A team of researchers studying the one-celled organism Tetrahymena has discovered a cellular structure that acts as a selective garbage disposal for unneeded DNA. The recently identified cellular component, dubbed a "dumposome," may prove to be an entirely new organelle. It's introduced in the cover story of the October 4 issue of Cell.
"The dumposome surrounds the DNA it's gobbling up, and then spits it out of the nucleus," says C. David Allis, the University of Rochester professor of biology who led the group of researchers. "It's basically a little garbage disposal for eating extra DNA."
Dumposomes appear only briefly during the life cycle of Tetrahymena. Over a five-hour period during reproduction, molecules of a structural protein known as Pdd1p coalesce to form about a dozen circular dumposomes -- each only one- thousandth of a millimeter in diameter -- on the inner edge of the nucleus.
"When Tetrahymena nuclei are examined during reproduction, they look like cereal bowls with floating Cheerios clinging to their inner edges," Allis says. "But these 'Cheerios' are actually dumposomes that are breaking down DNA within their middles."
"Other researchers had thought that the structure was just a developing nucleolus, which is the part of the nucleus that is a workbench for building ribosomes," adds James Smothers, a graduate student in Allis' lab. "We now know that the structure is unique with a distinct function -- which could make it an organelle in its own right."
Although it has only one cell, the mature Tetrahymena has two nuclei. One of these acts as a permanent genetic library; its DNA is left intact and is passed along to offspring. The second nucleus carries the DNA used by the organism during its own lifetime. The dumposomes break down 15 to 20 percent of this DNA during reproduction.