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Photosystem II (PSII) is the water-splitting enzyme central to oxygenic photosynthesis. To drive water oxidation, the energy from light is harvested by accessory pigments, mostly chlorophyll (Chl) a molecules, which absorb visible light (400-700 nm). Some cyanobacteria facultatively acclimate to shaded environments by altering their photosynthetic machinery to additionally absorb far-red light (FRL, 700-800 nm), a process termed far-red light photoacclimation, or FaRLiP. During FaRLiP, FRL-PSII is assembled with FRL-specific isoforms of the subunits PsbA, PsbB, PsbC, PsbD, and PsbH, and some Chl-binding sites contain Chls d or f instead of the usual Chl a. The structure of an apo-FRL-PSII monomer lacking the FRL-specific PsbH subunit has previously been determined, but visualization of the complete dimeric complex has remained elusive. Here, we report the cryo-electron microscopy structure of a dimeric FRL-PSII complex. The site assignments for Chls d and f are consistent with those assigned in the previous apo-FRL-PSII monomeric structure. All sites that bind Chl d or Chl f at high occupancy exhibit a FRL-specific interaction of the formyl moiety of the Chl d or Chl f with the protein environment, which in some cases involves a phenylalanine sidechain. Furthermore, the structure retains the FRL-specific PsbH2 subunit, which appears to alter the energetic landscape of FRL-PSII to redirect energy transfer from the phycobiliprotein complex to a Chl f molecule bound by PsbB2 that acts as a bridge for energy transfer to the electron transfer chain. Collectively, these observations extend our previous understanding of the structure-function relationship that allows PSII to function using lower energy FRL.

Far-red light (FRL) photoacclimation (FaRLiP) in cyanobacteria provides a selective growth advantage for some terrestrial cyanobacteria by expanding the range of photosynthetically active radiation to include far-red/near-infrared light (700 to 800 nm). During this photoacclimation process, photosystem II (PSII), the water:plastoquinone photooxidoreductase involved in oxygenic photosynthesis, is modified. The resulting FRL-PSII is comprised of FRL-specific core subunits and binds chlorophyll (Chl) d and Chl f molecules in place of several of the Chl a molecules found when cells are grown in visible light. These new Chls effectively lower the energy canonically thought to define the “red limit” for light required to drive photochemical catalysis of water oxidation. Changes to the architecture of FRL-PSII were previously unknown, and the positions of Chl d and Chl f molecules had only been proposed from indirect evidence. Here, we describe the 2.25-A resolution cryo-EM structure of a monomeric FRL-PSII core complex from Synechococcus sp. PCC 7335 cells that were acclimated to FRL. We identify one Chl d molecule in the ChlD1 position of the electron transfer chain, and four Chl f molecules in the core antenna. We also make observations that enhance our understanding of PSII biogenesis, especially on the acceptor side of the complex where a bicarbonate molecule is replaced by a glutamate sidechain in the absence of the assembly factor Psb28. In conclusion, these results provide a structural basis for the lower energy limit required to drive water oxidation, which is the gateway for most solar energy utilization on Earth.

Despite the high level of symmetry between the PsaA and PsaB polypeptides in Photosystem I, some amino acids pairs are strikingly different, such as PsaA-Gly693 and PsaB-Trp673, which are located near a cluster of 11 water molecules between the A1A and A1B quinones and the FX iron-sulfur cluster. In this work, we changed PsaB-Trp673 to PsaB-Phe673 in Synechocystis sp. PCC 6803. The variant contains ~ 85% of wild-type (WT) levels of Photosystem I but is unable to grow photoautotrophically. Both time-resolved and steady-state optical measurements show that in the PsaB-W673F variant less than 50% of the electrons reach the terminal iron-sulfur clusters FA and FB; the majority of the electrons recombine from A1A− and A1B−. However, in those reaction centers which pass electrons forward the transfer is heterogeneous: a minor population shows electron transfer rates from A1A− and A1B− to FX slightly slower than that of the WT, whereas a major population shows forward electron transfer rates to FX slowed to the ~ 10 µs time range. Competition between relatively similar forward and backward rates of electron transfer from the quinones to the FX cluster account for the relatively low yield of long-lived charge separation in the PsaB-W673F variant. A higher water content and its increased mobility observed in MD simulations in the interquinone cavity of the PsaB-W673F variant shifts the pK of PsaB-Asp575 and allows its deprotonation in situ. The heterogeneity found may be rooted in protonation state of PsaB-Asp575, which controls whether electron transfer can proceed beyond the phylloquinone cofactors.

Despite the high level of symmetry between the PsaA and PsaB polypeptides in Photosystem I, some amino acids pairs are strikingly different, such as PsaA-Gly693 and PsaB-Trp673, which are located near a cluster of 11 water molecules between the A

The Photosystem I structure from far-red light cells illuminates an ecologically important acclimation process in photosynthesis., Phototrophic organisms are superbly adapted to different light environments but often must acclimate to challenging competition for visible light wavelengths in their niches. Some cyanobacteria overcome this challenge by expressing paralogous photosynthetic proteins and by synthesizing and incorporating ~8% chlorophyll f into their Photosystem I (PSI) complexes, enabling them to grow under far-red light (FRL). We solved the structure of FRL-acclimated PSI from the cyanobacterium Fischerella thermalis PCC 7521 by single-particle, cryo–electron microscopy to understand its structural and functional differences. Four binding sites occupied by chlorophyll f are proposed. Subtle structural changes enable FRL-adapted PSI to extend light utilization for oxygenic photosynthesis to nearly 800 nm. This structure provides a platform for understanding FRL-driven photosynthesis and illustrates the robustness of adaptive and acclimation mechanisms in nature.

This work aims to fully elucidate the effects of a trehalose glassy matrix on electron transfer reactions in cyanobacterial Photosystem I (PS I). Forward and backward electron transfer rates from A1A− and A1B− to FX, and charge recombination rates from A0−, A1B−, A1A−, FX−, and [FA/FB]− to P700+ were measured in P700–FA/FB complexes, P700–FX cores, and P700–A1 cores, both in liquid and in a trehalose glassy matrix at 11% humidity. By comparing CONTIN-resolved kinetic events over 6 orders of time in increasingly simplified versions of PS I at 480 nm, a wavelength that reports primarily A1A−/A1B− oxidation, and over 9 orders of time at 830 nm, a wavelength that reports P700+ reduction and A0− oxidation, assignments could be made for nearly all of the resolved kinetic phases. Trehalose-embedded PS I samples demonstrated partially arrested forward electron transfer. The fractions of complexes in which electron transfer did not proceed beyond A0, A1 and FX were 53%, 16% and 22%, respectively, with only 10% of electrons reaching the terminal FA/FB clusters. The ~10 μs and ~150 μs components in both liquid and trehalose-embedded PS I were assigned to recombination between A1B− and P700+ and between A1A− and P700+, respectively. The kinetics and amplitudes of these resolved kinetic phases in liquid and trehalose-embedded PS I samples could be well-fitted by a kinetic model that allowed us to calculate the asymmetrical contribution of the A1A− and A1B− quinones to the electrochromic signal at 480 nm. Possible reasons for these effects are discussed.

Chloracidobacterium thermophilum is a microaerophilic, anoxygenic member of the green chlorophototrophic bacteria. This bacterium is the first characterized oxygen-requiring chlorophototroph with chlorosomes, the FMO protein, and homodimeric type-1 reaction centers (RCs). The RCs of C. thermophilum are also unique because they contain three types of chlorophylls, bacteriochlorophyll aP esterified with phytol, Chl aPD esterified with Δ2,6-phytadienol, and Zn-BChl aP′ esterified with phytol, in the approximate molar ratio 32:24:4. The light-induced difference spectrum of these RCs had a bleaching maximum at 839 nm and also revealed an electrochromic bandshift that is probably derived from a BChl a molecule near P840+. The FX [4Fe–4S] cluster had a midpoint potential of ca. − 581 mV, and the spectroscopic properties of the P+ F X − spin-polarized radical pair were very similar to those of reaction centers of heliobacteria and green sulfur bacteria. The data further indicate that electron transfer occurs directly from A0− to FX, as occurs in other homodimeric type-1 RCs. Washing experiments with isolated membranes suggested that the PscB subunit of these reaction centers is more tightly bound than PshB in heliobacteria. Thus, the reaction centers of C. thermophilum have some properties that resemble other homodimeric reaction centers but also have specific properties that are more similar to those of Photosystem I. These differences probably contribute to protection of the electron transfer chain from oxygen, contributing to the oxygen tolerance of this microaerophile.

Certain cyanobacteria can thrive in environments enriched in far-red light (700–800 nm) due to an acclimation process known as far-red light photoacclimation (FaRLiP). During FaRLiP, about 8% of the Chl a molecules in the photosystems are replaced by Chl f and a very small amount of Chl d. We investigated the spectroscopic properties of Photosystem I (PSI) complexes isolated from wild-type (WT) Synechococcus sp. PCC 7335 and a chlF mutant strain (lacking Chl f synthase) grown in white and far-red light (WL–PSI and FRL–PSI, respectively). WT–FRL–PSI complexes contain Chl f and Chl a but not Chl d. The light-minus dark difference spectrum of the trapping center at high spectral resolution indicates that the special pair in WT–FRL–PSI consists of Chl a molecules with maximum bleaching at 703–704 nm. The action spectrum for photobleaching of the special pair showed that Chl f molecules absorbing at wavelengths up to 800 nm efficiently transfer energy to the trapping center in FRL–PSI complexes to produce a charge-separated state. This is ~ 50 nm further into the near IR than WL–PSI; Chl f has a quantum yield equivalent to that of Chl a in the antenna, i.e., ~ 1.0. PSI complexes from Synechococcus 7002 carrying 3.8 Chl f molecules could promote photobleaching of the special pair by energy transfer at wavelengths longer than WT PSI complexes. Results from these latter studies are directly relevant to the issue of whether introduction of Chl f synthase into plants could expand the wavelength range available for oxygenic photosynthesis in crop plants.

In diverse terrestrial cyanobacteria, Far-Red Light Photoacclimation (FaRLiP) promotes extensive remodeling of the photosynthetic apparatus, including photosystems (PS)I and PSII and the cores of phycobilisomes, and is accompanied by the concomitant biosynthesis of chlorophyll (Chl) d and Chl f. Chl f synthase, encoded by chlF, is a highly divergent paralog of psbA; heterologous expression of chlF from Chlorogloeopsis fritscii PCC 9212 led to the light-dependent production of Chl f in Synechococcus sp. PCC 7002 (Ho et al., Science 353, aaf9178 (2016)). In the studies reported here, expression of the chlF gene from Fischerella thermalis PCC 7521 in the heterologous system led to enhanced synthesis of Chl f. N-terminally [His]10-tagged ChlF7521 was purified and identified by immunoblotting and tryptic-peptide mass fingerprinting. As predicted from its sequence similarity to PsbA, ChlF bound Chl a and pheophytin a at a ratio of ~ 3-4:1, bound β-carotene and zeaxanthin, and was inhibited in vivo by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Cross-linking studies and the absence of copurifying proteins indicated that ChlF forms homodimers. Flash photolysis of ChlF produced a Chl a triplet that decayed with a lifetime (1/e) of ~ 817 µs and that could be attributed to intersystem crossing by EPR spectroscopy at 90 K. When the chlF7521 gene was expressed in a strain in which the psbD1 and psbD2 genes had been deleted, significantly more Chl f was produced, and Chl f levels could be further enhanced by specific growth-light conditions. Chl f synthesized in Synechococcus sp. PCC 7002 was inserted into trimeric PSI complexes.

This work aims to fully elucidate the effects of a trehalose glassy matrix on electron transfer reactions in cyanobacterial Photosystem I (PS I). Forward and backward electron transfer rates from A

The light-induced functioning of photosystem 2 (PS 2) is directly linked to the translocation of both electrons and protons across the membrane, which results in the formation of transmembrane electric potential difference (ΔΨ). Generation of ΔΨ due to S-state transitions of the water oxidation complex was demonstrated for the first time in Mn-depleted and reconstituted PS 2 core complexes incorporated into liposomes. The kinetics and relative amplitudes of the electrogenic reactions in dark-adapted samples during S1→S2, S2→S3, and S4→S0 transitions in response to the first, second and third laser flashes were comparable to those obtained in the intact PS 2 core particles. These results expand current understanding of the nature and mechanisms of electrogenic (vectorial) reactions due to a charge transfer on the donor side of PS 2.

The protein-pigment complex of photosystem 2 (PS2) localized in the thylakoid membranes of higher plants, algae, and cyanobacteria is the main source of oxygen on Earth. The light-induced functioning of PS2 is directly linked to electron and proton transfer across the membrane, which results in the formation of transmembrane electric potential difference (ΔΨ). The major contribution to ΔΨ of the PS2 reaction center is due to charge separation between the primary chlorophyll donor P(680) and the quinone acceptor Q(A), accompanied by re-reduction of P(680)(+) by the redox-active tyrosine residue Y(Z). The processes associated with the uptake and release of protons on the acceptor and donor sides of the enzyme, respectively, are also coupled with ΔΨ generation. The objective of this work was to describe the mechanisms of ΔΨ generation associated with the S-state transitions of the water-oxidizing complex in intact PS2 complex and in PS2 preparation depleted of Mn(4)Ca cluster in the presence of artificial electron donors. The findings elucidate the mechanisms of electrogenic reactions on the PS2 donor side and may be a basis for development of an effective solar energy conversion system.

The pigment-protein complex of photosystem 1 (PS1) isolated from cyanobacterium Synechocystis sp. PCC 6803 has been adsorbed on a solid mesoporous film made from TiO2 nanoparticles. was on The TiO2 film supported on a glass substrate with a surface area of 1 cm2 adsorbs up to 0.045 nmol of PS1. PS1 molecules are distributed in the pores of the mesoporous support. Immobilization has an insignificant effect on the optical and photochemical properties of PS1. A reversible photoinduced EPR signal from the oxidized primary electron donor P700 of immobilized PS1 has been detected. It has been shown by photoelectrochemical methods that the photoexcitation of PS1 results in electron injection from PS1 to the conduction band of TiO2.

This mini review presents a general comparison of structural and functional peculiarities of three types of photosynthetic reaction centers (RCs)--photosystem (PS) II, RC from purple bacteria (bRC) and PS I. The nature and mechanisms of the primary electron transfer reactions, as well as specific features of the charge transfer reactions at the donor and acceptor sides of RCs are considered. Comparison of photosynthetic RCs shows general similarity between the core central parts of all three types, between the acceptor sides of bRC and PS II, and between the donor sides of bRC and PS I. In the latter case, the similarity covers thermodynamic, kinetic and dielectric properties, which determine the resemblance of mechanisms of electrogenic reduction of the photooxidized primary donors. Significant distinctions between the donor and acceptor sides of PS I and PS II are also discussed. The results recently obtained in our laboratory indicate in favor of the following sequence of the primary and secondary electron transfer reactions: in PS II (bRC): Р(680)(Р(870)) → Chl(D1)(В(А)) → Phe(bPhe) → Q(A); and in PS I: Р(700) → А(0А)/A(0B) → Q(A)/Q(B).

The effect of dehydration on the kinetics of forward electron transfer (ET) has been studied in cyanobacterial photosystem I (PS I) complexes in a trehalose glassy matrix by time-resolved optical and EPR spectroscopies in the 100 fs to 1 ms time domain. The kinetics of the flash-induced absorption changes in the subnanosecond time domain due to primary and secondary charge separation steps were monitored by pump–probe laser spectroscopy with 20-fs low-energy pump pulses centered at 720 nm. The back-reaction kinetics of P700 were measured by high-field time-resolved EPR spectroscopy and the forward kinetics of A 1A • − / A 1 B • − → F X ${\rm{A}}_{{\rm{1A}}}^{ \bullet - }/{\rm{A}}_{1{\rm{B}}}^{ \bullet - } \to {{\rm{F}}_{\rm{X}}}$ by time-resolved optical spectroscopy at 480 nm. The kinetics of the primary ET reactions to form the primary P 700 • + A 0 • − ${\rm{P}}_{700}^{ \bullet + }{\rm{A}}_0^{ \bullet - }$ and the secondary P 700 • + A 1 • − ${\rm{P}}_{700}^{ \bullet + }{\rm{A}}_1^{ \bullet - }$ ion radical pairs were not affected by dehydration in the trehalose matrix, while the yield of the P 700 • + A 1 • − ${\rm{P}}_{700}^{ \bullet + }{\rm{A}}_1^{ \bullet - }$ was decreased by ~20%. Forward ET from the phylloquinone molecules in the A 1 A • − ${\rm{A}}_{1{\rm{A}}}^{ \bullet - }$ and A 1 B • − ${\rm{A}}_{1{\rm{B}}}^{ \bullet - }$ sites to the iron–sulfur cluster FX slowed from ~220 ns and ~20 ns in solution to ~13 μs and ~80 ns, respectively. However, as shown by EPR spectroscopy, the ~15 μs kinetic phase also contains a small contribution from the recombination between A 1 B • − ${\rm{A}}_{1{\rm{B}}}^{ \bullet - }$ and P 700 • + . ${\rm{P}}_{700}^{ \bullet + }.$ These data reveal that the initial ET reactions from P700 to secondary phylloquinone acceptors in the A- and B-branches of cofactors (A1A and A1B) remain unaffected whereas ET beyond A1A and A1B is slowed or prevented by constrained protein dynamics due to the dry trehalose glass matrix.

We show that semiconductor nanocrystals (quantum dots, QD) can be used to increase the absorption capacity of pigment-protein complexes. In a mixture of photosystem 2 core complex (PS2) and QD, the fluorescence of the latter decreases several-fold due to the transfer of the absorbed energy to the PS2 core complex. We discuss Förster's inductive-resonance mechanism as a possible way of energy transfer in donor-acceptor pairs QD-PS2 core complex. Calculations based on the experimental data show that the enhancement of PS2 fluorescence and the rate of Q(A) reduction increase up to 60% due to efficient energy migration from QD to PS2.