In recent years humans have learned how detector molecules in neurons in our noses chemically detect odors. The detector proteins initiative a combination of additional chemical reactions, flows of ions through flood gates spanning the neuron membrane and an accompanying electrical Voltage spike. This electrical/chemical wave carries the signal along the surface of the long nerve axon to synapses in the brain where the information is processed, recorded, and sometimes responded to. (See investigations Touch & Hearing, Tasting & Smelling, Pain, Nerve Messaging, Contact by Nerve, Brain Chemistry and Memories for process details.)
Human noses aren't very sensitive compared to other creatures. So after the terrorist attacks on 9/11, dogs were trained to detect the faint odor of vapors from explosive compounds. But the process of training and using dogs to detect explosives remains relatively expensive. So Danny N. Dhanasekaran and his colleagues at Temple University set out to develop cheaper detectors: yeast cells.
First they tried to modify yeast so it had the chemistry of a nerve cell. So the team transplanted into the yeast DNA the portions of rat DNA which produce the large proteins embedded in the nerve membranes which perform key processes in rat noses. In a five-year effort, Dhanasekaran's team engineered into yeast the ability to manufacture 7 rat proteins required for sensing and producing a biochemical signal after exposure to an odor molecule. One of the small molecules released in abundance by these large neuron proteins when they detect an odor is cyclic adenosine monophosphate (cAMP, the DNA nucleotide often labeled as A, shown at left, contains the adenine also in ATP, the sugar ribose, but with only one phosphate group attached as the ring at the lower left).
Next they searched for a way that the yeast could communicate when they had detected a particular molecule. Activating the rat olfactory system leads to marked increased cellular levels of cyclic adenosine monophosphate (cAMP). So the researchers genetically engineered the yeast to make a green fluorescent protein whenever the cAMP concentration increased.
The final major challenge was to figure out a way to make the yeast cells detect the odor molecule. Once the yeast's transplanted olfaction system was working, the researchers screened 1,000 rat olfactory receptor proteins to find one that could specifically detect an explosive. They found a rat receptor protein which detects 2,4-dinitrotoluene (DNT), a byproduct of the manufacture of explosives with usually accompanies the explosive TNT, trinitrotoluene. They spliced the DNA segment for the DNT-detecting protein into the existing olfactory system in the yeast to complete the sensor. So when the yeast smells the TNT-associated compound, the cell makes cAMP and then turns a fluorescent green.
While this is not yet a perfected detection system, it overcomes the major challenges and demonstrates how simple organisms can be bioengineered to perform tasks where society has identified a need. Potential applications range from diagnosing pathologies associated with odors in body fluids of sick people to monitoring environmental or industrial processes. For this immediate task, the researchers still need to improve the yeast's sensitivity and specificity for odors in order for it to achieve its full potential.
A critical part of bioengineering is to determine what is desired, and the stepwise processes needed to achieve the desired end result.
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