Yeast Sniffs Out Explosives

ANYONE WHO HAS BAKED bread or brewed beer knows the smell of yeast in action. Now, yeast has been engineered to do its own smelling—specifically, sniffing out the odor of explosives.

The new sensor is a yeast cell that detects 2,4-dinitrotoluene, or DNT, using rat olfactory proteins that have been inserted into its outer cell membrane. When the yeast smells the TNT-associated compound, the cell turns a fluorescent green (Nat. Chem. Biol., DOI: 10.1038/nchembio882).

Making sensors "from the receptors that animals use to detect odors is one of the most exciting avenues of sensory science," comments Randall R. Reed, a director at Johns Hopkins University's Center for Sensory Biology.

In a five-year effort, molecular biologist Danny N. Dhanasekaran and his colleagues at Temple University engineered into yeast seven rat proteins required for sensing and transducing a biochemical signal after exposure to an odorant. The next challenge was to figure out a way to make the yeast cells signal detection of the odorant, says Dhanasekaran.

Activating the rat olfactory system leads to increased cellular levels of a molecule called cyclic adenosine monophosphate (cAMP). So the researchers genetically engineered the yeast to make green fluorescent protein whenever the cAMP switch was turned on by odor sensing.

Once they knew the yeast's transplanted olfaction system was working, the researchers screened 1,000 rat olfactory receptor proteins to find one that could specifically detect DNT. Then they spliced the DNT-detecting protein segment into the existing olfactory circuit in yeast to complete the sensor.

"Any event related to the presence or production of a specific odorant can potentially be followed" with this system, comments Edith Pajot-Augy, a neurobiologist at the University of Orsay, in France. Potential applications range from diagnosing pathologies associated with odors in body fluids to monitoring environmental or industrial processes. But she cautions that the researchers would have to improve the system's sensitivity and specificity for odors in order for it to achieve its full potential.