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GENETIC engineers have just got a new set of tools. One group of scientists
in the US has designed switches that can flick genes into and out of action,
while another group has devised timers that activate genes like clockwork.

Tools like these should shed light on how genes interact. They also open up
the possibility of building complex genetic 鈥渃ircuits鈥� to treat disease.

For more than a decade, biologists have had only two ways of activating
engineered genes. Either the gene is permanently active and produces a protein
all the time, or it is activated in response to some chemical that has to be
permanently present to keep the gene switched on. It has not been possible to
build from scratch a reliable gene switch with two stable states: on and
off.

This leaves molecular biologists with few design choices, says Tim Gardner, a
biomedical engineer at Boston University in Massachusetts. 鈥淚magine if you only
had electronic switches that were stuck on or that you had to keep pushing to
stay lit,鈥� he says.

But now Gardner and his Boston colleagues Charles Cantor and James Collins
have engineered a stable on-off switch into Escherichia colibacteria.
They took a pair of genes that have an antagonistic relationship
(see Diagram).
Gene A makes a 鈥渞epressor鈥� protein that binds to gene B and switches it off, and
the protein from gene B likewise switches off gene A. When one gene is active,
the other must be inactive.

The researchers then added a marker gene that would produce a glowing green
protein when gene A was active. This is the 鈥渙n鈥� state of their switch. To flip
the switch, they used chemicals or heat to temporarily inactivate the protein
that was repressing gene B until the opposite state was established.

The position of the switch turned out to be stable indefinitely. Both states
were constant for at least 20 hours, even though the bacteria divided about
every 40 minutes.

Meanwhile, Michael Elowitz and Stanislas Leibler, biophysicists at Princeton
University in New Jersey, have built a device that makes E. colimark
time. In their design, three genes form a cycle of repression. Gene A makes a
repressor that shuts off gene B. Gene B鈥檚 repressor shuts off gene C, which
encodes a repressor to shut off gene A.

This creates a cycle in which the three repressors trade dominance in the
cell. When gene A鈥檚 repressor is dominant it clamps down on gene B. Gene B can
no longer repress gene C which begins to dominate the cell and repress gene A.
Once gene A is shut down the repressor from B starts to take over the cell and
shut down gene C. Gene A then ramps up again and the cycle repeats.

The researchers followed this molecular merry-go-round by using the same
glowing green protein as a marker and linking its production to gene A. They
found that the cells glowed brightly on average every 160 minutes. And this
molecular 鈥渃lock鈥� kept ticking even as it was passed from generation to
generation during cell division.

However, it turned out that this synthetic clock lost time more quickly than
natural body clocks that keep track of daily rhythm. 鈥淭hat鈥檚 really
interesting,鈥� says Elowitz. 鈥淣ow that we鈥檝e built a clock, we can tinker with
it, and ask why natural clocks are so reliable.鈥� Indeed, all the researchers
feel that a benefit from their work will be a greater understanding of how
natural networks of genes and proteins work together and evolve to accomplish
tasks.

But both groups also foresee practical uses for their new tools. By making
many of them interact, they believe that complex genetic circuits could be built
and 鈥減rogrammed鈥� to treat diseases.

For example, in a diabetic patient, a circuit might control the precise
amount of insulin released in response to the concentration of sugar in the
blood. Another circuit might release painkilling proteins to a cancer patient
every few hours. 鈥淭his adds a whole new dimension to the way we can think about
genetic engineering,鈥� says Gardner.