Principle of the Biosensor Test

Many bacteria possess effective resistance mechanisms for the detoxification of arsenic ions. One relatively well studied system is that encoded by the ars operon found on the Escherichia coli plasmid R773. It involves three well known proteins: an arsenate reductase that reduces arsenate to arsenite, a membrane protein complex that removes arsenite from the interior of the cell to the outside and - last but not least - the arsenic sensing protein ArsR which also has a regulatory function.

From nature....

In the absence of arsenic ions, ArsR binds to a specific site on the bacterial DNA. It thereby prevents the synthesis of the arsenate reductase and the arsenite pump. When arsenic ions enter the cell, ArsR changes its habits immediately. It interacts with the arsenic compounds and dissociates from the DNA. As a consequence, ArsR no longer represses the formation of the two other proteins. The result: the arsenate reductase and the arsenic pump are produced in large amounts.

....to the laboratory

For the development of our arsenic biosensor, we did not need the arsenate reductase and the arsenic pump. But we took advantage of the biochemical capacities of the arsenic sensing protein ArsR. By genetic engineering, we coupled the gene coding for the ArsR protein, the DNA binding site for ArsR, and a reporter gene coding for an easily detectable protein, like luciferase, beta-galactosidase or green fluorescent protein. This construction was inserted into the host bacteria Escherichia coli DH5a and the biosensor was in principle already ready! In the presence of arsenic ions, the modified bacterial cells no longer activate the resistance mechanisms but now trigger expression of the reporter genes.