Of Fireflies and Tobacco Plants

It sounds like a child's riddle: What do you get when you cross a firefly with a tobacco plant? Answer: tobacco that lights itself. That is essentially what a team of scientists at the University of California at San Diego has done. By outfitting a fragment of a plant virus with the gene that tells firefly cells to produce a protein central to generating light, the researchers have created a plant that literally glows in the dark.

The technique, reported in last week's issue of the journal Science, is significant not so much as a demonstration of virtuoso genetic engineering, but because it will provide scientists with a valuable research tool for studying how genes go about their business. By fusing the firefly gene to the genetic material of other plants and animals, biologists gain a visual cue $ that will help them understand in detail how genes -- strands of DNA whose structure acts as a sort of coded instruction manual -- tell different cells what their duties are within an organism. Armed with such specific knowledge, researchers may someday understand exactly why these instructions are occasionally garbled and, perhaps, why cancer and other gene-influenced diseases occur. Predicts Stephen Howell, a plant molecular biologist and a member of the research team: "The scientific community will be able to exploit this tool for as many purposes as one can imagine."

In studying genes, scientists deal basically with two components: one part supplies the code for the production of a particular protein, and the other, a sort of regulatory switch, turns the protein-producing mechanism on and off. In the human body, as in all organisms, every cell contains the complete genetic code and, in theory, has the potential to serve any function. A liver cell has the instructions necessary to grow hair, for example, and a bone cell to transmit information as a nerve does. The reason these things do not happen is that the instructions -- the genes -- are switched on only under very specific conditions. If researchers can fuse the firefly gene to specific plant or animal genes, they will be able to monitor the "expression," or turning on, of those genes simply by looking at what parts of the organism light up, and when.

The initial impetus for the research came from a rather oblique direction. UCSD Biochemist Marlene DeLuca has been investigating for 20 years how the firefly protein -- in this case, an enzyme called luciferase -- produces light. But the process of collecting and grinding up fireflies to extract the enzyme was laborious and costly. She and Donald Helinski, a molecular geneticist, decided to isolate the luciferase gene, cloning exact copies of it and splicing it into the genetic machinery of the common bacterium E. coli. The E. coli could then massproduce luciferase by the vat. DeLuca and Helinski accomplished this task by using standard recombinant DNA techniques developed over the past 20 years and now widely employed in industrial microbiology laboratories.

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."