Chemists at Emory University have reprogrammed bacteria to seek and degrade the herbicide atrazine (Nat. Chem. Biol., DOI: 10.1038/nchembio.369). Such bacteria could prove useful for bioremediation of atrazine, which is toxic to animals and possibly humans as well.
Justin P. Gallivan, Joy Sinha, and Samuel J. Reyes engineer Escherichia coli to produce RNA molecules called riboswitches that change conformation when they bind atrazine. The switching activates the translation of a protein called CheZ that allows the bacteria to move and chase atrazine in their surroundings. “The riboswitch acts like a molecular brake,” Gallivan says. “When you add atrazine, you release the brake and the cells can move.”
The usual way to find riboswitches is to start with an RNA that tightly binds the target. “You can imagine some RNAs that might bind atrazine very well but be incapable of undergoing a conformational change that allows you to get a change in gene expression,” Gallivan says. He and his coworkers screened a library of RNAs with moderate affinity for atrazine to find the best riboswitch. “By doing in vivo selection, we can find switches that work the way we intend,” he says.
The team also rewired the bacteria to produce an enzyme that converts atrazine to hydroxyatrazine, which does not act as an herbicide and is not as toxic.
The riboswitch binds atrazine but not its degradation product. “If you’re chasing something nasty in the environment, you want to break it down and keep looking for more of the nasty thing rather than get hung up on the product,” Gallivan says.
“This work represents a clever use of riboswitches to engineer E. coli cells to show a chemotactic response to the herbicide atrazine,” says Lawrence P. Wackett, a biochemist at the University of Minnesota, who identified the enzymes that degrade atrazine. “It is currently unclear what advantage this offers in bioremediation applications, but it may be useful as a methodology for selecting atrazine-metabolizing clones with differential activity.”