London, April 10 : Researchers have designed a bacterium in such a way that it can be used to search out pesticides, thus helping improve efforts to clean up the environment.
According to a report in Nature News, researchers hacked into the navigation system of the bacterium Escherichia coli, causing it to hunt down a widely used herbicide called atrazine.
Escherichia coli has receptor proteins on its cell surface that can identify chemicals of interest, enabling the bacterium to follow a chemical along its concentration gradient to its source.
The recognition information is passed along the cell, eventually triggering its whip-like tail, or flagellum, to rotate either one way to move forward or the other way to tumble randomly.
Engineered E. coli bacteria follow an S shape delineated by the molecule theophylline. But, this chain of events can be intercepted by tailoring the bacterium's RNA.
To do this, the researchers used a strain of E. coli that lacked the gene needed to move cells forward.
Using these modified cells, the group engineered a segment of RNA, or riboswitch, containing the gene. In the presence of atrazine, the switch was turned on, allowing the bacterium to move toward higher concentrations of the chemical.
Some bacteria can metabolize atrazine, creating another chemical by-product as waste. The team members showed that they could create a riboswitch that recognized atrazine but not the by-product.
"By incorporating genes from atrazine-eating bacteria, it should be fairly straightforward to engineer a bacterium that can seek out and destroy the pesticide," said Justin Gallivan of Emory University in Atlanta, Georgia.
The biggest advantage to using riboswitches may be the speed with which they can be identified.
"By contrast, re-engineering existing receptor proteins to become sensitive to a different molecule is time-intensive, depends on unpredictable molecular folding, and sometimes succeeds only in making a receptor sensitive to a wider range of molecules," said Gallivan.
But the number of possible riboswitch sequences is fairly small, and Gallivan's team can sort through all of them in a single experiment.
If there is a riboswitch that fits a small molecule of interest, Gallivan says, "you're reasonably certain you'll find it".
The best riboswitches can then be selected by looking for E. coli that move the most.
"The really cool thing about what he's doing is that he's able to get these switches quickly using this method," said Mark Goulian, a biophysicist at the University of Pennsylvania in Philadelphia.