Abstract
The versatile anaerobic metabolism of the Gram-negative bacterium Shewanella oneidensis MR-1 (SOMR-1) relies on a multitude of redox proteins found in its periplasm. Most are multiheme cytochromes that carry electrons to terminal reductases of insoluble electron acceptors located at the cell surface, or bona fide terminal reductases of soluble electron acceptors. In this study, the interaction network of several multiheme cytochromes was explored by a combination of NMR spectroscopy, activity assays followed by UV-visible spectroscopy and comparison of surface electrostatic potentials. From these data the small tetraheme cytochrome (STC) emerges as the main periplasmic redox shuttle in SOMR-1. It accepts electrons from CymA and distributes them to a number of terminal oxidoreductases involved in the respiration of various compounds. STC is also involved in the electron transfer pathway to reduce nitrite by interaction with the octaheme tetrathionate reductase (OTR), but not with cytochrome c nitrite reductase (ccNiR). In the main pathway leading the metal respiration STC pairs with flavocytochrome c (FccA), the other major periplasmic cytochrome, which provides redundancy in this important pathway. The data reveals that the two proteins compete for the binding site at the surface of MtrA, the decaheme cytochrome inserted on the periplasmic side of the MtrCAB–OmcA outer-membrane complex. However, this is not observed for the MtrA homologues. Indeed, neither STC nor FccA interact with MtrD, the best replacement for MtrA, and only STC is able to interact with the decaheme cytochrome DmsE of the outer-membrane complex DmsEFABGH. Overall, these results shown that STC plays a central role in the anaerobic respiratory metabolism of SOMR-1. Nonetheless, the trans-periplasmic electron transfer chain is functionally resilient as a consequence of redundancies that arise from the presence of alternative pathways that bypass/compete with STC.
Highlights
Shewanella oneidensis MR-1 is a Gram-negative bacterium that can use a wide range of terminal electron acceptors in the absence of oxygen, including fumarate, nitrite, nitrate, trimethylamine oxide (TMAO), dimethyl sulfoxide (DMSO), sulfur compounds and a variety of metal compounds including radionuclides (Myers and Nealson, 1988; Nealson and Saffarini, 1994; Myers and Myers, 2000; Gralnick and Newman, 2007; Burns and DiChristina, 2009)
Nuclear magnetic resonance (NMR) Titrations and Binding Affinities For electron transfer to occur at physiologically relevant rates between two cytochromes, the heme groups of donor and acceptor must be in close proximity (Zhang et al, 2008; Gray and Winkler, 2010)
The electron transfer pathways of the Shewanella oneidensis MR-1 (SOMR-1) to reduce Fe(III), DMSO, fumarate and nitrite are established by a variety of multiheme c-type cytochromes located at the inner membrane, periplasm and outer membrane
Summary
Shewanella oneidensis MR-1 is a Gram-negative bacterium that can use a wide range of terminal electron acceptors in the absence of oxygen, including fumarate, nitrite, nitrate, trimethylamine oxide (TMAO), dimethyl sulfoxide (DMSO), sulfur compounds and a variety of metal compounds including radionuclides (Myers and Nealson, 1988; Nealson and Saffarini, 1994; Myers and Myers, 2000; Gralnick and Newman, 2007; Burns and DiChristina, 2009). With the exception of thiosulfate and TMAO reduction, all forms of anaerobic respiration described in SOMR-1 are routed via the tetraheme c-type cytochrome CymA, anchored to the inner membrane of the cell (Myers and Myers, 1997; Saffarini et al, 2002) This protein collects electrons from the menaquinone pool in the cytoplasmic membrane and distributes them among periplasmic proteins. It was recently shown that both STC and FccA can accept electrons from CymA and transfer them to outer-membrane metal reductases, which lead to the identification of two independent redox pathways across the periplasm (Fonseca et al, 2013) This finding confirmed the functional redundancy already observed for other multiheme cytochromes (Coursolle and Gralnick, 2010; Richardson et al, 2012), the physiological reason for this is still unknown. It appears to play a similar role to MtrCAB–OmcA complex because of their high similarity (Fredrickson et al, 2008; Coursolle and Gralnick, 2010)
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