Article

Engineering of a synthetic electron conduit in living cells. Proc Natl Acad Sci USA

Department of Chemistry and Howard Hughes Medical Institute, University of California, Berkeley, CA 94720, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 10/2010; 107(45):19213-8. DOI: 10.1073/pnas.1009645107
Source: PubMed

ABSTRACT

Engineering efficient, directional electronic communication between living and nonliving systems has the potential to combine the unique characteristics of both materials for advanced biotechnological applications. However, the cell membrane is designed by nature to be an insulator, restricting the flow of charged species; therefore, introducing a biocompatible pathway for transferring electrons across the membrane without disrupting the cell is a significant challenge. Here we describe a genetic strategy to move intracellular electrons to an inorganic extracellular acceptor along a molecularly defined route. To do so, we reconstitute a portion of the extracellular electron transfer chain of Shewanella oneidensis MR-1 into the model microbe Escherichia coli. This engineered E. coli can reduce metal ions and solid metal oxides ∼8× and ∼4× faster than its parental strain. We also find that metal oxide reduction is more efficient when the extracellular electron acceptor has nanoscale dimensions. This work demonstrates that a genetic cassette can create a conduit for electronic communication from living cells to inorganic materials, and it highlights the importance of matching the size scale of the protein donors to inorganic acceptors.

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    • "Since the electron transfer mechanism of S. oneidensis is intensively studied and one of the best understood electrogenic pathways, a genetic cassette containing a part of the electron transfer chain of S. oneidensis can be used to create an electron conduit in E. coli. Expression of the mtrCAB genes of S. oneidensis in E. coli gives E. coli the ability to reduce both metal ions and solid metal oxides by transfer of electrons along a well-defined path from the cell interior to extracellular inorganic materials[Jensen et al. 2010]. This study demonstrates that through a synthetic biology approach, cells can be engineered to obtain electrical communication between living cells and non-living systems. "
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    ABSTRACT: Recent developments demonstrate that the combination of microbiology with micro- and nanoelectronics is a successful approach to develop new miniaturized sensing devices and other technologies. In the last decade, there has been a shift from the optimization of the abiotic components, for example, the chip, to the improvement of the processing capabilities of cells through genetic engineering. The synthetic biology approach will not only give rise to systems with new functionalities, but will also improve the robustness and speed of their response towards applied signals. To this end, the development of new genetic circuits has to be guided by computational design methods that enable to tune and optimize the circuit response. As the successful design of genetic circuits is highly dependent on the quality and reliability of its composing elements, intense characterization of standard biological parts will be crucial for an efficient rational design process in the development of new genetic circuits. Microengineered devices can thereby offer a new analytical approach for the study of complex biological parts and systems. By summarizing the recent techniques in creating new synthetic circuits and in integrating biology with microdevices, this review aims at emphasizing the power of combining synthetic biology with microfluidics and microelectronics.
    Preview · Article · Nov 2013 · ACM Journal on Emerging Technologies in Computing Systems
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    • "MtrC copurifies with MtrAB from S. oneidensis, and the functionality of the MtrABC complex in transmembrane electron transfer was demonstrated in sealed proteoliposomes (Hartshorne et al., 2009). Further support for a role for MtrCAB in trans-membrane electron transfer comes from expression of mtrCAB in E. coli that confers a capacity for hematite reduction on the recipient organism (Jensen et al., 2010). It should be noted though that the iron-reducing activity reported was very low com- Microbe-to-mineral electron transfer 203 pared to S. oneidensis and so the question of whether MtrCAB on its own is sufficient to confer high rates on mineral iron respiration remains. "
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    ABSTRACT: Many species of bacteria can couple anaerobic growth to the respiratory reduction of insoluble minerals containing Fe(III) or Mn(III/IV). It has been suggested that in Shewanella species electrons cross the outer membrane to extracellular substrates via 'porin-cytochrome' electron transport modules. The molecular structure of an outer-membrane extracellular-facing deca-haem terminus for such a module has recently been resolved. It is debated how, once outside the cells, electrons are transferred from outer-membrane cytochromes to insoluble electron sinks. This may occur directly or by assemblies of cytochromes, perhaps functioning as 'nanowires', or via electron shuttles. Here we review recent work in this field and explore whether it allows for unification of the electron transport mechanisms supporting extracellular mineral respiration in Shewanella that may extend into other genera of Gram-negative bacteria.
    Full-text · Article · May 2012 · Molecular Microbiology
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    • "The purified MtrABC complex can transfer electrons across a lipid bilayer following incorporation into proteoliposomes, providing direct evidence that together, MtrABC serve as an electron conduit between the periplasm of S. oneidensis MR-1 cells and its extracellular environments (Hartshorne et al., 2009). Consistent with these results, heterologous co-expression of MtrABC enables E. coli to reduce solid-phase Fe(III) oxides (Jensen et al., 2010). Furthermore, while MtrAB can form a stable complex in the absence of MtrC, an MtrBC complex cannot be isolated in the absence of MtrA. "
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    ABSTRACT: In the absence of O2 and other electron acceptors, the Gram-negative bacterium Shewanella oneidensis MR-1 can use ferric [Fe(III)] (oxy)(hydr)oxide minerals as the terminal electron acceptors for anaerobic respiration. At circumneutral pH and in the absence of strong complexing ligands, Fe(III) oxides are relatively insoluble and thus are external to the bacterial cells. S. oneidensis MR-1 has evolved the machinery (i.e., metal-reducing or Mtr pathway) for transferring electrons across the entire cell envelope to the surface of extracellular Fe(III) oxides. The protein components identified to date for the Mtr pathway include CymA, MtrA, MtrB, MtrC and OmcA. CymA is an inner-membrane tetraheme c-type cytochrome (c-Cyt) that is proposed to oxidize the quinol in the inner-membrane and transfers the released electrons to redox proteins in the periplasm. Although the periplasmic proteins receiving electrons from CymA during Fe(III) oxidation have not been identified, they are believed to relay the electrons to MtrA. A decaheme c-Cyt, MtrA is thought to be embedded in the trans outer-membrane and porin-like protein MtrB. Together, MtrAB deliver the electrons across the outer-membrane to the MtrC and OmcA on the outmost bacterial surface. Functioning as terminal reductases, the outer membrane and decaheme c-Cyts MtrC and OmcA can bind the surface of Fe(III) oxides and transfer electrons directly to these minerals. To increase their reaction rates, MtrC and OmcA can use the flavins secreted by S. oneidensis MR-1 cells as diffusible co-factors for reduction of Fe(III) oxides. MtrC and OmcA can also serve as the terminal reductases for soluble forms of Fe(III). Although our understanding of the Mtr pathway is still far from complete, it is the best characterized microbial pathway used for extracellular electron exchange. Characterizations of the Mtr pathway have made significant contributions to the molecular understanding of microbial reduction of Fe(III) oxides.
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