Everybody recognises the earthy smell of a field that's been freshly tilled, but how this smell forms has so far remained a mystery. A team at Brown University has now identified the bacterial genes responsible for the scent.
The smell of earth comes from a combination of two harmless chemicals called geosmin and methylisoborneol. Both belong to a class of compounds called terpenes and are synthesised by soil bacteria.
Last year, David Cane discovered the gene that helps make geosmin, but methylisoborneol remained elusive. Unlike the other terpenes, which all contain either 10 or 15 carbons, it contains 11.
Cane and his colleague Chieh-Mieh Wang scanned a database containing all 8000 genes from a soil bacterium called Streptomyces. They came across one that looked like it coded for a terpene catalyst, but when they inserted the gene into another bacterium, nothing happened – no terpenes were formed.
"We then noticed another gene, right next to the first one, which looked like it might code for a catalyst that adds a single carbon to chemical compounds," says Cane.
That was the "eureka" moment. Cane realised that, together, the two genes could theoretically produce an 11-carbon terpene – the elusive methylisoborneol. And indeed, when they genetically engineered an E. coli bacterium to express both genes, they obtained the scented compound.
Methylisoborneol is also responsible for the muddy smell that tap water sometimes takes on when reservoirs are invaded by blooms of blue-green algae. Cane believes these algae probably have both genes as well, and says that knowing their sequence could make it possible to detect the early signs of blooming, saving the millions of dollars that are sometimes spent on treating full-blown blooms.
Journal reference: Journal of the American Chemical Society (DOI: 10.1021/ja80803639g)
By General Disarray
Fri Jul 18 19:11:10 BST 2008
Soon there will be no mysteries left.By Allan Mckeown
Fri Jul 18 20:00:02 BST 2008
When I worked as a microbiology medical lab manager in Canada I kept a culture of Streptomyces on the bench to remind me of the "outdoors". Somewhat eccentric I suppose but helpful during times of stress etc!All comments should respect the New Scientist House Rules. If you think a particular comment breaks these rules then please use the "Report" link in that comment to report it to us.
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