Abstract
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.
Highlights
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
Several anoxygenic phototrophs can grow by coupling oxidation of ferrous iron to carbon dioxide (CO2) fixation using the energy of light, a process called photoferrotrophy [1]
This N-terminal extension is conserved in PioA homologs annotated in the genomes of different phototrophic bacteria and does not harbor any clear protein domains (Fig. S2)
Summary
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 use poised electrodes as their electron donor via phototrophic extracellular electron uptake (phototrophic EEU). 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. A deletion mutant lacking pioABC cannot perform photoferrotrophy [6] and has a partial defect in phototrophic EEU [15] Beyond these genetic studies, only the iron-sulfur protein PioC has been biochemically characterized as a periplasmic electron transfer protein and proposed to shuttle electrons from PioA to the photosynthetic reaction center [33, 34]. Where Pio proteins oxidize Fe(II) during photoferrotrophy and how they transfer electrons across the outer membrane from poised electrodes during phototrophic EEU are unknown
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