Team:Cornell/project/wetlab/assembly

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Shewanella oneidensis MR-1’s metal reduction pathway can be likened to a simple switch analogy. Without MtrB, no extracellular transfer of electrons is possible, and therefore no current is produced. However, when reintroduced, MtrB closes the switch therefore allowing extracellular reduction and current generation.

To sensitize Shewanella’s metal reduction pathway to our analytes, we decided to use a complementation strategy. By using the Shewanella MtrB knockout strain, JG 700, which was graciously provided by Professor Jeffery Gralnick from the University of Minnesota, we are able to reintroduce MtrB on a plasmid under the control of inducible promoters sensitive to the analytes we want to detect. Thus, MtrB—and therefore current—should only be produced in the presence of analyte.

One of the greatest strengths of this approach is its modularity; by simply switching out the sensing region on the plasmid, we can sensitize MtrB production to any analyte for which genetic parts exist.

Because of its essential role in extracellular electron transfer, MtrB can be thought of as an electric switch: When absent, the switch is open, disallowing current production in microbial electrochemical systems; when present, the switch is closed and current may be produced. We took advantage of this in the development of our biosensing strains: Using a dmtrB strain (JG700) as a host, we adopted a complementation strategy wherein the capability for extracellular electron shuttling was reintroduced via the expression of MtrB from a plasmid. Because we designed our engineered plasmids so that mtrB transcription could be induced in response to our analyte of interest, we were able to construct strains that produce more current in response to analyte—as the MtrB ‘switches’ close.