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Bacteria are terrific chemists, but they normally
synthesize only molecules they need for their own survival, says
Gallivan. His research team is interested in making bacteria
synthesize molecules that they would otherwise not make on their own,
resulting in molecules that may someday benefit humans. The Emory team
reasoned that if a bacterium needs a particular molecule to survive,
it has a strong incentive to help make it, so the goal was to make
bacteria depend on a molecule that they wouldn't normally need.
In their first major breakthrough, the Emory
researchers have coupled the life of a bacterium to the presence of
theophylline, a compound that is used to treat asthma, and is produced
by the breakdown of caffeine in both coffee and tea plants. One of the
reasons that coffee has a high level of caffeine is that in the plant,
caffeine is synthesized very quickly, but breaks down to theophylline
very slowly.
"We know that there is an enzyme that breaks
caffeine down into theophylline, but we don't know much about it,"
says Gallivan, an assistant professor of chemistry. "What we do know
is that it works very slowly. Ideally, we would like to speed it up a
bit so that we could create coffee plants that are low in caffeine.
That's where the bacteria come in. They now need the breakdown product
of the enzyme (theophylline) for survival, but they can't do much with
caffeine."
Gallivan says that the idea is to supply these
bacteria with caffeine, and give each bacterium a piece of DNA from
coffee plants that may encode the enzyme that will allow the bacterium
to convert the caffeine to the theophylline it needs to survive.
"At the end of the day, we will know that all of
the surviving bacteria have 'learned' to convert caffeine to
theophylline, and thus have the enzyme that we're interested in. We
can then learn about the enzyme and how it works," Gallivan says. "We
hope to use a process known as 'directed evolution' to help speed up
the enzyme to break down caffeine faster. Since the bacteria need
theophylline for their survival, they're partners in the whole process."
Eventually, the faster enzyme could be introduced into coffee plants
to produce decaffeinated coffee, he says.
To develop bacteria that are addicted to
theophylline, Gallivan and Desai used a piece of the genetic material
RNA, known as an aptamer, which was known to bind to theophylline
tightly. The remaining challenge was to couple this binding to a vital
function of the bacteria -- the production of a protein. To do this,
the Emory team created a new sequence of RNA known as a "riboswitch."
In bacteria, riboswitches normally recognize
essential molecules, such as vitamin B12, and switch the production of
proteins on or off. The Emory team created a synthetic riboswitch that
recognizes theophylline, and turns on the production of a protein
known as "cat" which allows the cells to survive in the presence of an
antibiotic known as chloramphenicol. Most bacteria die when exposed to
chloramphenicol. However, bacteria containing the synthetic riboswitch
survive when exposed to chloramphenicol as long as theophylline is
present because theophylline turns on the production of the "cat"
protein.
Gallivan says not to expect good-tasting, naturally
decaffeinated coffee anytime soon. "We're still at the earliest stages
of this work. There are many hurdles to overcome," he says. "As a
scientist, I'm excited about the future. As a caffeinated coffee
addict, part of me is not in a hurry to solve this one." |
