|Strings of pearls: The genome sequence of Anabaena|
By Birgit Reinert
January 18, 2002
An international team of researchers has reported the complete genomic sequence of Anabaena, a cyanobacteriumor blue-green algae with beadlike cells. The organism can be found as plankton in shallow water and damp soil. Anabaena is valued for its nitrogen-fixing abilities and the researchers identified numerous genes involved in this biochemical process.
The sequencing project was started in February 1999 as a collaboration between Japanese researchers led by Takakazu Kaneko, of the Kazusa DNA Research Institute in Chiba, Japan, and C. Peter Wolk, of the Michigan State University in East Lansing.
The complete genome of Anabaena is 7.2 million base pairs long. The sequenced strain, known as Anabaena sp. strain PCC 7120, has a single chromosome of 6.4 million base pairs and six smaller DNA structures called plasmids. The researchers identified 5,368 genes in the chromosome, of which 45 percent could be assigned a function.
The analysis focused on the genes involved in nitrogen fixation and the formation of heterocysts, which are special cells where atmospheric nitrogen is converted into ammonia. "More than 60 genes involved in various processes of heterocyst formation and nitrogen fixation were assigned to the chromosome based on their similarity to the reported genes," Kaneko's team writes in Genome Research.
The researchers compared the sequencing data to a close relative of Anabaena, another cyanobacterium called Synechocystis whose genome was sequenced by Kaneko's team in 1996. To their surprise, only 37 percent of the Anabaena genes were similar to those of Synechocystis. "The results indicate that significant portions of the gene components in the genomes are unique to the species," the researchers write.
Some species of Anabaena, which can be found worldwide, have been successfully used to provide nitrogen to rice crops in flooded paddy fields. By increasing the nitrogen content of the soil, cyanobacteria provide an important source of natural fertilizer.
Sequence data of the complete Anabaena genome can be retrieved online at the Kazusa DNA Research Institute.
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