Protozoan machinery grinds parental genes.

Teenagers often wonder how they can circumvent the outdated commandments handed down by their parents. Lacking a conscience, one tiny protozoan resolves the issue forcefully, dissolving many of its parents' genes as it charts its own course, according to new research.

"We've found the protein machinery a protozoan uses to turn off, and get rid of, unwanted genes," says C. David Allis of the University of Rochester (N.Y.). He and his colleagues at Rochester and at the Fred Hutchinson Cancer Research Center in Seattle report in the Oct. 4 Cell their studies of tetrahymena, a relative of the paramecium that stars in high school biology classes.

Most single-celled creatures have one nucleus that contains all their genetic information, but tetrahymena has two. A micronucleus holds the creature's full set of genes, while a macronucleus has a restricted set that carries only essential living instructions.

When food is plentiful, tetrahymena operates under the command of the macronucleus and splits repeatedly into identical copies of itself. If stressed by lack of food, however, the micronucleus takes over and the protozoan starts hunting for a sex partner. Each individual wants to exchange genes to improve its chances of survival, says Allis.

When two famished tetrahymena encounter each other, their cell walls dissolve and the pair joins. Their micronuclei break apart and fuse into a larger nucleus. There, some genes exchange places, moving from a chromosome of one parent to a chromosome of the other. Finally, the nucleus splits into eight parts, four of which become macronuclei and four micronuclei. The nuclei are parceled out in such a way that the four offspring have identical genes.

Researchers knew that during this division, some unknown actor strips the new macronuclei of all genes related to sexual reproduction and simultaneously destroys the parental macronuclei. The new micronuclei emerge unscathed. To ferret out the culprit, Allis and his colleagues labeled the doomed genes with a fluorescent molecule.

"We looked for anything hanging around in the nuclei at this time," says Allis. What they found was a suspicious protein that attached to the tagged genes in the new cells and to all of the genes inside the parental macronuclei. The researchers suspect that the protein harbors another molecule, an enzyme that chews up DNA. It kills the old macronuclei and leaves the new macronuclei with only basic living instructions.

"The protein serves as master control switch, making regions of chromosomes inactive," says biologist Robert B. Dickson of Georgetown University Medical Center in Washington, D.C. "It's an important discovery for basic biology."

Tetrahymena belongs to a family of protozoans called ciliates, and biologists wonder if other members share this ruthless ability to dispose of parental genes.

Allis describes the newly discovered protein as a permanent cellular organ and calls it a "dumposome." That term isn't mentioned in the study, however, and his coauthors downplay the idea of a new organelle. In their view, the young cells shoot out the protein to clean up their new homes only briefly, during sexual reproduction.

The researchers are now creating a version of tetrahymena that is incapable of expressing the gene-wrecking protein and plan to see what happens to new generations that lack it. They think the mechanism of gene disposal might be related to control of cell death in higher animals. - D. Vergano