Accepted Manuscript online January 16, 2017.Geobacter bacteria usually prevail among other microorganisms in soils and sediments where Fe(III) reduction has a central role. This reduction is achieved by extracellular electron transfer (EET), where the electrons are exported from the interior of the cell to the surrounding environment. Periplasmic cytochromes play an important role in establishing an interface between inner and outer membrane electron transfer components. In addition, periplasmic cytochromes, in particular nanowire cytochromes that contain at least 12 haem groups, have been proposed to play a role in electron storage in conditions of an environmental lack of electron acceptors. Up to date, no redox partners have been identified in Geobacter sulfurreducens, and concomitantly, the EET and electron storage mechanisms remain unclear. In this work, NMR chemical shift perturbation measurements were used to probe for an interaction between the most abundant periplasmic cytochrome PpcA and the dodecahaem cytochrome GSU1996, one of the proposed nanowire cytochromes in G. sulfurreducens. The perturbations on the haem methyl signals of GSU1996 and PpcA showed that the proteins form a transient redox complex in an interface that involves haem groups from two different domains located at the C-terminal of GSU1996. Overall, the present study provides for the first time a clear evidence for an interaction between periplasmic cytochromes that might be relevant for the EET and electron storage pathways in G. sulfurreducens.1D, one-dimensional; CbcL, c- and b-type cytochrome for low potential; EET, extracellular electron transfer; HP, His-patch; ImcH, inner membrane c-type cytochrome; MacA, metal-reduction-associated cytochrome; NaPi, sodium phosphate; NBAF, acetate-fumarate medium; NMR, nuclear magnetic resonance; PpcA, periplasmic c-type cytochrome; SDS–PAGE, sodium dodecyl sulphate–polyacrylamide gel electrophoresis; STC, small tetrahaem cytochrome.

The purple bacterium Rhodopseudomonas palustris TIE-1 expresses multiple small high-potential redox proteins during photoautotrophic growth, including two high-potential iron-sulfur proteins (HiPIPs) (PioC and Rpal_4085) and a cytochrome c2. We evaluated the role of these proteins in TIE-1 through genetic, physiological, and biochemical analyses. Deleting the gene encoding cytochrome c2 resulted in a loss of photosynthetic ability by TIE-1, indicating that this protein cannot be replaced by either HiPIP in cyclic electron flow. PioC was previously implicated in photoferrotrophy, an unusual form of photosynthesis in which reducing power is provided through ferrous iron oxidation. Using cyclic voltammetry (CV), electron paramagnetic resonance (EPR) spectroscopy, and flash-induced spectrometry, we show that PioC has a midpoint potential of 450 mV, contains all the typical features of a HiPIP, and can reduce the reaction centers of membrane suspensions in a light-dependent manner at a much lower rate than cytochrome c2. These data support the hypothesis that PioC linearly transfers electrons from iron, while cytochrome c2 is required for cyclic electron flow. Rpal_4085, despite having spectroscopic characteristics and a reduction potential similar to those of PioC, is unable to reduce the reaction center. Rpal_4085 is upregulated by the divalent metals Fe(II), Ni(II), and Co(II), suggesting that it might play a role in sensing or oxidizing metals in the periplasm. Taken together, our results suggest that these three small electron transfer proteins perform different functions in the cell.