In one system, Noel and his collaborators are comparing enzymes used by members of the nightshade (Solanaceae) family—which includes tobacco, tomatoes, potatoes, and eggplant—to produce compounds closely related to volatile terpene compounds (found throughout the plant kingdom) to ward off fungal infections. In one case, the amino acids that make up two enzyme “cousins” in Egyptian henbane and tobacco plants, which diverged from their common ancestor about 10 million years ago, are 80 percent identical. But a subtle difference in the enzymes' structures means that while one produces a compound that acts against a fungus in Egyptian henbane's habitat, the other produces a chemically distinct natural compound that defends tobacco against its own fungal menace.

The gene sequence of the two enzymes is so similar, Noel says, that “in a traditional sense we would say they're the same enzyme.” Yet, he and his colleagues have used structural analysis to zero in on nine amino acids, of the nearly 560 comprising each enzyme, that determine which antifungal agent is produced. “When we take these nine positions in the tobacco enzyme and change them to the nine amino acids found in the Egyptian henbane enzyme, the enzyme completely alters its characteristics,” Noel explains. “It makes the chemical that the Egyptian henbane makes, and it does so with the same efficiency. The reverse experiment also works nicely.”

The next step for Noel and his team is to use this information to discern the structure of the enzyme that must have existed millions of years ago in the plants' shared ancestor. “We don't have molecular fossils,” he says, “but with the information now available we might be able to re-create what the ancient enzyme looked like and more importantly, how it behaved.” They are proceeding by making more than 1,000 specific versions of each of the related enzymes, reflecting every possible combination of the tobacco and Egyptian henbane amino acids in the critical positions identified thus far. “We are also studying each of these variants structurally to see how the shape and dynamics change and together with assessing the cocktail of chemicals each produces,” he says.

Eventually, glimpses like these into the structural and biosynthetic history of enzymes of secondary metabolism may help Noel and other scientists alter biosynthetic pathways to create new pharmaceuticals or other compounds with desirable properties.

		Joe Noel's lab has encoded onto a single gene the entire set of enzymes that grape, peanut, and blueberry plants use to convert the common amino acid tyrosine into resveratrol—a compound particularly abundant in red wine that is known to dramatically enhance muscle tissue's ability to metabolize fat for energy. In collaboration with fellow HHMI investigator Ronald M. Evans, also at the Salk Institute for Biological Studies, Noel plans to create a mouse that expresses this “natural chemical factory” gene only in muscle tissue. Muscle cells in the mouse will then use the enzymes to transform a small amount of the tyrosine in the animal's diet to resveratrol, allowing the scientists to study how the newly-produced compound affects muscle without having to feed large amounts of it to the animal. Noel is undertaking a similar project with Salk colleague Fred H. Gage to create mice that express the chemical factory in the brain—overcoming the challenges of getting a compound consumed in the diet across the blood-brain barrier—to learn how resveratrol might slow aging by altering the behavior of neural stem cells. Experiments like these may eventually enable “genetically-encoded medicinal chemistry”—allowing scientists to target enzymes to specific tissues and cells in living animals, where they can create chemical variants from materials available in the diet.

	PAGE 2 OF 2	Back