The 2016 iGEM Toronto team has developed a synthetic biological sensor for detecting gold. It will take advantage of the natural transcriptional pathway which is induced by the gold-binding promoter, GolS. A reporter gene attached the promoter produces a colour change which will indicate the presence of gold. Furthermore, the entire construct has been removed from the rest of the cellular machinery, and is in fact paper based. This cell-free mechanism is cheaper and more convenient than traditional biosensing methods.
GolS is a member of the MerR family of transcriptional regulators. Members of the MerR family have a helix-turn-helix DNA binding domain on the N-terminus and similar effector binding domains on the C-terminus. There are a variety of inducers for the different MerR-family transcription factors, including oxidative stress, certain dyes, and metals. GolS in particular is sensitive to the gold ion, Au(I), at its C-terminal binding domain. However, it also has a tendency to bind to other metal ions, in particular Cu(I). We have thus developed several mutants which will increase gold-specificity: A113T and P118A.
Predicted model of GolS using Rosetta.
The GolS and reporter gene machinery was moved to a paper-based platform in place of a cellular one. Escheria coli, the cell expressing the protein, was lysed, and the extract was substituted with free amino acids and plasmid constructs which would help it retain protein function while on paper. Furthermore, this paper construct could be freeze-dried then stored for upwards of a year at room temperature.
To complement our sensor, we developed a smartphone application capable of quantifying color changes from the sensor. Our application uses advanced colormetric analytic algorithms to control for variations between images. Overall, this application is a convient, user-friendly method for translating color changes into quantitative data.