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Akhtar Parveen

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Lambrev Petar
Light energy harvesting by photosystem I in cyanobacterial cells

Aug 30 - szerda

15:30 – 17:00

II. Poszterszekció

P45

Light energy harvesting by photosystem I in cyanobacterial cells

Petar Lambrev¹, Parveen Akhtar¹, Avratanu Biswas¹, Ivo van Stokkum²

¹ Biological Research Centre, Szeged, Institute of Plant Biology

² Universiteit Amsterdam, 1081 HV Amsterdam, The Netherlands

Photosystem I (PSI) is a crucial component of the light-dependent reactions of oxygenic photosynthesis occurring in cyanobacteria, algae and plants. It is a multi-subunit pigment-protein complex binding more than a hundred pigment – chlorophylls (Chls) and carotenoids, as well as cofactors carrying out photoinduced electron transport with a quantum yield of near unity. Whereas PSI in plants and eukaryotic algae is attached to peripheral membrane-intrinsic light-harvesting complexes, cyanobacteria utilize the membrane extrinsic phycobilisomes (PBS) as the main light-harvesting antenna complex¹. While the structural and energetic interaction between the PBS and photosystem II (PSII) is well established, less is known about the connectivity of PBS and PSI and the ability of the PBS to transfer energy directly to PSI is debated. We investigated the pathways and dynamics of energy transfer from PBS to the photosystems in Synechocystis sp. PCC 6803. The excitation kinetics of PBS and PSI were followed by picosecond time-resolved fluorescence spectroscopy in the wild-type strain and in a mutant devoid of PSII². We found that PBS are capable of directly transferring energy to PSI in the PSII-deficient mutant, in a time scale of about 20 ps at room temperature. Based on an earlier model of energy transfer in Synechocystis sp. PCC 6803³ and simultaneous fitting to the measured data of isolated complexes and intact cells, a detailed model of energy transfer between different PBS, PSI and PSII chromophore groups was obtained.

Many cyanobacterial species, when exposed to iron limitation conditions, produce a specialized pigment-protein complex, IsiA, that is known to associate into rings around PSI⁴. The physiological function of IsiA is not fully understood. In isolated PSI-IsiA complexes IsiA efficiently transfers absorbed photon energy to the PSI core⁵, which can extend the absorption cross-section of the photosystem and help reduce the number of iron-rich PSI core complexes in the cells. However, IsiA has also been proposed to have a photoprotective, energy-dissipating role or to serve as a Chl depot⁶. To find more about the light-harvesting role of IsiA in vivo, we followed the cellular content of IsiA in cells of Synechocystis sp. PCC 6803 under iron limitation and investigated the energy transfer from IsiA to PSI by time-resolved spectroscopy. IsiA formed PSI-IsiA supercomplexes in vivo having similar energy transfer characteristics as isolated supercomplexes – confirming the primary role of IsiA as an accessory light-harvesting antenna to PSI. However, a significant fraction (40%) remained unconnected to PSI, supporting the notion of a dual functional role of IsiA. Moreover, we found that Synechocystis mutants containing only monomeric PSI contained far fewer IsiA units per PSI compared to the wild-type strain. We conclude that the trimeric organization of PSI in wild-type Synechocystishas role both in the accumulation and the energy transfer capabilities of IsiA under iron stress.

References

1. Blankenship, R. E., Molecular mechanisms of photosynthesis. John Wiley & Sons: 2021.

2. Bittersmann, E.; Vermaas, W., Fluorescence lifetime studies of cyanobacterial photosystem II mutants. Biochim. Biophys. Acta 1991, 1098 (1), 105–116.

3. van Stokkum, I. H.; Gwizdala, M.; Tian, L.; Snellenburg, J. J.; van Grondelle, R.; van Amerongen, H.; Berera, R., A functional compartmental model of the Synechocystis PCC 6803 phycobilisome. Photosynth. Res. 2018, 135 (1-3), 87–102.

4. Toporik, H.; Li, J.; Williams, D.; Chiu, P.-L.; Mazor, Y., The structure of the stress-induced photosystem I–IsiA antenna supercomplex. Nat. Struct. Mol. Biol. 2019, 26 (6), 443–449.

5. Andrizhiyevskaya, E. G.; Frolov, D.; van Grondelle, R.; Dekker, J. P., Energy transfer and trapping in the photosystem I complex of Synechococcus PCC 7942 and in its supercomplex with IsiA. Biochim. Biophys. Acta 2004, 1656 (2–3), 104–113.

6. Jia, A.; Zheng, Y.; Chen, H.; Wang, Q., Regulation and functional complexity of the chlorophyll-binding protein IsiA. Frontiers in Microbiology 2021, 12, 774107.

Magyar Melinda
Rate-limiting steps in the dark-to-light transition of photosystem II: Dependence on the temperature and the lipidic environment of the reaction center

Aug 31 - csütörtök

10:45 – 11:00

Bioenergetika és fotobiofizika

E41

Rate-limiting steps in the dark-to-light transition of photosystem II: Dependence on the temperature and the lipidic environment of the reaction center

Melinda Magyar¹, Gábor Sipka¹, Parveen Akhtar¹, Guangye Han², Petar H. Lambrev¹, Jian-Ren Shen^2,3 and Győző Garab^1,4

¹Institute of Plant Biology, Biological Research Centre, Szeged, Hungary

²Photosynthesis Research Center, Key Laboratory of Photobiology, Institute of Botany, Chinese Academy of Sciences, Beijing, China

³Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University, Okayama, Japan

⁴Faculty of Science, University of Ostrava, Ostrava, Czech Republic

Photosystem II (PSII) is the redox-active pigment–protein complex embedded in the thylakoid membrane (TM) that catalyzes the oxidation of water and the reduction of plastoquinone. We performed single-turnover saturating flash-induced (STSF) variable chlorophyll-a fluorescence transient measurements on PSII, and we have identified rate-limiting steps in the dark-to-light transition of PSII [1]. It was demonstrated in diuron-treated samples that the first STSF – generating the closed state (PSII_C) – induces only an F₁(<F_m) fluorescence level, and additional excitations with sufficiently long Δτ waiting times between them are required to reach the maximum (F_m) level. We also revealed that the F₁-to-F_m transition is linked to the gradual formation of the light-adapted charge-separated state, PSII_L, which possesses an increased stabilization of charges [2]. Recently, we studied the effects of different physicochemical environments of PSII on the half-rise time (Δτ_1/2) and probed its presence during later steps (F₂, F₃ etc.) [3, 4]. In particular, we investigated the influence of the lipidic environment [3] and the temperature dependence of Δτ_1/2 of PSII core complexes (CC) of Thermosynechococcus (T.) vulcanus and pea TMs [4]. We showed that (i) while non-native lipids has no effect, TM lipids shorten the Δτ_1/2 of PSII CCs of T. vulcanus to that of TMs – uncovering the role of lipid matrix in the rate limiting steps; (ii) PSII CCs of T. vulcanus and spinach TMs exhibit very similar temperature dependences, with enhanced values at low temperatures; and (iii) the Δτ_1/2 values in PSII CC are essentially independent at all temperatures on the number of the STSF-induced increments. These data suggest that the same physical mechanism is involved during the PSII_C-PSII_Ltransition.

References

[1] Magyar M et al. (2018) Sci Rep 8: 2755

[2] Sipka G et al. (2021) Plant Cell 33: 1286-1302.

[3] Magyar M et al. (2022) Photosynthetica 60: 147-156.

[4] Magyar M et al. (2022) IJMS 24: 94-104.