DEPENDING on your viewpoint it鈥檚 either the ultimate in cooperation or it鈥檚 slavery. The mitochondria that provide our cells with energy and the chloroplasts in plant cells that harness light energy were once free-living bacteria. After becoming symbiotic, hundreds of millions of years ago, they lost their ability to survive independently.
Now two species of bacteria that live in insects have been found in the process of transition from free-living organism to organelle. Their genomes are the smallest ever sequenced.
One of the bacteria, Carsonella ruddii, (see above) has the fewest genes of any cell known in the world 鈥� a mere 182, taking up about 160,000 base pairs of DNA. Humans have around 20,000 genes and 3 billion base pairs.
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C. ruddii has lost many basic functions, including the ability to replicate independently; the host cell now takes care of that. The bacterium pays for its keep, however, supplying its sap-eating insect host with amino acids, including leucine (Science, DOI: 10.1126/science.1134196). Given what C. ruddii has lost, 鈥渋t鈥檚 just amazing that it has anything extra, even for the host鈥�, says Nancy Moran of the University of Arizona in Tucson, who led the research.
The other bacterium that has been caught in the process of becoming symbiotic relies on the hospitality of an aphid. Buchnera aphidicola bacterium has 362 genes and a genome 420,000 bases long (Science, DOI: 10.1126/science1130441).
C. ruddii relies on its host so heavily that it could qualify as an organelle. Moran does not call it this, however, because it exists in only a subset of cells in the insect. Organelles are found in all cells.
The findings show how few genes an organism needs to get by, but only when it can rely on help from a host. Earlier this year Anthony Forster of Vanderbilt University Medical Center in Nashville, Tennessee, and George Church of Harvard Medical School calculated that the theoretical minimum number of genes a cell needs is 151. The new results dramatically change the concept of a minimal bacterial genome, Forster says, and has implications for understanding the function of so-called 鈥渏unk鈥� DNA, which doesn鈥檛 code for proteins.
鈥�C. ruddii has lost many basic functions, including the ability to replicate independently. The host cell takes care of that鈥�
鈥淭his organism contains almost no intergenic DNA, illustrating that this so-called 鈥榡unk DNA鈥� can be dispensed with,鈥� says Forster.
