Textbooks brace for nuclear challenge.

New data threaten to shake up 30 years of scientific dogma regarding how a cell carries out one of its most basic tasks: the translation of the genetic code into proteins.

According to a study appearing in an upcoming SCIENCE, the cell's nucleus takes part in the task of manufacturing proteins. To date, researchers have thought that that process takes place only outside the nucleus, in the cytoplasm.

"It's certainly an unexpected finding, if true," says Joseph G. Gall of the Carnegie Institution of Washington in Baltimore.

Under any scenario, the nucleus provides the template for protein production. There, a single-stranded nucleic acid, RNA, is spooled from DNA. In the next step, enzyme machines called ribosomes latch onto the RNA molecules and translate them into proteins. Three decades' worth of scientific convention has placed that step outside the nucleus.

Ribosomes work by adding amino acid building blocks one by one into a chain. Peter R. Cook at Oxford University in England and his colleagues took advantage of this fact to visualize newly made proteins under a microscope.

They incubated cells in amino acids labeled with a molecular tag that made them glow. To their surprise, the researchers observed that about 15 percent of the glowing dots in the cell lit up within the nucleus. "I didn't believe it for 2 years," says Cook of his own results.

After additional experiments, he concluded that the dots in the nucleus indeed represent sites of new protein synthesis.

"The evidence is pretty good," says Thoru Pederson of the University of Massachusetts in Worcester. He expects that many scientists will question the new results. Chief among the criticisms will be the argument that the nuclear dots represent proteins initially made in the cytoplasm and then transported back into the nucleus.