (native PSI complex) by mutagenesis or chemical manipulation yields a PSI core (P700-F(X) core) that also lacks subunits PsaD and PsaE and the two iron-sulfur clusters F(A) and F(B), which constitute an integral part of PsaC. In this P700-F(X) core, the redox potentials (E(m)) of the two quinones A(1A/B) and the iron-sulfur cluster F(X) as well as the corresponding protonation patterns are investigated by evaluating the electrostatic energies from the solution of the linearized Poisson-Boltzmann equation. The B-side specific Asp-B558 changes its protonation state significantly upon isolating the P700-F(X) core, being mainly protonated in the native PSI complex but ionized in the P700-F(X) core. In the P700-F(X) core, E(m)(A(1A/B)) remains practically unchanged, whereas E(m)(F(X)) is upshifted by 42 mV. With these calculated E(m) values, the electron transfer rate from A(1) to F(X) in the P700-F(X) core is estimated to be slightly faster on the A(1A) side than that of the wild type, which is consistent with kinetic measurements.

Modelling of the electron transfer reactions in Photosystem I by electron tunnelling theory: The phylloquinones bound to the PsaA and the PsaB reaction centre subunits of PS I are almost isoenergetic to the iron–sulfur cluster FX

Photosystem I is a large macromolecular complex located in the thylakoid membranes of chloroplasts and in cyanobacteria that catalyses the light driven reduction of ferredoxin and oxidation of plastocyanin. Due to the very negative redox potential of the primary electron transfer cofactors accepting electrons, direct estimation by redox titration of the energetics of the system is hampered. However, the rates of electron transfer reactions are related to the thermodynamic properties of the system. Hence, several spectroscopic and biochemical techniques have been employed, in combination with the classical Marcus theory for electron transfer tunnelling, in order to access these parameters. Nevertheless, the values which have been presented are very variable. In particular, for the case of the tightly bound phylloquinone molecule A(1), the values of the redox potentials reported in the literature vary over a range of about 350 mV. Previous models of Photosystem I have assumed a unidirectional electron transfer model. In the present study, experimental evidence obtained by means of time resolved absorption, photovoltage, and electron paramagnetic resonance measurements are reviewed and analysed in terms of a bi-directional kinetic model for electron transfer reactions. This model takes into consideration the thermodynamic equilibrium between the iron-sulfur centre F(X) and the phylloquinone bound to either the PsaA (A(1A)) or the PsaB (A(1B)) subunit of the reaction centre and the equilibrium between the iron-sulfur centres F(A) and F(B). The experimentally determined decay lifetimes in the range of sub-picosecond to the microsecond time domains can be satisfactorily simulated, taking into consideration the edge-to-edge distances between redox cofactors and driving forces reported in the literature. The only exception to this general behaviour is the case of phylloquinone (A(1)) reoxidation. In order to describe the reported rates of the biphasic decay, of about 20 and 200 ns, associated with this electron transfer step, the redox potentials of the quinones are estimated to be almost isoenergetic with that of the iron sulfur centre F(X). A driving force in the range of 5 to 15 meV is estimated for these reactions, being slightly exergonic in the case of the A(1B) quinone and slightly endergonic, in the case of the A(1A) quinone. The simulation presented in this analysis not only describes the kinetic data obtained for the wild type samples at room temperature and is consistent with estimates of activation energy by the analysis of temperature dependence, but can also explain the effect of the mutations around the PsaB quinone binding pocket. A model of the overall energetics of the system is derived, which suggests that the only substantially irreversible electron transfer reactions are the reoxidation of A(0) on both electron transfer branches and the reduction of F(A) by F(X).

Effect of phosphatidylglycerol on molecular organization of photosystem I

Phosphatidylglycerol (PG) is the only anionic phospholipid in photosynthetic membrane. In this study, photosystem I (PSI) particles obtained from plant spinach were reconstituted into PG liposomes at a relatively high concentration. The results from visible absorption, fluorescence emission, and circular dichroism (CD) spectra reveal an existence of the interactions of PSI with PG. PG effect causes blue-shift and intensity decrease of Chl a peak bands in the absorption and 77 K fluorescence emission. The visible CD spectra indicate that the excitonic interactions for Chl a and Chl b molecules were enhanced upon reconstitution. Furthermore, more or less blue- or red-shift of the peaks characterized by Chl a, Chl b, and carotenoid molecules are also occurred. Simultaneously, an increase in alpha-helix and a decrease particularly in the disordered conformations of protein secondary structures are observed. In addition, the same effect also leads to somewhat more tryptophan (Trp) residues exposed to the polar environment. These results demonstrate that some alteration of molecular organization occurs within both the external antenna LHCI and PSI core complex after PSI reconstitution.

Characterization of the bonding interactions of QB upon photoreduction via A-branch or B-branch electron transfer in mutant reaction centers from Rhodobacter sphaeroides

In Rhodobacter sphaeroides reaction centers (RCs) containing the mutation Ala M260 to Trp (AM260W), transmembrane electron transfer along the full-length of the A-branch of cofactors is prevented by the loss of the Q(A) ubiquinone, but it is possible to generate the radical pair P(+)H(A)(-) by A-branch electron transfer or the radical pair P(+)Q(B)(-) by B-branch electron transfer. In the present study, FTIR spectroscopy was used to provide direct evidence for the complete absence of the Q(A) ubiquinone in mutant RCs with the AM260W mutation. Light-induced FTIR difference spectroscopy of isolated RCs was also used to probe the neutral Q(B) and the semiquinone Q(B)(-) states in two B-branch active mutants, a double AM260W-LM214H mutant, denoted WH, and a quadruple mutant, denoted WAAH, in which the AM260W, LM214H, and EL212A-DL213A mutations were combined. The data were compared to those obtained with wild-type (Wt) RCs and the double EL212A-DL213A (denoted AA) mutant which exhibit the usual A-branch electron transfer to Q(B). The Q(B)(-)/Q(B) spectrum of the WH mutant is very close to that of Wt RCs indicating similar bonding interactions of Q(B) and Q(B)(-) with the protein in both RCs. The Q(B)(-)/Q(B) spectra of the AA and WAAH mutants are also closely related to one another, but are very different to that of the Wt complex. Isotope-edited IR fingerprint spectra were obtained for the AA and WAAH mutants reconstituted with site-specific (13)C-labeled ubiquinone. Whilst perturbations of the interactions of the semiquinone Q(B)(-) with the protein are observed in the AA and WAAH mutants, the FTIR data show that the bonding interaction of neutral Q(B) in these two mutants are essentially the same as those for Wt RCs. Therefore, it is concluded that Q(B) occupies the same binding position proximal to the non-heme iron prior to reduction by either A-branch or B-branch electron transfer.

Redox potential of quinones in both electron transfer branches of photosystem I

The redox potentials of the two electron transfer (ET) active quinones in the central part of photosystem I (PSI) were determined by evaluating the electrostatic energies from the solution of the Poisson-Boltzmann equation based on the crystal structure. The calculated redox potentials are -531 mV for A1A and -686 mV for A1B. From these results we conclude the following. (i) Both branches are active with a much faster ET in the B-branch than in the A-branch. (ii) The measured lifetime of 200-290 ns of reduced quinones agrees with the estimate for the A-branch and corroborates with an uphill ET from this quinone to the iron-sulfur cluster as observed in recent kinetic measurements. (iii) The electron paramagnetic resonance spectroscopic data refer to the A-branch quinone where the corresponding ET is uphill in energy. The negative redox potential of A1 in PSI is primarily because of the influence from the negatively charged FX, in contrast to the positive shift on the quinone redox potential in bacterial reaction center and PSII that is attributed to the positively charged non-heme iron atom. The conserved residue Asp-B575 changes its protonation state after quinone reduction. The difference of 155 mV in the quinone redox potentials of the two branches were attributed to the conformation of the backbone with a large contribution from Ser-A692 and Ser-B672 and to the side chain of Asp-B575, whose protonation state couples differently with the formation of the quinone radicals.

The binding of cofactors to photosystem I analyzed by spectroscopic and mutagenic methods

This review focuses on cofactor-ligand and protein-protein interactions within the photosystem I reaction center. The topics include a description of the electron transfer cofactors, the mode of binding of the cofactors to protein-bound ligands, and a description of intraprotein contacts that ultimately allow photosystem I to be assembled (in cyanobacteria) from 96 chlorophylls, 22 carotenoids, 2 phylloquinones, 3 [4Fe-4S] clusters, and 12 polypeptides. During the 15 years that have elapsed from the first report of crystals to the atomic-resolution X-ray crystal structure, cofactor-ligand interactions and protein-protein interactions were systematically being explored by spectroscopic and genetic methods. This article charts the interplay between these disciplines and assesses how good the early insights were in light of the current structure of photosystem I.

Bidirectional electron transfer in photosystem I: electron transfer on the PsaA side is not essential for phototrophic growth in Chlamydomonas

We have used pulsed electron paramagnetic resonance (EPR) measurements of the electron spin polarised (ESP) signals arising from the geminate radical pair P700(z.rad;+)/A(1)(z.rad;-) to detect electron transfer on both the PsaA and PsaB branches of redox cofactors in the photosystem I (PSI) reaction centre of Chlamydomonas reinhardtii. We have also used electron nuclear double resonance (ENDOR) spectroscopy to monitor the electronic structure of the bound phyllosemiquinones on both the PsaA and PsaB polypeptides. Both these spectroscopic assays have been used to analyse the effects of site-directed mutations to the axial ligands of the primary chlorophyll electron acceptor(s) A(0) and the conserved tryptophan in the PsaB phylloquinone (A(1)) binding pocket. Substitution of histidine for the axial ligand methionine on the PsaA branch (PsaA-M684H) blocks electron transfer to the PsaA-branch phylloquinone, and blocks photoaccumulation of the PsaA-branch phyllosemiquinone. However, this does not prevent photoautotrophic growth, indicating that electron transfer via the PsaB branch must take place and is alone sufficient to support growth. The corresponding substitution on the PsaB branch (PsaB-M664H) blocks kinetic electron transfer to the PsaB phylloquinone at 100 K, but does not block the photoaccumulation of the phyllosemiquinone. This transformant is unable to grow photoautotrophically although PsaA-branch electron transfer to and from the phyllosemiquinone is functional, indicating that the B branch of electron transfer may be essential for photoautotrophic growth. Mutation of the conserved tryptophan PsaB-W673 to leucine affects the electronic structure of the PsaB phyllosemiquinone, and also prevents photoautotrophic growth.

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.

Electron transfer in cyanobacterial photosystem I: I. Physiological and spectroscopic characterization of site-directed mutants in a putative electron transfer pathway from A0 through A1 to FX

The Photosystem I (PS I) reaction center contains two branches of nearly symmetric cofactors bound to the PsaA and PsaB heterodimer. From the x-ray crystal structure it is known that Trp697PsaA and Trp677PsaB are pi-stacked with the head group of the phylloquinones and are H-bonded to Ser692PsaA and Ser672PsaB, whereas Arg694PsaA and Arg674PsaB are involved in a H-bonded network of side groups that connects the binding environments of the phylloquinones and FX. The mutants W697FPsaA, W677FPsaB, S692CPsaA, S672CPsaB, R694APsaA, and R674APsaB were constructed and characterized. All mutants grew photoautotrophically, yet all showed diminished growth rates compared with the wild-type, especially at higher light intensities. EPR and electron nuclear double resonance (ENDOR) studies at both room temperature and in frozen solution showed that the PsaB mutants were virtually identical to the wild-type, whereas significant effects were observed in the PsaA mutants. Spin polarized transient EPR spectra of the P700+A1- radical pair show that none of the mutations causes a significant change in the orientation of the measured phylloquinone. Pulsed ENDOR spectra reveal that the W697FPsaA mutation leads to about a 5% increase in the hyperfine coupling of the methyl group on the phylloquinone ring, whereas the S692CPsaA mutation causes a similar decrease in this coupling. The changes in the methyl hyperfine coupling are also reflected in the transient EPR spectra of P700+A1- and the CW EPR spectra of photoaccumulated A1-. We conclude that: (i) the transient EPR spectra at room temperature are predominantly from radical pairs in the PsaA branch of cofactors; (ii) at low temperature the electron cycle involving P700 and A1 similarly occurs along the PsaA branch of cofactors; and (iii) mutation of amino acids in close contact with the PsaA side quinone leads to changes in the spin density distribution of the reduced quinone observed by EPR.

Type I photosynthetic reaction centres: structure and function

We review recent advances in the study of the photosystem I reaction centre, following the determination of a spectacular 2.5 A resolution crystal structure for this complex of Synechococcus elongatus. Photosystem I is proving different to type II reaction centres in structure and organization, and the mechanism of transmembrane electron transfer, and is providing insights into the control of function in reaction centres that operate at very low redox potentials. The photosystem I complex of oxygenic organisms has a counterpart in non-oxygenic bacteria, the strictly anaerobic phototrophic green sulphur bacteria and heliobacteria. The most distinctive feature of these type I reaction centres is that they contain two copies of a large core polypeptide (i.e. a homodimer), rather than a heterodimeric arrangement of two related, but different, polypeptides as in the photosystem I complex. To compare the structural organization of the two forms of type I reaction centre, we have modelled the structure of the central region of the reaction centre from green sulphur bacteria, using sequence alignments and the structural coordinates of the S. elongatus Photosystem I complex. The outcome of these modelling studies is described, concentrating on regions of the type I reaction centre where important structure-function relationships have been demonstrated or inferred.

Insights into the evolution of the antenna domains of Type-I and Type-II photosynthetic reaction centres through homology modelling

The (bacterio)chlorophylls of photosynthetic antenna and reaction centre complexes are bound to the protein via a fifth, axial ligand to the central magnesium atom. A number of the amino acids identified as providing such ligands are conserved between the large antenna of the cyanobacterial Type-I reaction centre and smaller antennas of the Type-I reaction centres of green sulphur bacteria and heliobacteria, and these numbers match closely the estimated number of antenna bacteriochlorophylls in the latter. The possible organisation of the antenna in the latter reaction centres is discussed, as is the mechanism by which the more pigment-rich antenna of the cyanobacterial reaction centre evolved. The homology modelling approach is also extended to the six-helix antenna proteins CP47 and CP43 associated with the Photosystem II reaction centre.

Orientation selection in photosynthetic PS I multilayers: structural investigation of the charge separated state P(700)(+z.rad;)A(1)(-z.rad;) by high-field/high-frequency time-resolved EPR at 3.4 T/95 GHz

The radical-pair state of the primary electron donor and the secondary electron acceptor (P(700)(+z.rad;)A(1)(-z.rad;)) of the photosynthetic reaction center (RC) photosystem I (PS I) of Synechocystis PCC 6803 was studied by time-resolved electron paramagnetic resonance (TREPR) at high field/high frequency (3.4 T/95 GHz) using orientation selection in multilayers. The goal of the present article is to work out the basis for future studies, in which the improved resolution of such multilayers may be used to detect mutation-induced structural changes of PS I in membrane preparations. This approach is particularly interesting for systems that cannot be prepared as single crystals. However, in order to use such multilayers for structural investigations of protein complexes, it is necessary to know their orientation distribution. PS I was chosen as a test example because the wild type was recently crystallized and its X-ray structure determined to 2.5 A resolution [Nature 411 (2001) 909]. On the basis of our experimental results we determined the orientation distribution. Furthermore, a simulation model for the general case in which the orientation distribution is not axially symmetric about the C(2) symmetry axis of the RC is developed and discussed. Spectra simulations show that changes in the TREPR spectra of PS I are much more significant for these oriented multilayers than for disordered samples. In this way the use of oriented multilayers, in conjunction with multifrequency TREPR measurements on oriented as well as on disordered samples, is a promising approach for studies of structural changes of PS I systems that are induced by point mutations.

Photoaccumulation of the PsaB phyllosemiquinone in photosystem I of Chlamydomonas reinhardtii

Photoaccumulation of membrane preparations of Chlamydomonas reinhardtii at pH 8 and 220 K reduces the primary and secondary electron acceptors in the Photosystem I (PSI) reaction centre, and produces a maximum of two spins per P700(z.rad;+). Proton electron nuclear double resonance (ENDOR) spectra demonstrate that the phyllosemiquinone produced is that attributed to the PsaA branch of electron transfer. Photoaccumulation at pH 10 and 220 K produces a maximum of four spins per P700(z.rad;+), and proton ENDOR spectra indicate that a second phyllosemiquinone is being photoaccumulated, with markedly different proton hyperfine couplings (hfcs). This phyllosemiquinone is unaffected by mutation of PsaAW693, confirming that it does not arise from the PsaA branch of electron transfer, and we therefore attribute it to the PsaB phyllosemiquinone.

Reaction centres: the structure and evolution of biological solar power

Reaction centres are complexes of pigment and protein that convert the electromagnetic energy of sunlight into chemical potential energy. They are found in plants, algae and a variety of bacterial species, and vary greatly in their composition and complexity. New structural information has highlighted features that are common to the different types of reaction centre and has provided insights into some of the key differences between reaction centres from different sources. New ideas have also emerged on how contemporary reaction centres might have evolved and on the possible origin of the first chlorophyll-protein complexes to harness the power of sunlight.

Light-induced spin polarization in type I photosynthetic reaction centres

The use of light-induced spin polarization to study the structure and function of type I reaction centres is reviewed. The absorption of light by these systems generates a series of sequential radical pairs, which exhibit spin polarization as a result of the correlation of the unpaired electron spins. A description of how the polarization patterns can be used to deduce the relative orientation of the radicals is given and the most important structural results from such studies on photosystem I (PS I) are summarized. Quinone exchange experiments which demonstrate the influence of protein-cofactor interactions on the polarization patterns are discussed. The results show that there are significant differences between the binding sites of the primary quinone acceptors in PS I and purple bacterial reaction centres and suggest that pi-pi interactions probably play a more important role in PS I. Studies using spin-polarized EPR transients and spectra to investigate the electron transfer pathway and kinetics are also reviewed. The results from PS I, green-sulphur bacteria and Heliobacteria are compared and the controversy surrounding the role of a quinone in the electron transfer in the latter two systems is 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.

Mutations in both sides of the photosystem I reaction center identify the phylloquinone observed by electron paramagnetic resonance spectroscopy

The core of photosystem I (PS1) is composed of the two related integral membrane polypeptides, PsaA and PsaB, which bind two symmetrical branches of cofactors, each consisting of two chlorophylls and a phylloquinone, that potentially link the primary electron donor and the tertiary acceptor. In an effort to identify amino acid residues near the phylloquinone binding sites, all tryptophans and histidines that are conserved between PsaA and PsaB in the region of the 10th and 11th transmembrane alpha-helices were mutated in Chlamydomonas reinhardtii. The mutant PS1 reaction centers appear to assemble normally and possess photochemical activity. An electron paramagnetic resonance (EPR) signal attributed to the phylloquinone anion radical (A(1)(-)) can be observed either transiently or after illumination of reaction centers with pre-reduced iron-sulfur clusters. Mutation of PsaA-Trp(693) to Phe resulted in an inability to photo-accumulate A(1)(-), whereas mutation of the analogous tryptophan in PsaB (PsaB-Trp(673)) did not produce this effect. The PsaA-W693F mutation also produced spectral changes in the time-resolved EPR spectrum of the P(700)(+) A(1)(-) radical pair, whereas the analogous mutation in PsaB had no observable effect. These observations indicate that the A(1)(-) phylloquinone radical observed by EPR occupies the phylloquinone-binding site containing PsaA-Trp(693). However, mutation of either tryptophan accelerated charge recombination from the terminal Fe-S clusters.

Evidence from time resolved studies of the P700(.+)/A1(.-) radical pair for photosynthetic electron transfer on both the PsaA and PsaB branches of the photosystem I reaction centre

Kinetic analysis using pulsed electron paramagnetic resonance (EPR) of photosynthetic electron transfer in the photosystem I reaction centres of Synechocystis 6803, in wild-type Chlamydomonas reinhardtii, and in site directed mutants of the phylloquinone binding sites in C. reinhardtii, indicates that electron transfer from the reaction centre primary electron donor, P700, to the iron-sulphur centres, Fe-S(X/A/B), can occur through either the PsaA or PsaB side phylloquinone. At low temperature reaction centres are frozen in states which allow electron transfer on one side of the reaction centre only. A fraction always donates electrons to the PsaA side quinone, the remainder to the PsaB side.