NEW YORK (GenomeWeb News) – A team of Japanese researchers reported today that they have sequenced the genome of a yet-uncultured bacterial endosymbiont that lives inside a protist called Pseudotrichonympha grassii, single-celled organisms found within the gut of termites.
In a paper scheduled to appear online today in Science, a team of researchers led by Moriya Ohkuma at Japan’s RIKEN Advanced Science Institute isolated a single P. grassii cell and then collected the bacteria inside, using whole-genome amplification to sequence the endosymbiont DNA. In so doing, they came up with the genome sequence for a bacterial species known as CfPt1-2, which accounts for an estimated 70 percent of the bacterial cells found in the termite Coptotermes formosanus.
“This highly evolved symbiotic system probably underlies the ability of the worldwide pest termite Coptotermes to use wood as its sole food,” Ohkuma and colleagues wrote.
C. formosanus, also known as the Formosan subterranean termite, causes hundreds of millions of dollars worth of damage each year in Japan and a billion dollars a year in the US. The termites typically occur in temperate and sub-tropical areas.
Because they can digest lignocellulose, termites are also of interest to those keen on developing biofuels from woody plant material. The termites’ ability to perform this feat seems to depend on symbiotic organisms, though these relationships are complicated and poorly understood.
Among the organisms in the termite’s gut, P. grassii cells help the termite break down wood particles. Each of these single-celled protists, in turn, contains roughly 100,000 CfPt1-2 cells, a bacterial phylotype in the Bacteroidales order.
For this paper, Ohkuma and his team used whole-genome amplification to sequence the 1,114,206 basepair genome CfPt1-2 — contained in a single circular chromosome — as well as four circular plasmids. Earlier this year, the team sequenced the genome of another termite gut bacteria, Rs-D17 using a similar approach.
The researchers determined that the CfPt1-2 genome houses 758 putative protein-coding genes, 38 transfer RNA genes, and four ribosomal RNA genes. Their subsequent analysis revealed that CfPt1-2 had more DNA repair and recombination genes than other intracellular symbionts that have been characterized to date.
When they turned their attention to metabolic pathways, the team uncovered several genes involved in nitrogen fixation. That was surprising, the authors noted, since no other species in this phyla have been shown capable of nitrogen fixation. Based on their subsequent analysis, they suggested that CfPt1-2 is likely responsible for the bulk of nitrogen fixation occurring in termite guts.
The researchers determined that CfPt1-2, which they dubbed Candidatus Azobacteroides pseudotrichonymphae, likely fixes nitrogen and recycles nitrogen wastes from its host in order to generate amino acids and cofactors, importing both glucose and xylose to use as carbon sources. That suggests that the bacteria can couple their nitrogen fixation and cellulolysis with the biological processes occurring in the protists’ cells.
“Endosymbionts of cellulolytic protists are probably required by their protist hosts for the biosynthesis of nitrogen compounds that are deficient in woody materials,” the authors concluded. “The cellulolytic protist with its N2-fixing endosymbionts, in turn, enables highly efficient growth of the host termite and its colony without being limited by nitrogen deficiency. The combined metabolic capacity of these organisms has allowed termites to take advantage of a nutrient-limited resource to humankind’s detriment.”