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Project Overview

Welcome to Yale-iGEM 2010! In our inaugural year of iGEM competition, we have designed a system to harness biology to construct conductive circuits.

By enabling E. coli to affect local redox chemistry, we can use bacteria to catalyze metal deposition. When this activity is controlled spatially and temporally, this method could be used to construct circuit elements in a specified geometry. This would allow the manufacture of electrical components under biological conditions.

What would it take to...

Build a circuit using microbially catalyzed metal sulfide deposition?

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Background: SRB, H₂S, and Copper Sulfides

The inspiration for this idea came from an ecological observation made of copper biomineralization by a species of Sulfate Reducing Bacteria. Sulfate Reducing Bacteria (SRB) comprise a family of chemolithotrophic bacteria that use sulfate as the terminal electron acceptor in anaerobic metabolism. As a result of sulfate reduction, these bacteria produce gaseous H₂S. Subsequently, microbially generated H₂S reduces copper in solution and precipitates soluble copper ions in the form of insoluble copper sulfide (CuS). It was discovered that by this method, some strains of SRB formed a CuS compound nearly identical to covellite - a natural superconductor (Weber, 2009). If this activity could be enhanced under spatial and temporal control, bacteria could be harnessed to deposit metal sulfide in specified geometries for manufacturing and engineering applications.

Building H₂S Production Activity into E. coli

Investigation of H₂S production in bacteria has been well documented in E. coli. Work by Dr. Jay Keasling at University of California, Berkeley, has shown that a gene encoding Thiosulfate Reductase from Salmonella enterica serovar Typhimurium has previously been expressed in E. coli to overproduce hydrogen sulfide from thiosulfate.

The gene phsABC functionally encodes Thiosulfate reductase. The gene itself contains an open reading frame for each of the three genes phsA, phsB, and phsC. The resulting complex of three transmembrane proteins constitutes Thiosulfate reductase and catalyzes the reduction of inorganic thiosulfate to hydrogen sulfide.





This activity can be monitored by growing bacteria on Triple Sugar Iron (TSI) Agar. Evolution of H₂S as a gas, results in the reduction of Iron Sulfate to an Iron Sulfide. Iron sulfide precipitates as a black solid and can be easily identified visually on the macroscopic scale. Bacteria that are actively producing H₂S will reduce the Iron in the medium, and turn the agar black.

Growing Bacteria In Copper Medium

Bacteria expressing Thiosulfate Reductase can then be grown in media containing both a source of sulfur and free copper in solution. In order to maximize While high copper concentrations are

Correlating Bacterial Growth to Copper Deposition

Localizing Copper Deposition

Modeling the Construction of Metal Sulfides

Project

In an effort to achieve our goal, we have designed a bacterial model using synthetic biology. A gene encoding the protein Thisulfate Reductase has been inserted Our initial research indicated that several observations have been made of natural mechanisms for metal deposition and precipitation in a family of microorganisms called Sulfate Reducing Bacteria (SRB). These SRB comprise a class of chemolithotrophic microbes that couple anaerobic electron transport to ATP synthesis using sulfate as the terminal electron acceptor. Importantly, as a consequence of their metabolism, SRB produce hydrogen sulfide: a gas that can be used to reduce metals in solution.
These H2S Producing bacteria were found to form significant concentrations of solid metals on and around their cell surface. The H2S gas reduces free metal ions in solution to form a metal sulfide, which subsequently precipitates out of solution. The metal sulfide can self-associate to form clusters of had the capability to reduce could be used to catalyze the precipitation of metal solids in solutions containing metal in the appropriate ionic state. Biochemistry of Sulfate Reducing Bacteria
Papers on Sulfate Reducing Bacteria
Sulfate Reducing Bacteria (SRB) comprise a class of chemolithotrophic microorganisms that couple anaerobic electron transport to ATP synthesis, using sulfate as a terminal electron acceptor.