Jellyfish's glow reveals head's beginnings.

Four months ago, a fluorescent green worm appeared on the cover of SCIENCE. Columbia University's Martin Chalfie and Douglas C. Prasher, now at the U.S. Department of Agriculture at Otis Air National Guard Base in Massachusetts, had succeeded in turning a fluorescing jellyfish protein into a new kind of biological tracer. With it, they had lit up parts of the nervous system of Caenorhabditis elegans.

That report also lit up Columbia's switchboard. The university and Chalfie have received some 1,000 requests for the gene that codes for this marker, called green fluorescent protein (GFP).

This week, a second Columbia team used GFP to illuminate the movements of molecules that help eggs transform into complex organisms. "Everyone can see this technique is going to be amazingly useful," comments Edwin Stephenson, a developmental geneticist at the University of Alabama in Tuscaloosa.

In cells, DNA passes instructions for making proteins to RNA, a messenger molecule that travels from the nucleus to where the cell eventually manufactures the proteins. In the egg chambers of fruit flies, that RNA also moves from nourishing cells called nurse cells to the maturing egg, traveling through cytoplasmic pathways called ring canals, says Columbia's Tulle Hazelrigg.

The egg concentrates certain RNA molecules in specific places. There they wait until the embryo begins to develop. Then, those RNA messages call for the production of a protein called bicoid. A high concentration of bicoid in one part of the embryo activates the genes needed for development of the head and thorax, says Hazelrigg.

To get where they're supposed to go, the RNA molecules encoding the bicoid message seem to need the help of a protein called exuperantia (exu). Without exu, RNA disperses evenly throughout the egg rather than gathering near the anterior region, says Hazelrigg.

For their experiments, Columbia's Shengxian Wang and Hazelrigg joined the gene for GFP to the fruit fly gene that codes for the exu protein. They then inserted this hybrid gene into mutant fruit fly germ cells that lacked the exu gene. With the new gene, the germ cells developed eggs and nurse cells. As the nurse cells made exu, they also made fluorescing protein, lighting up this exu, the researchers report in the June 2 NATURE.

"Every molecule of exu is tagged," Hazelrigg says. Usually, scientists use tags attached to antibodies, molecules that home in on particular molecular targets. But antibodies miss some targets and may show up in places where no target exists, Hazelrigg says. Also, to use labeled antibodies, scientists must kill the cells. "We can see [GFP] in fixed and live cells," she adds.

In some experiments, Wang and Hazelrigg added a drug that disrupts microtubules, part of the cell's internal transport system. In those cells, exu failed to localize. The addition of a different drug led to the formation of new microtubules -- and subsequently an accumulation of glowing exu -- at those sites, they report. They suggest that microtubules are important to exu's movement to particular spots.

"It's really nice work because it gives a more detailed and higher-resolution picture of RNA localization mechanisms," says Stephenson. "The technique is going to allow people to look at processes in real time, in living organisms."

Scientists want to use this jellyfish protein in many organisms because it needs no helper enzymes to get it to glow, just blue light, says Chalfie.