Photoferrotrophy is a form of anoxygenic photosynthesis whereby bacteria utilize soluble or insoluble forms of ferrous iron as an electron donor to fix carbon dioxide using light energy. They can also use poised electrodes as their electron donor via phototrophic extracellular electron uptake (phototrophic EEU). The electron uptake mechanisms underlying these processes are not well understood. Using Rhodopseudomonas palustris TIE-1 as a model, we show that a single periplasmic decaheme cytochrome c, PioA, and an outer membrane porin, PioB, form a complex allowing extracellular electron uptake across the outer membrane from both soluble iron and poised electrodes. We observe that PioA undergoes postsecretory proteolysis of its N terminus to produce a shorter heme-attached PioA (holo-PioAC, where PioAC represents the C terminus of PioA), which can exist both freely in the periplasm and in a complex with PioB. The extended N-terminal peptide controls heme attachment, and its processing is required to produce wild-type levels of holo-PioAC and holo-PioACB complex. It is also conserved in PioA homologs from other phototrophs. The presence of PioAB in these organisms correlate with their ability to perform photoferrotrophy and phototrophic EEU.IMPORTANCE Some anoxygenic phototrophs use soluble iron, insoluble iron minerals (such as rust), or their proxies (poised electrodes) as electron donors for photosynthesis. However, the underlying electron uptake mechanisms are not well established. Here, we show that these phototrophs use a protein complex made of an outer membrane porin and a periplasmic decaheme cytochrome (electron transfer protein) to harvest electrons from both soluble iron and poised electrodes. This complex has two unique characteristics: (i) it lacks an extracellular cytochrome c, and (ii) the periplasmic decaheme cytochrome c undergoes proteolytic cleavage to produce a functional electron transfer protein. These characteristics are conserved in phototrophs harboring homologous proteins.

中文翻译：

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光铁营养体产生用于细胞外电子摄取的 PioAB 电子导管。

光铁营养是缺氧光合作用的一种形式，细菌利用可溶或不溶形式的亚铁作为电子供体，利用光能固定二氧化碳。他们还可以通过光养细胞外电子摄取（光养 EEU）使用平衡电极作为电子供体。这些过程背后的电子吸收机制尚不清楚。使用沼泽红假单胞菌 TIE-1 作为模型，我们发现单个周质十血红素细胞色素 c PioA 和外膜孔蛋白 PioB 形成复合物，允许从可溶性铁和平衡电极穿过外膜摄取细胞外电子。我们观察到 PioA 的 N 末端经历分泌后蛋白水解，产生较短的血红素附着 PioA（holo-PioAC，其中 PioAC 代表 PioA 的 C 末端），它可以自由存在于周质中，也可以与 PioB 形成复合物。延伸的 N 端肽控制血红素附着，其加工是产生野生型水平的 Holo-PioAC 和 Holo-PioACB 复合物所必需的。它在其他光养生物的 PioA 同系物中也是保守的。这些生物体中 PioAB 的存在与它们进行光养铁和光养 EEU 的能力相关。 重要性 一些缺氧光养生物使用可溶性铁、不溶性铁矿物质（例如铁锈）或其代理（平衡电极）作为光合作用的电子供体。然而，潜在的电子吸收机制尚未明确。在这里，我们证明这些光养生物使用由外膜孔蛋白和周质十血红素细胞色素（电子转移蛋白）组成的蛋白质复合物从可溶性铁和平衡电极收集电子。该复合物具有两个独特的特征：(i) 它缺乏细胞外细胞色素 c，以及 (ii) 周质十血红素细胞色素 c 经历蛋白水解裂解以产生功能性电子转移蛋白。这些特征在含有同源蛋白质的光养生物中是保守的。