Fall 2002 -- Vol. 20, No. 1

Sequencing of Bacterial Genome Reveals Distant Evolutionary Link to Plants

A team of scientists have, for the first time, completely deciphered the genome sequence of a green-sulfur bacterium—a finding that may provide new insights into the evolution of photosynthesis and energy production in plant cells. The team, which includes Penn State’s Donald A. Bryant, the Ernest C. Pollard Professor of Biotechnology and a professor of biochemistry and molecular biology published its achievement in the Proceedings of the National Academy of Sciences. The report is the result of a collaboration between Bryant’s laboratory and The Institute for Genomic Research in Rockville, Maryland.

The bacterium, named Chlorobium tepidum, originally was discovered in New Zealand living in sulfur-rich, oxygen-depleted hot springs. “C. tepidum has a single chromosome that contains almost 2.2 million base pairs encoding a total of 2,288 genes,” Bryant explains. The genes involved in photosynthesis are of particular interest to scientists studying the evolution of plants because members of the phylum Chlorobia perform photosynthesis without evolving oxygen, a trait that sets them apart from the more familiar plants and cyanobacteria—organisms to which Chlorobia are not closely related. “All Chlorobia are adapted to perform photosynthesis in very-low-light environments and form unique light-harvesting structures called chlorosomes, each of which can contain up to 250,000 chlorophyll molecules,” Bryant says. The light energy harvested by chlorosomes is efficiently transferred to reaction centers that distantly resemble one of the two types found in plants and cyanobacteria. Chlorobia are also among the few organisms that oxidize sulfur compounds while converting carbon dioxide from the atmosphere into energy-rich biomolecules. “The results of this study may help to elucidate the evolution of photosynthesis, to define new biochemical pathways for energy production, and to understand better the global cycling of sulfur, nitrogen, and carbon by microorganisms,” Bryant says.

Barbara K. Kennedy

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