Far-red light photoacclimation exhibited by some cyanobacteria allows these organisms to use the far-red region of the solar spectrum (700-800 nm) for photosynthesis. Part of this process includes the replacement of six photosystem I (PSI) subunits with isoforms that confer the binding of chlorophyll (Chl) f molecules that absorb far-red light (FRL). However, the exact sites at which Chl f molecules are bound are still challenging to determine. To aid in the identification of Chl f-binding sites, we solved the cryo-EM structure of PSI from far-red light-acclimated cells of the cyanobacterium Synechococcus sp. PCC 7335. We identified six sites that bind Chl f with high specificity and three additional sites that are likely to bind Chl f at lower specificity. All of these binding sites are in the core-antenna regions of PSI, and Chl f was not observed among the electron transfer cofactors. This structural analysis also reveals both conserved and nonconserved Chl f-binding sites, the latter of which exemplify the diversity in FRL-PSI among species. We found that the FRL-PSI structure also contains a bound soluble ferredoxin, PetF1, at low occupancy, which suggests that ferredoxin binds less transiently than expected according to the canonical view of ferredoxin-binding to facilitate electron transfer. We suggest that this may result from structural changes in FRL-PSI that occur specifically during FRL photoacclimation.

中文翻译：

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来自海洋蓝藻的光系统 I-铁氧还蛋白复合物的结构提供了对远红光光适应的见解。

一些蓝藻表现出的远红光光适应允许这些生物使用太阳光谱的远红区域（700-800 nm）进行光合作用。该过程的一部分包括用同种型替换六个光系统 I (PSI) 亚基，这些亚型赋予吸收远红光 (FRL) 的叶绿素 (Chl) f 分子结合。然而，叶绿素分子结合的确切位点仍然难以确定。为了帮助识别 Chl f 结合位点，我们从蓝细菌 Synechococcus sp的远红光适应细胞中解析了 PSI 的低温 EM 结构。PCC 7335. 我们确定了六个以高特异性结合 Chl f 的位点和三个可能以较低特异性结合 Chl f 的位点。所有这些结合位点都在 PSI 的核心天线区域，在电子转移辅因子中未观察到 Chl f。这种结构分析还揭示了保守和非保守的 Chl f 结合位点，后者体现了物种间 FRL-PSI 的多样性。我们发现 FRL-PSI 结构还包含结合的可溶性铁氧还蛋白 PetF1，在低占用率下，这表明根据铁氧还蛋白结合促进电子转移的规范观点，铁氧还蛋白的结合比预期的更短暂。我们认为这可能是由于在 FRL 光驯化过程中特别发生的 FRL-PSI 的结构变化。我们发现 FRL-PSI 结构还包含结合的可溶性铁氧还蛋白 PetF1，在低占用率下，这表明根据铁氧还蛋白结合促进电子转移的规范观点，铁氧还蛋白的结合比预期的更短暂。我们认为这可能是由于在 FRL 光驯化过程中特别发生的 FRL-PSI 的结构变化。我们发现 FRL-PSI 结构还包含结合的可溶性铁氧还蛋白 PetF1，在低占用率下，这表明根据铁氧还蛋白结合促进电子转移的规范观点，铁氧还蛋白的结合比预期的更短暂。我们认为这可能是由于在 FRL 光驯化过程中特别发生的 FRL-PSI 的结构变化。