•Plants utilize super/megacomplexes to survive severe light conditions on land.•Several LHCII trimers can bind to the PSI–PSII megacomplex via the LHCI belt.•Excitation energy is diverted ...automatically to PSI in the megacomplex.•Megacomplexes help regulate the excitation energy in photosystems.
Traditionally, two types of photosystem reaction centers (PSI and PSII) are thought to be spatially dispersed in the plant thylakoid membrane. In this model, PSI and PSII independently accept excitation energy from their own peripheral light-harvesting complexes, LHCI and LHCII, respectively, and form supercomplexes (PSI–LHCI and PSII–LHCII). However, recent studies using a combination of mild detergent treatment and spectroscopic analysis have revealed the existence of various megacomplexes such as a PSI–PSII megacomplex and a PSII megacomplex. Flexibility in the formation of supercomplexes and megacomplexes is important for land plants to regulate excitation energy to survive under strong and fluctuating sunlight on land.
•P. patens LHC proteins are redundantly diversified.•Lhcb9 is able to associate with PSI to form a larger supercomplex.•Multiple pathways for electron flow in chloroplast stroma exist in P. ...patens.•Both LHCSR and PsbS are functional to induce qE in P. patens.
Plants have successfully adapted to a vast range of terrestrial environments during their evolution. To elucidate the evolutionary transition of light-harvesting antenna proteins from green algae to land plants, the moss Physcomitrella patens is ideally placed basally among land plants. Compared to the genomes of green algae and land plants, the P. patens genome codes for more diverse and redundant light-harvesting antenna proteins. It also encodes Lhcb9, which has characteristics not found in other light-harvesting antenna proteins. The unique complement of light-harvesting antenna proteins in P. patens appears to facilitate protein interactions that include those lost in both green algae and land plants with regard to stromal electron transport pathways and photoprotection mechanisms. This review will highlight unique characteristics of the P. patens light-harvesting antenna system and the resulting implications about the evolutionary transition during plant terrestrialization.
Fucoxanthin chlorophyll (Chl) a/c-binding proteins (FCPs) function as light harvesters in diatoms. The structure of a diatom photosystem II-FCPII (PSII-FCPII) supercomplex have been solved by ...cryo-electron microscopy (cryo-EM) previously; however, the FCPII subunits that constitute the FCPII tetramers and monomers are not identified individually due to their low resolutions. Here, we report a 2.5 Å resolution structure of the PSII-FCPII supercomplex using cryo-EM. Two types of tetrameric FCPs, S-tetramer, and M-tetramer, are identified as different types of hetero-tetrameric complexes. In addition, three FCP monomers, m1, m2, and m3, are assigned to different gene products of FCP. The present structure also identifies the positions of most Chls c and diadinoxanthins, which form a complicated pigment network. Excitation-energy transfer from FCPII to PSII is revealed by time-resolved fluorescence spectroscopy. These structural and spectroscopic findings provide insights into an assembly model of FCPII and its excitation-energy transfer and quenching processes.
Diatoms are a major group of microalgae in marine and freshwater environments. To utilize the light energy in blue to green region, diatoms possess unique antenna pigment–protein complexes, ...fucoxanthin chlorophyll
a
/
c
-binding proteins (FCPs). Depending on light qualities and quantities, diatoms form FCPs with different energies: normal-type and red-shifted FCPs. In the present study, we examined changes in light-harvesting and energy-transfer processes of a diatom
Chaetoceros gracilis
cells grown using white- and single-colored light-emitting diodes (LEDs), by means of time-resolved fluorescence spectroscopy. The blue LED, which is harvested by FCPs, modified energy transfer involving CP47, and suppressed energy transfer to PSI. Under the red-LED conditions, which is absorbed by both FCPs and PSs, energy transfer to PSI was enhanced, and the red-shifted FCP appeared. The red-shifted FCP was also recognized under the green- and yellow-LEDs, suggesting that lack of the shorter-wavelength light induces the red-shifted FCP. Functions of the red-shifted FCPs are discussed.
The fucoxanthin chlorophyll (Chl) a/c-binding protein (FCP) is responsible for excellent light-harvesting strategies that enable survival in fluctuating light conditions. Here, we report the ...light-harvesting and quenching states of two FCP complexes, FCP-A and FCP-B/C, isolated from the diatom Chaetoceros gracilis. Pigment analysis revealed that FCP-A is enriched in Chl c, whereas FCP-B/C is enriched in diadinoxanthin, reflecting differences in low-temperature steady-state absorption and fluorescence spectra of each FCP complex. Time-resolved fluorescence spectra were measured at 77 K, and the characteristic lifetimes were determined using global fitting analysis of the spectra. Tens of picosecond (ps) components revealed energy transfer to low-energy Chl a from Chls a and c, whereas the other components showed only fluorescence decay components with no concomitant rise components. The normalized amplitudes of hundreds of picosecond components were relatively 30% in the total fluorescence, whereas those of longest-lived components were 60%. The hundreds of picosecond components were assigned as excitation energy quenching, whereas the longest-lived components were assigned as fluorescence from the final energy traps. These results suggest that 30% of FCP complex forming quenching state and the other 60% of FCP complex forming light-harvesting state exist heterogeneously in each FCP fraction under continuous low-light condition.
The light-harvesting complex II (LHCII) trimer in plants functions as a major antenna complex and a quencher to protect it from photooxidative damage. Theoretical studies on the structure of an LHCII ...trimer have demonstrated that excitation energy transfer between chlorophylls (Chls) in LHCII can be modulated by its exquisite conformational fluctuation. However, conformational changes depending on its binding location have not yet been investigated, even though reorganization of protein complexes occurs by physiological regulations. In this study, we investigated conformational differences in LHCII by comparing published structures of an identical LHCII trimer in the three different photosystem supercomplexes from the green alga Chlamydomonas reinhardtii. Our results revealed distinct differences in Chl configurations as well as polypeptide conformations of the LHCII trimers depending on its binding location. We propose that these configurational differences readily modulate the function of LHCII and possibly lead to a change in excitation-energy flow over the photosynthetic supercomplex.
Phylogenies based on entire genomes are a powerful tool for reconstructing the Tree of Life. Several methods have been proposed, most of which employ an alignment-free strategy. Average sequence ...similarity methods are different than most other whole-genome methods, because they are based on local alignments. However, previous average similarity methods fail to reconstruct a correct phylogeny when compared against other whole-genome trees. In this study, we developed a novel average sequence similarity method. Our method correctly reconstructs the phylogenetic tree of in silico evolved E. coli proteomes. We applied the method to reconstruct a whole-proteome phylogeny of 1,087 species from all three domains of life, Bacteria, Archaea, and Eucarya. Our tree was automatically reconstructed without any human decisions, such as the selection of organisms. The tree exhibits a concentric circle-like structure, indicating that all the organisms have similar total branch lengths from their common ancestor. Branching patterns of the members of each phylum of Bacteria and Archaea are largely consistent with previous reports. The topologies are largely consistent with those reconstructed by other methods. These results strongly suggest that this approach has sufficient taxonomic resolution and reliability to infer phylogeny, from phylum to strain, of a wide range of organisms.
Excitation-energy transfer in photosystem I (PSI) is changed by a cation formation of a special pair chlorophyll P700 in the PSI core; however, it remains unclear how light-harvesting pigment–protein ...complexes are involved in the P700-related energy-transfer mechanisms. Here, we report effects of the redox changes of P700 on excitation-energy dynamics in diatom PSI-fucoxanthin chlorophyll a/c-binding protein (PSI-FCPI) and PSI core complexes by means of time-resolved fluorescence (TRF) spectroscopy. For the TRF measurements, the PSI-FCPI and PSI were adapted under P700 neutral and cation conditions using chemical reagents. Upon the P700+ formation, fluorescence decay-associated (FDA) spectra constructed from the TRF spectra exhibit a larger fluorescence decay amplitude relative to a fluorescence rise magnitude within 100 ps in each of the PSI-FCPI and PSI. The decay components are shifted to lower wavelengths in each of the P700-cation PSI-FCPI and PSI than in the P700-neutral PSIs. The rapid fluorescence decays upon the P700+ formation are clearly verified by mean lifetimes reconstructed from the FDA spectra. Because the P700-cation PSI does not cause charge-separation reactions, the relatively strong decay components and rapid fluorescence decays observed are likely attributed to excitation-energy quenching. These observations suggest that chlorophylls in PSI and around/within FCPI are involved in the energy-quenching events by the redox changes of P700.
pH influences excitation-energy-relaxation processes in photosynthetic light-harvesting complexes. Here, we report the excitation-energy dynamics by pH changes in fucoxanthin chlorophyll
/
-binding ...proteins (FCPs) isolated from a diatom
, probed by time-resolved fluorescence spectroscopy at 77 K. The fluorescence curve measured at pH 5.0 showed a shorter lifetime component than that measured at pH 6.5 and 8.0. The rapid decay component at pH 5.0 is supported by fluorescence decay-associated (FDA) spectra, where strong fluorescence decays relative to fluorescence rises appear in the pH-5.0 FDA spectrum with 70 ps. These results indicate that the diatom FCPs switch their function from light-harvesting to energy-quenching via arrangements of the energy-transfer pathways under acidic pHs. Based on the crystal structure of the diatom FCPs, we propose a model for the energy-quenching machinery through structural changes of the pigment environments, thus providing insights into the pH-dependent light-harvesting strategy in the diatom FCPs.
Photosynthetic organisms handle solar energy precisely to achieve efficient photochemical reactions. Because there are a wide variety of light-harvesting antennas in oxyphototrophs, the excitation ...energy transfer mechanisms are thought to differ significantly. In this study, we compared excitation energy dynamics between photosystem I (PSI) cores and a complex between PSI and fucoxanthin chlorophyll (Chl) a/c-binding protein I (PSI−FCPI) isolated from a diatom, Chaetoceros gracilis, by means of picosecond time-resolved fluorescence analyses. Time-resolved spectra measured at 77 K clearly show that low-energy Chls in the FCPI transfer not only most of the excitation energy to the reaction center Chls in the PSI cores but also the remaining energy to carotenoids for quenching. Under room-temperature conditions, the energy in the low-energy Chls is rapidly equilibrated on Chls in the PSI cores by uphill energy transfer within a few tens of picoseconds. These findings provide solid evidence that the low-energy Chls in the FCPI contribute to the photochemical reactions in PSI.
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