Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University, Okayama, Japan.
Biostructural Mechanism Laboratory, RIKEN SPring-8 Center, Hyogo, Japan.
Elife. 2022 Apr 11;11:e73990. doi: 10.7554/eLife.73990.
Photosystem I (PSI) is a multi-subunit pigment-protein complex that functions in light-harvesting and photochemical charge-separation reactions, followed by reduction of NADP to NADPH required for CO fixation in photosynthetic organisms. PSI from different photosynthetic organisms has a variety of chlorophylls (Chls), some of which are at lower-energy levels than its reaction center P700, a special pair of Chls, and are called low-energy Chls. However, the sites of low-energy Chls are still under debate. Here, we solved a 2.04-Å resolution structure of a PSI trimer by cryo-electron microscopy from a primordial cyanobacterium PCC 7421, which has no low-energy Chls. The structure shows the absence of some subunits commonly found in other cyanobacteria, confirming the primordial nature of this cyanobacterium. Comparison with the known structures of PSI from other cyanobacteria and eukaryotic organisms reveals that one dimeric and one trimeric Chls are lacking in the PSI. The dimeric and trimeric Chls are named Low1 and Low2, respectively. Low2 is missing in some cyanobacterial and eukaryotic PSIs, whereas Low1 is absent only in . These findings provide insights into not only the identity of low-energy Chls in PSI, but also the evolutionary changes of low-energy Chls in oxyphototrophs.
光系统 I（PSI）是一个多亚基的色素-蛋白复合物，其功能是在光捕获和光化学电荷分离反应中，随后将 NADP 还原为光合作用生物中 CO 固定所需的 NADPH。来自不同光合生物的 PSI 具有各种叶绿素（Chls），其中一些叶绿素的能量水平低于其反应中心 P700，一对特殊的叶绿素，称为低能量 Chls。然而，低能量 Chls 的位置仍存在争议。在这里，我们通过冷冻电镜从原始蓝细菌 PCC 7421 中解决了一个 PSI 三聚体的 2.04 Å 分辨率结构，该蓝细菌没有低能量 Chls。该结构显示出一些在其他蓝细菌中常见的亚基缺失，证实了这种蓝细菌的原始性质。与其他蓝细菌和真核生物的 PSI 已知结构的比较表明，PSI 中缺少一个二聚体和一个三聚体 Chls。二聚体和三聚体 Chls 分别命名为 Low1 和 Low2。低 2 在一些蓝细菌和真核 PSI 中缺失，而低 1 仅在 中缺失。这些发现不仅为 PSI 中低能量 Chls 的身份提供了深入了解，也为氧光合作用生物中低能量 Chls 的进化变化提供了深入了解。
