Some one-and-a-half-billion years ago, biologists believe, one of our single-celled ancestors engulfed a unique bacterium. Instead of becoming a meal, the swallowed microbe somehow managed to hang on inside that ancient cell and in its descendants, eventually evolving into a mitochondrion--the vital, energy-producing component now found in all animal and plant cells, as well as in fungi. There is plenty of evidence to back up the theory: mitochondria carry their own protein-coding DNA, and they divide by splitting in two, just like bacteria. Moreover, some mitochondrial genes and enzymes more closely resemble those of bacteria than those of their host organism. Now researchers have discovered what may be the theory’s clincher: the most bacterium-like mitochondrion ever found.
The ur-mitochondrion lives inside a unicellular mud-dwelling protozoan called Reclinomonas americana. Its circular string of DNA contains 62 protein-encoding genes, one-tenth the number found in any bacteria but far more than in any other known mitochondria. (Human mitochondria have only 13 genes.) Many of the Reclinomonas mitochondrial genes are typically found in mitochondria of other species, but 18 have never been seen in any mitochondrion. Some of those 18 are very similar to genes found in bacteria. The mitochondrion, then, seems to be a missing link between a free-living bacterial ancestor and its cell-bound, gene- lean descendants.
Among the mitochondrion’s bacteria-like features are four genes, each of which codes for parts of an enzyme called rna polymerase, which helps copy DNA into messenger rna, the molecule that directs protein synthesis. All other mitochondria lack these genes; instead they import an enzyme from the cell’s nucleus to do the job. But the Reclinomonas mitochondrion makes it from scratch, as do bacteria. Another gene not found in other mitochondria but common in bacteria codes for a protein that helps move proteins into the mitochondrion; a third bacteria-like enzyme aids in protein construction.
The discovery suggests that all mitochondria descend from one ancestor, says Michael Gray, a biochemist at Dalhousie University in Halifax, Nova Scotia, and one of the researchers who identified the new mitochondrion. If, eons ago, many different kinds of bacteria had been engulfed and gone on to become mitochondria, biologists would expect to find far greater variation in mitochondrial genes than what they observe today. The gene content and organization are similar enough between Reclinomonas and other mitochondrial DNA, says Gray, that it’s clear that they had to have come from a common ancestor.