Laurel Hamers in Science:
It’s one of the big mysteries of cell biology. Why do mitochondria—the oval-shaped structures that power our cells—have their own DNA, and why have they kept it when the cell itself has plenty of its own genetic material? A new study may have found an answer. Scientists think that mitochondria were once independent single-celled organisms until, more than a billion years ago, they were swallowed by larger cells. Instead of being digested, they settled down and developed a mutually beneficial relationship developed with their hosts that eventually enabled the rise of more complex life, like today’s plants and animals. Over the years, the mitochondrial genome has shrunk. The nucleus now harbors the vast majority of the cell’s genetic material—even genes that help the mitochondria function. In humans, for instance, the mitochondrial genome contains just 37 genes, versus the nucleus’s 20,000-plus. Over time, most mitochondrial genes have jumped into the nucleus. But if those genes are mobile, why have mitochondria retained any genes at all, especially considering that mutations in some of those genes can cause rare but crippling diseases that gradually destroy patients’ brains, livers, hearts, and other key organs. Scientists have tossed around some ideas, but there haven't been hard data to pick one over another.
…Mitochondria make energy through a series of chemical reactions that pass electrons along a membrane. Key to this process is a series of protein complexes, large protein globs that embed in the internal membrane of the mitochondria. All of the mitochondria’s remaining genes help produce energy in some way. But the team found that a gene was more likely to stick around if it created a protein that was central to one of these complexes. Genes responsible for more peripheral energy-producing functions, meanwhile, were more likely to be outsourced to the nucleus, the group reports today in Cell Systems. “Keeping those genes locally in the mitochondria gives the cell a way to individually control mitochondria,” Johnston says, because pivotal proteins are created in the mitochondria themselves. That local control means the cell can more quickly and efficiently regulate energy production moment-to-moment in individual mitochondria, instead of having to make sweeping changes to the hundreds or thousands of mitochondria it contains. For instance, out-of-whack mitochondrion can be fixed individually rather than triggering a blanket, cell-wide response that might then throw something else off balance.