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The UCSD team quickly realized that the successful harnessing of luciferase might yield other benefits. If the firefly gene was a simple, straightforward and easily manipulated one-gene-one-enzyme system (some enzymes require the cooperative efforts of several genes), it might be possible to use it as a $ marker, or "reporter," gene. "We lucked out," says Helinski. "It did turn out to be a single gene that we could manipulate."
They enlisted Howell and a colleague, David Ow, who began trying to package the gene in a way that could prove useful to the research of gene expression. The resulting procedure, though the simplest available, might have been designed by Rube Goldberg. The luciferase gene was spliced to the regulatory switch of a gene belonging to a virus that infects plants. The altered two- part piece of DNA was then inserted into a circular strand of DNA, called a plasmid, from the bacterium Agrobacterium. The bacterial plasmid was incubated with tobacco-leaf cells, and the cells were nurtured into full-fledged plants.
Why choose tobacco? Says Howell: "Tobacco is the laboratory rat of plant molecular biologists. It's a model system that we use in these sorts of experiments." Responding to orders from the firefly-virus gene, the plants dutifully produced their own luciferase.
The final step was to irrigate the plants with a solution containing luciferin, another substance found in fireflies, which must combine with luciferase, oxygen and adenosine triphosphate, a substance found in all cells, to produce the familiar luminescence. The plant's well-being is unaffected by the glow, which can be seen only with sensitive video equipment, photographic time exposures or eyes that have become accustomed to the dark.
Response from the scientific community is already enthusiastic; labs in the U.S., Europe and Asia, as well as several biotechnology companies, have requested samples of the tailored gene containing the firefly-virus DNA for use in their own research. Another UCSD team has taken this technique a step further, transferring the luciferase gene into monkey cells growing in laboratory culture.
Howell expects little outcry from anti-genetic-engineering activists about the plant experiments' danger to the environment. "At this time, this is only a laboratory creature. And plants don't fly or crawl across the floor or creep into mouseholes. You can set one down and be pretty sure that's where it's going to be when you look again."