5.B.5.2.1
The secreted phototrophic iron (Fe²⁺) oxidase (CO₂ reducing), PioABC (Jiao and Newman, 2007). 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). Gupta et al. 2019 showed that the single periplasmic decaheme cytochrome c, PioA, and the outer membrane porin, PioB, form a complex allowing extracellular electron uptake across the outer membrane from both soluble iron and poised electrodes. They observed that PioA undergoes postsecretory proteolysis of its N terminus to produce a shorter heme-attached PioA (holo-PioA_C, where PioA_C 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 the holo-PioA_C and holo-PioA_CB complex. It is also conserved in PioA homologs from other phototrophs (Gupta et al. 2019).