Part:BBa_K1895999

Arabinose controlled 'resistor'

As part of our iGEM project we have attempted to engineer E. coli to behave like a variable resistor. We hoped to do this by using E. coli to vary the amount of free ions in an electrolyte. Ion uptake will be controlled by the expression of a protein, smtA. SmtA is a metallothionein that can bind to heavy metal ions like cadmium (II), Zinc (II) and Copper (II).

In this construct we have placed smtA under the control of an AraC regulated promoter allowing the expression of smtA to be controlled by the addition or removal of arabinose.

Usage and Biology

[http://www.rcsb.org/pdb/explore.do?structureId=1JJD SmtA is a metallothionein from Cyanobacteria]. It is thought to play a role in preventing heavy metal toxicity by binding excess heavy metal ions such as Cadmium (II) or Zinc (II). As SmtA is expressed it can bind to Zinc molecules, 4 ions per protein (Blindauer et al., 2002). For example, the addition of Zinc sulfate to growth media provides a source of Zn2+ ions as it dissasociates into (Zn)2+ and (SO4)2- ions in solution. The ion binding takes place intracellularly after the ions have entered the cell. Similarly, as the protein is degraded the Zinc is released again and can participate in electrolysis and permit electricty to flow.

The half equations for Zinc sulfate electrolysis are:

At the cathode: Zn2+(aq) +2e− ⟶ Zn(s) At the anode: 4OH− ⟶ 2H2O+O2 + 4e−

Electrons flow around the circuit from the anode to the cathode.

If you are building a circuit using this system you may choose to use either platimum or carbon electrodes as both are unreactive in the system. Additionally Zinc can be used for the cathode only as that is where it is being deposited.

Note that because Zinc is used in a number of cellular processes (DNA synthesis, transcription, etc.) the over production of the SmtA protein might be harmful to the organism by it limiting zinc availiability. This being said, SmtA has been used in a number of iGEM projects and is in the registry (BBa_K519010). It has previously been used in experiments for Cadmium (II) uptake, see [http://2011.igem.org/Team:Tokyo-NoKoGen/metallothionein Tokyo-NokoGen] 2011 with no ill effects.