The World's Smallest Genome Just Got Smaller

To make living organisms, a bundle of genes are needed. To make a human, for instance, you would need about 23,000. Now, scientists in Japan and the Unites States report new information about the minimum number of genes you need for life.

And the number is remarkably small.

Scientists used to think the number was around 300, but now researchers have found a bacterium that gets by with only 182 genes. The bacteria live inside tiny insects called psyllids. Researchers posit that the bacterium is in the evolutionary process of becoming a part of the insect.

Some of the smallest bacterial genomes are in bacteria that have what is called a symbiotic relationship with a host. The bacteria make something the host needs, and the host can take care of many of the bacteria's biological needs.

Nancy Moran, who studies these symbiotic bacteria at the University of Arizona, first got interested in a bacterium called Carsonella a few years ago.

"We did some sequencing to characterize what kind of bacterium it was," Moran says, "and found that it was really truly bizarre, it had really extreme sequence features when you sequenced the DNA."

To learn more about the bacterial genome, Moran sent samples of Carsonella to a genome sequencing center in Yokohama, Japan. The results she got back were astonishing. As Moran reports in the new issue of Science, Carsonella is made up of only 160,000 DNA letters. And even more stunning, it contained only 182 genes.

Moran says that prior to Carsonella, no one had seen an independent cellular organism with fewer than about 400 genes.

A symbiotic bacterium like Carsonella can afford to give up some genes; it can rely on the cells of its host to supply its needs. But giving up too many genes can give a bacterium an identity crisis.

"It no longer becomes independent on any level and becomes an integral part of the cell," says Patricia Johnson, who works on bacterial evolution at UCLA.

Johnson says most scientists believe that cells of higher organisms achieved their lofty status by enslaving bacteria that gave up a few too many genes. For example, our cells have something in them called mitochondria. Mitochondria are known as the power plants inside cells.

Johnson says mitochondria were once bacteria that lost too many genes.

"Of course, these events occurred a very, very long time ago," she says, "and what I think is extremely exciting about this new research is that we might be seeing this happen in real time."

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