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Light-harvesting bacterium C. tepidum is sequenced


Scientists have sequenced the complete genome of a green bacterium that lives in sulfuric environments and has an unusual capacity for generating energy from light. The organism, Chlorobium tepidum, has for years been a model species for researchers studying green-sulfur bacteria. Now, scientists are using its genome sequence to investigate the origin and mechanisms of photosynthesis and how these microbes might be used as alternative sources of energy.

An electron micrograph of the green sulfur bacterium Chlorobium tepidum.

"As a model species, Chlorobium tepidum has everything in one package," says Jonathan A. Eisen, who led the sequencing project at The Institute for Genomic Research (TIGR) in Rockville, Maryland. "The bacterium has a relatively small genome, it is easy to grow in the lab and has interesting biological systems for photosynthesis and processing nitrogen and sulfur."

Isolated from a hot spring in New Zealand, the bacterium carries out photosynthesis in ways that are different from plants and other bacteria. Unlike plants, the green bacteria do not produce oxygen from photosynthesis—they do not even like to grow in environments with oxygen.

Photosynthesis may have its evolutionary origins in organisms like C. tepidum, according to some researchers. Such species would have been able to harvest energy from light at a time when the Earth's atmosphere had little oxygen. In addition, the organisms' ability to grow in low-light environments may have helped them limit their exposure to UV irradiation, which was likely at higher levels in the early days of Earth.

The ability of C. tepidum to fix atmospheric nitrogen, and its reliance on sulfur compounds for its photosynthetic processes make this species an important model for understanding the role microbes play in global nutrient cycles.

"Sulfur and nitrogen are very important elements for biological systems," says Eisen. "They are both integral components of cells as well as the carriers of energy within cells and even between species. If we are going to find ways to produce biofuels in the future, we first need to better understand the diversity of ways that sulfur and nitrogen are processed in individual cells." These sorts of investigations can now go forward in the laboratory using the genetic tools and genomic information on C. tepidum.

The US Department of Energy funded the sequencing project as part of a larger effort to understand microbes that play a role in global energy and nutrient cycles. Chlorobium tepidum TLS is the first member of the green-sulfur group to be fully sequenced, and an analysis of the genome appears in Proceedings of the National Academy of Sciences.

The complete sequence of C. tepidum is available at the TIGR Website

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Eisen, J.A. et al. The complete genome sequence of Chlorobium tepidum TLS, a photosynthetic, anaerobic, green-sulfur bacterium. Proc Natl Acad Sci USA 99, 9509-9514 (July 9, 2002).

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