The chemical structure of atrazine
The bacteria are controlled using riboswitches - little RNA pieces that can bind directly to a ligand (or signal molecule) and cause a change in gene expression, changes which can include switching on or off the genes involved in cell movement. Atrazine is a good molecule to start with because as well as being a relevant pollutant it also contains plenty of N-H bonds which are good for forming hydrogen-bond interactions with RNA. As well as that it has a well-characterized breakdown pathway, all components of which have been expressed successfully in E. coli.
The first stage in creating these seek and destroy bacteria was finding RNA sequences that would bind to atrazine. This was done by attaching the atrazine to a solid support and running bits of RNA past it, to see which ones would bind. They then took these successful binders and tested for riboswitch activity, i.e whether the binding to atrazine caused a conformational change in the RNA that lead to the turning on of a gene. They did this by putting a sequence complimentary to the isolated RNA upstream of the DNA for the CheZ gene, which controls motility in E. coli and then carrying out the selection process shown below:
In the absence of atrazine the CheZ is not synthesized and the bacteria stay where they are. When atrazine is added to the plate, the bacteria start to move...
Dose-dependent assays were then done on the successful RNA sequences, to characterise the reaction and check that it actually was the atrazine levels that lead to movement rather than some other confounding factor. Sequencing and examination of the binding site also helped to characterize the riboswitch and determine how it was working. The genes for atrazine-consuming ability were then added to the bacteria that moved towards the atrazine, leading to a little search-and destroy module capable of seeking out a dangerous pollutant and removing it from the environment.
Sinha J, Reyes SJ, & Gallivan JP (2010). Reprogramming bacteria to seek and destroy an herbicide. Nature chemical biology, 6 (6), 464-70 PMID: 20453864
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