Biology subjects

Viola, S.

Publications and source records attributed to Viola, S..

8 recordsLinked to original sources

bioRxiv2025

The Recombination Triplet State in the Far-Red Light Adapted Photosystem II is Located at the ChlD1 Site and Resides on the Red-Most Chlorophyll of the Reaction Center.

The energetic limits of Photosystem II (PSII) photochemical reactivity required reconsideration after the discovery of far-red light acclimation responses in cyanobacteria. Insights on PSII functionality following the inclusion of the red shifted Chlorophylls d and f can be obtained by extending the current knowledge on spectroscopic and structural properties of its reaction center (RC). The photo-induced triplet states, that represent selective endogenous probes, were therefore investigated in far-red adapted PSII by magnetic resonance techniques. Zero-field splitting tensor analysis combined with spin-polarization dynamics arising from radical pair recombination unambiguously identify an intrinsically low-energy-absorbing chlorophyll participating to charge separation reactions. The triplet-minus-singlet (T-S) spectrum associated to the recombination triplet state, obtained by microwave selection, showed a sharp 725 nm bleaching demonstrating the dominant involvement of this red-shifted chlorophyll in the lowest RC exciton. Moreover, spectral simulations provided strong evidence in favor of its localization at the ChlD1 position, making it the most likely site of primary photochemistry.

biochemistry↗

bioRxiv2025

Acclimation of photosynthesis began with a Cu-binding superoxide detoxifying enzyme

Plant acclimation is a growing scientific concept, at molecular, cellular and global scales. All photosynthetic organisms that created an oxic atmosphere on earth possess a gene of unknown function "Acclimation of Photosynthesis to the Environment 1". Here we show that APE1 encodes a thylakoid-bound protein with a unique motif that binds copper and detoxifies the superoxide anion radical, O2*-. Maturation of the recombinant APE1 protein from Chlamydomonas reinhardtii requires formation of cysteine disulfide bonds after copper binding or via a high affinity interaction with a copper chaperone (Plastid Copper Chaperone 1) that boosts its scavenging capacity for O2*-. APE1 co-occurs in evolution with Photosystem II oxygen evolving proteins and it is the archaic O2*- detoxifying enzyme for acclimating photosynthesis to an oxygenic environment.

plant biology↗

bioRxiv2024

Investigation of electrochromic band-shifts in the Soret region induced by the formation of TyrD, TyrZ, and QA- in Photosystem II

The effects of TyrD*, TyrZ*, and QA*- radical formation on the absorption spectrum in the Soret region were studied in Mn-depleted Photosystem II at pH 8.6 (in order to be in the TyrD state after dark adaptation). Flash-induced difference spectra were recorded in several PSII samples from: i) Thermosynechococcus vestitus (formerly T. elongatus), ii) Synechocystis sp. PCC 6803, iii) Chroococcidiopsis thermalis PCC 7203 grown under far-red light, and iv) Acaryochloris marina. In the case of T. vestitus, mutants D1/H198Q, D1/T1789H, D2/I178H, and D2/Y160F, with PsbA1/Q130 instead of PsbA3/E130, were also studied for possible contributions from PD1, ChlD1, ChlD2, and PheD1, respectively. For a possible contribution from PD2, the D2/H197A mutant was studied in S. 6803. While PD1 is clearly the species whose spectrum is blue-shifted by [~]3nm in the presence of QA*-, as has already been well documented in the literature, the species whose spectra shift upon the formation of TyrD* and TyrZ* remain to be clearly identified, as they appear different from PD1, PD2, PheD1, ChlD1, and ChlD2, as concluded by the lack of different light-induced difference spectra in the mutants listed above. Although we cannot rule out a weak effect, considering the accuracy of the experiments, it is proposed that other pigments, such as antenna Chl and/or Car, near the reaction center are involved. Additionally, it is shown that: i) there is no proton release into the bulk upon the oxidation of TyrD at pH 8.6, and ii) the rearrangement of the electrostatic environment of the pigments involved in the light-induced difference spectra in the samples studied, upon the formation of TyrD*, TyrZ*, and QA*-, likely occurs differently from both a kinetic and structural perspective.

biophysics↗

bioRxiv2024

Locating the Missing Chlorophylls f in Far-red Photosystem I

The discovery of chlorophyll f-containing oxygenic photosynthesis, with its long-wavelength photochemistry, represented a new low-energy paradigm. However, subsequent structural studies on chlorophyll f-containing Photosystem I (PSI) found five chlorophylls f but none among the photochemically active pigments and concluded that chlorophyll f plays no photochemical role. Here we report a cryo-EM structure (2.01 [A]) of far-red PSI from Chroococcidiopsis thermalis PCC 7203, showing all eight chlorophylls f, including the redox active A-1B. Simulations of absorption difference spectra induced by charge separation indicate that the A-1B chlorophyll f absorbs at 755 nm. The chlorophyll f sites, some wavelength assignments, and conserved far-red-specific amino acids, provide functional insights, including redox tuning of chlorophyll f as the primary donor and far-red excitation energy-sharing over the PSI trimer.

biochemistry↗

bioRxiv2024

In vivo ElectroChromic Shift measurements of photosynthetic activity in far-red absorbing cyanobacteria

Some cyanobacteria can do photosynthesis using not only visible but also far-red light that is unused by most other oxygenic photoautotrophs because of its lower energy content. These species have a modified photosynthetic apparatus containing red-shifted pigments. The incorporation of red-shifted pigments decreases the photochemical efficiency of photosystem I and, especially, photosystem II, and it might affect the distribution of excitation energy between the two photosystems with possible consequences on the activity of the entire electron transport chain. To investigate the in vivo effects on photosynthetic activity of these pigment changes, we present here the adaptation of a spectroscopic method, based on a physical phenomenon called ElectroChromic Shift (ECS), to the far-red absorbing cyanobacteria Acaryochloris marina and Chroococcidiopsis thermalis PCC7203. ECS measures the electric field component of the trans-thylakoid proton motive force generated by photosynthetic electron transfer. We show that ECS can be used in these cyanobacteria to investigate in vivo the stoichiometry of photosystem I and photosystem II and their absorption cross-section, as well as the overall efficiency of light energy conversion into electron transport. Our results indicate that both species use visible and far-red light with similar efficiency, despite significant differences in their light absorption characteristics. ECS thus represents a new non-invasive tool to study the performance of naturally occurring far-red photosynthesis.

biophysics↗

bioRxiv2024

Redox regulation by the CDSP32 thioredoxin of ATP-synthase activity and enzymatic antioxidant network in Solanum tuberosum

Plant thioredoxins (TRXs) form a complex family involved in numerous metabolic and signalling pathways, such as the regulation of photosynthetic metabolism in relation with light conditions. The atypical CDSP32, chloroplastic drought-induced stress protein of 32 kDa, TRX includes two TRX-fold domains, one of which has an atypical redox-active HCGPC motif, and has been initially reported to participate in responses to oxidative stress as an electron donor to peroxiredoxins and methionine sulfoxide reductases. Here, we further characterized potato lines modified for CDSP32 expression to clarify the physiological roles of the TRX. Upon high salt treatments, modified lines displayed changes in the abundance and redox status of CDSP32 antioxidant partners, and exhibited sensitivity to NaHCO3, but not to NaCl. In non-stressed plants overexpressing CDSP32, a lower abundance of photosystem II PsbO and D1 subunits and ATP-synthase {gamma} subunit was noticed. The CDSP32 co-suppressed line showed altered chlorophyll a fluorescence induction and modified regulation of the plastidial ATP-synthase activity during dark/light and light/dark transitions, revealing the involvement of CDSP32 in the control of the photosynthetic machinery. In agreement with the previously reported interaction in planta between CDSP32 and the ATP-synthase {gamma} subunit, our data show that CDSP32 participates in the regulation of the transthylakoid membrane potential. Consistently, modeling of protein complex 3-D structure indicates that the CDSP32 TRX constitutes a suitable partner of ATP-synthase {gamma} subunit. We discuss the roles of CDSP32 in chloroplast redox homeostasis through the regulation of both photosynthetic activity and enzymatic antioxidant network.

plant biology↗

bioRxiv2022

Absorption changes in Photosystem II in the Soret band region upon the formation of the chlorophyll cation radical +

Flash-induced absorption changes in the Soret region arising from the [PD1PD2]+ state, the chlorophyll cation radical formed upon light excitation of Photosystem II (PSII), were measured in Mn-depleted PSII cores at pH 8.6. Under these conditions, TyrD is i) reduced before the first flash, and ii) oxidized before subsequent flashes. In wild-type PSII, when TyrD[bullet] is present, an additional signal in the [PD1PD2]+-minus-[PD1PD2] difference spectrum was observed when compared to the first flash when TyrD is not oxidized. The additional feature was "W-shaped" with troughs at 434 nm and 446 nm. This feature was absent when TyrD was reduced, but was present i) when TyrD was physically absent (and replaced by phenylalanine) or ii) when its H-bonding histidine (D2-His189) was physically absent (replaced by a Leucine). Thus, the simple difference spectrum without the double trough feature at 434 nm and 446 nm, seemed to require the native structural environment around the reduced TyrD and its H bonding partners to be present. We found no evidence of involvement of PD1, ChlD1, PheD1, PheD2, TyrZ, and the Cytb559 heme in the W-shaped difference spectrum. However, the use of a mutant of the PD2 axial His ligand, the D2-His197Ala, shows that the PD2 environment seems involved in the formation of "W-shaped" signal.

biophysics↗

bioRxiv2022

Impact of energy limitations on function and resilience in long-wavelength Photosystem II

Photosystem II (PSII) uses the energy from red light to split water and reduce quinone, an energy-demanding process based on chlorophyll a (Chl-a) photochemistry. Two kinds of cyanobacterial PSII can use Chl-d and Chl-f to perform the same reactions using lower energy, far-red light. PSII from Acaryochloris marina has Chl-d replacing all but one of its 35 Chl-a, while PSII from Chroococcidiopsis thermalis, a facultative far-red species, has just 4 Chl-f and 1 Chl-d and 30 Chl-a. From bioenergetic considerations, the far-red PSII were predicted to lose photochemical efficiency and/or resilience to photodamage. Here, we compare enzyme turnover efficiency, forward electron transfer, back-reactions and photodamage in Chl-f-PSII, Chl-d-PSII and Chl-a-PSII. We show that: i) all types of PSII have a comparable efficiency in enzyme turnover; ii) the modified energy gaps on the acceptor side of Chl-d-PSII favor recombination via PD1+Phe- repopulation, leading to increased singlet oxygen production and greater sensitivity to high-light damage compared to Chl-a-PSII and Chl-f-PSII; ii) the acceptor-side energy gaps in Chl-f-PSII are tuned to avoid harmful back reactions, favoring resilience to photodamage over efficiency of light usage. The results are explained by the differences in the redox tuning of the electron transfer cofactors Phe and QA and in the number and layout of the chlorophylls that share the excitation energy with the primary electron donor. PSII has adapted to lower energy in two distinct ways, each appropriate for its specific environment but with different functional penalties.

biochemistry↗