Every living thing on Earth stores the instructions for life as DNA, using the four genetic bases A, G, C, and T. All except one, that is. In the San Diego laboratory of Floyd Romesberg—and at a startup he founded—grow bacteria with an expanded genetic code. They have two more letters, an “unnatural” pair he calls X and Y.
The bacterium is termed a “semi-synthetic” organism, since while it harbors an expanded alphabet, the rest of the cell hasn’t been changed. Even so, Peter Carr, a biological engineer at MIT’s Lincoln Laboratory, says it suggests that scientists are only beginning to learn how far life can be redesigned, a concept known as synthetic biology.
“We don’t know what the ultimate limits are on our ability to engineer living systems, and this paper helps show we’re not limited to four bases,” he says. “I think it’s pretty impressive.”
The practical payoff of an organism with a bigger genetic alphabet is that it has a bigger vocabulary—it can assemble proteins with components not normally found in nature. That could solve some tricky problems in medicinal chemistry, which is the art of shaping molecules so they do exactly what’s wanted in the body, and nothing that isn’t.
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