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Science/Tech See other Science/Tech Articles Title: The sweet smell of testosterone, or not Smell is in the nose of the beholder as much as beauty is in the eye, according to a study released Sunday showing for the first time that variations in a single gene can determine whether a scent is perceived as fair or foul. It has long been known that smell and taste -- which are essentially the same thing -- are highly subjective. The fragrance that one person finds sublime could make someone else queasy, and one man's wine of the gods can be another man's plonk. A third person might not smell anything at all. But the exact mechanism accounting for these differences has remained largely a mystery, though genes were known to play a role. The US study, published online in the British journal Nature, fills in a piece of the puzzle. Subjects in a "smell survey" conducted by Leslie Vosshall at Rockefeller University in New York judged the intensity and pleasantness of dozens of odours, including a testosterone-derived steroid called androstenone found in human urine and sweat, especially in men. Most of the respondents said the compound smelled something like "stale urine". But a significant minority -- about 20 percent -- found the odour pleasing, saying it reminded them of vanilla or honey. Another research team at Duke University in North Carolina, meanwhile, had discovered in laboratory experiments led by Hiroaki Matsunami that androstenone switched on a human odour gene called OR7D4. Humans have several hundred functional odorant receptor genes, but olfactory sensory neurons in the nasal passage only express, or activate, them one at a time. In a collaborative effort, the two teams used DNA samples from each of the smell survey participants to sequence the gene that encodes the OR7D4 receptor. In some of the subjects, they discovered, the gene had undergone a slight mutation called single nucleotide polymorphisms (SNPs) in which a few of the basic, paired building blocks of DNA has changed. This gave rise to two variants of OR7D4. When they lined up the results of the DNA analysis with the subjective "smell survey," there was a very strong match between the two sets of groups: those who thought androstenone smelled like old cat pee had one variant of the gene, and those who smelled sweet vanilla had the other. "With this large data set, we are able to say that people who express different variants of this receptor perceive this odor differently," said the study's lead author, Andreas Keller. In another study released on Sunday, in Nature Chemical Biology, scientists at Brown University in the United States finally figured out why dirt smells like dirt. The structure of geosmin -- the chemical that yields the loamy smell of freshly-ploughed earth -- has been known for decades, but exactly how the micro-organisms that make it went their work remained obscure. David Cane and colleagues discovered that the Streptomyces coelicolor enzyme produces geosmin in a two step process rather than a single one, as previously thought. One half of this protein, they found, catalyses an intermediate chemical that is then transformed by the other half into the pungent chemical. Because geosmin can cause an undesirable musty smell in water, food and wine, understanding the biosynthesis of this earthy odour may help reduce these unwanted odours.
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