In oxygenic photosynthesis, the cytochrome b6f (cytb6f) complex links the linear electron transfer (LET) reactions occurring at photosystems I and II and generates a transmembrane proton gradient via the Q-cycle. In addition to this central role in LET, cytb6f also participates in a range of processes including cyclic electron transfer (CET), state transitions and photosynthetic control. Many of the regulatory roles of cytb6f are facilitated by auxiliary proteins that differ depending upon the species, yet because of their weak and transient nature the structural details of these interactions remain unknown. An apparent key player in the regulatory balance between LET and CET in cyanobacteria is PetP, a ∼10 kDa protein that is also found in red algae but not in green algae and plants. Here, we used cryogenic electron microscopy to determine the structure of the Synechocystis sp. PCC 6803 cytb6f complex in the presence and absence of PetP. Our structures show that PetP interacts with the cytoplasmic side of cytb6f, displacing the C-terminus of the PetG subunit and shielding the C-terminus of cytochrome b6, which binds the heme cn cofactor that is suggested to mediate CET. The structures also highlight key differences in the mode of plastoquinone binding between cyanobacterial and plant cytb6f complexes, which we suggest may reflect the unique combination of photosynthetic and respiratory electron transfer in cyanobacterial thylakoid membranes. The structure of cytb6f from a model cyanobacterial species amenable to genetic engineering will enhance future site-directed mutagenesis studies of structure-function relationships in this crucial ET complex.

中文翻译：

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集胞藻属的冷冻电镜结构。PCC 6803 细胞色素 b6f 复合物，有和没有调节性 PetP 亚基

在有氧光合作用中，细胞色素 b6f (cytb6f) 复合物连接光系统 I 和 II 发生的线性电子转移 (LET) 反应，并通过 Q 循环产生跨膜质子梯度。除了在 LET 中的核心作用外，cytb6f 还参与一系列过程，包括循环电子转移 (CET)、状态转换和光合控制。cytb6f 的许多调节作用是由因物种而异的辅助蛋白促进的，但由于它们的弱和短暂性，这些相互作用的结构细节仍然未知。蓝藻中 LET 和 CET 之间调节平衡的一个明显关键因素是 PetP，这是一种 10 kDa 的蛋白质，也存在于红藻中，但不存在于绿藻和植物中。这里，我们使用低温电子显微镜来确定集胞藻属的结构。在存在和不存在 PetP 的情况下 PCC 6803 cytb6f 复合物。我们的结构表明，PetP 与 cytb6f 的细胞质侧相互作用，取代 PetG 亚基的 C 端并屏蔽细胞色素 b6 的 C 端，细胞色素 b6 与被认为介导 CET 的血红素 cn 辅因子结合。这些结构还突出了蓝藻和植物 cytb6f 复合物之间质体醌结合模式的关键差异，我们认为这可能反映了蓝藻类囊体膜中光合和呼吸电子转移的独特组合。