Far-red light (FRL) photoacclimation in cyanobacteria provides a selective growth advantage for some terrestrial cyanobacteria by expanding the range of photosynthetically active radiation to include far-red/near-infrared light (700-800 nm). During this photoacclimation process, photosystem II (PSII), the water:plastoquinone photooxidoreductase involved in oxygenic photosynthesis, is modified. The resulting FRL-PSII is comprised of FRL-specific core subunits and binds chlorophyll (Chl) d and Chl f molecules in place of several of the Chl a molecules found when cells are grown in visible light. These new Chls effectively lower the energy canonically thought to define the "red limit" for light required to drive photochemical catalysis of water oxidation. Changes to the architecture of FRL-PSII were previously unknown, and the positions of Chl d and Chl f molecules had only been proposed from indirect evidence. Here, we describe the 2.25 Å resolution cryo-EM structure of a monomeric FRL-PSII core complex from Synechococcus sp. PCC 7335 cells that were acclimated to FRL. We identify one Chl d molecule in the ChlD1 position of the electron transfer chain and four Chl f molecules in the core antenna. We also make observations that enhance our understanding of PSII biogenesis, especially on the acceptor side of the complex where a bicarbonate molecule is replaced by a glutamate side chain in the absence of the assembly factor Psb28. In conclusion, these results provide a structural basis for the lower energy limit required to drive water oxidation, which is the gateway for most solar energy utilization on earth.

中文翻译：

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来自适应远红光的蓝藻的单体光系统 II 核心复合物的结构揭示了叶绿素 d 和 f 的功能。

蓝藻中的远红光 (FRL) 光驯化通过将光合有效辐射范围扩大到包括远红/近红外光 (700-800 nm) 为一些陆地蓝藻提供了选择性生长优势。在这个光适应过程中，光系统 II (PSII)，即参与有氧光合作用的水：质体醌光氧化还原酶，被修改。产生的 FRL-PSII 由 FRL 特异性核心亚基组成，并结合叶绿素 (Chl) d 和 Chl f 分子，代替细胞在可见光下生长时发现的几个 Chl a 分子。这些新的 Chls 有效地降低了通常被认为定义驱动水氧化的光化学催化所需的光的“红色极限”的能量。FRL-PSII 架构的变化以前是未知的，Chl d 和 Chl f 分子的位置只是从间接证据中提出的。在这里，我们描述了来自 Synechococcus sp 的单体 FRL-PSII 核心复合物的 2.25 Å 分辨率冷冻电镜结构。适应 FRL 的 PCC 7335 细胞。我们在电子转移链的 ChlD1 位置识别出一个 Chl d 分子，在核心天线中识别出四个 Chl f 分子。我们还进行了观察，增强了我们对 PSII 生物发生的理解，特别是在复合体的受体侧，在没有组装因子 Psb28 的情况下，碳酸氢盐分子被谷氨酸侧链取代。总之，这些结果为驱动水氧化所需的能量下限提供了结构基础，这是地球上大多数太阳能利用的门户。