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Shrinking DNA. This Prochlorococcus ocean microbe has a much smaller genome than its close relatives.

The Incredible Shrinking Genome

The number of genes in one of the world's most prolific marine microbes is shrinking. By comparing the recently deciphered genomes of three closely related photosynthetic microbes, researchers have discovered that the youngest of these single-cell mariners, evolutionarily speaking, has much less DNA than its relatives. The work suggests that prolific organisms can adapt to new environments by getting rid of genes.

Free-floating Prochlorococcus are some of the most abundant photosynthetic organisms on Earth. They inhabit the oceans, particularly nutrient-poor waters, where they play a major role in fueling the food chain. In 2003, gene sequencers wrapped up their efforts on three species--one adapted to surface life with lots of light and two that prefer low light.

Since then, Frédéric Partensky, Alexis Dufresne, and Laurence Garczarek of the University of Paris in Roscoff, France have been comparing the sequences to learn the secrets of this marine organism's success, despite having relatively small genomes for a free-living microbe. For the analysis, they compared the three genomes, as well as one from another marine microbe. They then translated the genes into the corresponding proteins.

The Prochlorococcus genomes are quite different, says Partensky. The biggest is 2.41 million bases, which includes 2300 genes; the smallest bottoms out at 1.66 million bases and 1700 genes. All of the genomes share about 1300 genes. The smallest genome belongs to the surface-dwelling bug, which also evolved more recently than the other two, about 150 million years ago. In addition, its proteins are evolving much faster than in the others, Partensky reports in the 14 January Genome Biology. This suggests that evolution is going full-speed, making possible rapid adaptation to new circumstances. Partensky says natural selection seems to be selecting for smaller genomes in this microbe to economize on DNA production and save space in the cell—necessary requirements to live where nutrients are scarce.

The work is quite exciting, says Wolfgang Hess, an applied bioinformaticist at the University of Freiburg, Germany. "It's the first example of genome minimization for a free-living organism."

Related Sites
The study
Prochlorococcus biology and genome project
Partensky lab homepage