Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University, Okayama, 700-8530, Japan.
Graduate School of Science, Kobe University, Hyogo, 657-8501, Japan.
Nat Commun. 2022 Mar 30;13(1):1679. doi: 10.1038/s41467-022-29303-7.
Photosystem I (PSI) is one of the two photosystems functioning in light-energy harvesting, transfer, and electron transfer in photosynthesis. However, the oligomerization state of PSI is variable among photosynthetic organisms. We present a 3.8-Å resolution cryo-electron microscopic structure of tetrameric PSI isolated from the glaucophyte alga Cyanophora paradoxa, which reveals differences with PSI from other organisms in subunit composition and organization. The PSI tetramer is organized in a dimer of dimers with a C2 symmetry. Unlike cyanobacterial PSI tetramers, two of the four monomers are rotated around 90°, resulting in a completely different pattern of monomer-monomer interactions. Excitation-energy transfer among chlorophylls differs significantly between Cyanophora and cyanobacterial PSI tetramers. These structural and spectroscopic features reveal characteristic interactions and excitation-energy transfer in the Cyanophora PSI tetramer, suggesting that the Cyanophora PSI could represent a turning point in the evolution of PSI from prokaryotes to eukaryotes.
光系统 I（PSI）是光合作用中光能收集、传递和电子转移的两个光系统之一。然而，PSI 的寡聚状态在光合生物中是可变的。我们展示了来自蓝藻门植物 Cyanophora paradoxa 的四聚 PSI 的 3.8 Å 分辨率冷冻电镜结构，该结构揭示了其在亚基组成和组织方面与其他生物体 PSI 的差异。PSI 四聚体以二聚体的二聚体形式组织，具有 C2 对称性。与蓝细菌 PSI 四聚体不同，四个单体中的两个绕 90°旋转，导致单体-单体相互作用的完全不同模式。叶绿素之间的能量转移在 Cyanophora 和蓝细菌 PSI 四聚体之间有显著差异。这些结构和光谱特征揭示了 Cyanophora PSI 四聚体中的特征相互作用和激发能量转移，表明 Cyanophora PSI 可能代表了 PSI 从原核生物向真核生物进化的一个转折点。
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