RDX is a common military explosive, and it’s dangerous not just because it explodes - it’s also toxic. Places where RDX is used, produced, or stored often present a serious hazardous waste problem, such at the Massachusetts Military Reservation on Cape Cod, where the local aquifer has been contaminated with RDX. A group of researchers from the University of York in the UK and Canada’s Biotechnology Research Institute have shown how it might be possible to clean up RDX with explosives-eating transgenic plants.
Plants that use man-made explosives as a nitrogen source are not so easy to find in nature. In this case, the researchers genetically engineered two key bacterial proteins into the thale cress Arabidopsis thaliana. Plants that produce these proteins are able to take up RDX from soil and metabolize it. But how did these researchers find such useful bacterial proteins?
The answer is that they searched in the obvious place - contaminated soil where bacteria that metabolize explosives are likely to evolve. Bacteria reproduce extremely quickly and live in fairly large populations, thus it is not uncommon for a bacterium with just the right mutation to turn up and outcompete its peers in a new environment. The researchers found a bacterial strain of R. rhodochrous that was able to use RDX as its sole nitrogen source. Explosives are nitrogen-rich compounds, so this bacterial strain has evolved to take advantage of an abundant nutrient in its environment.
And it turns out that not many mutational changes were required to make this strain of R. rhodochrous an explosives-eater. All organisms contain many versions of a versatile class of electron-shuttling proteins called cytochrome P450s. This class of proteins is involved in some of the most sophisticated chemistry in the cell, including the synthesis of steroids and various vitamins. These cytochromes have been tweaked by evolution to perform an astounding range of chemical jobs. The explosives-degrading R. rhodochrous harbors a variant cytochrome P450 that is able to break an N-NO2 bond, something rarely encountered in nature but present in RDX.
Once they had the bacterial gene in hand, the researchers attempted to express it in the Arabidopsis plant. Since the plant itself has plenty of cytochrome P450s, it was easily able to properly synthesize the bacterial version in quantities large enough to start metabolizing RDX from the soil. This was a straightforward use of evolution and biochemistry to create a useful genetically engineered organism. It's possible that this approach could be successful a range of environmental problems. Given the chemical sophistication and diversity of the metabolic pathways found in nature, the chances of finding a solution to other pollution problems like this one are good.