Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 112, Issue 33, Pages 10224-10230Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1423979112
Keywords
endosymbiosis; spirochetes; single-cell genomics; adaptive evolution; metabolic interaction
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Funding
- grant for Precursory Research for Embryonic Science and Technology from the Japan Science and Technology Agency
- Japan Society for the Promotion of Science (JSPS) [19380055, 23117003, 26292047]
- fund for Next Generation World-Leading Researchers from JSPS
- Grants-in-Aid for Scientific Research [26292047, 19380055] Funding Source: KAKEN
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Symbiotic associations of cellulolytic eukaryotic protists and diverse bacteria are common in the gut microbial communities of termites. Besides cellulose degradation by the gut protists, reductive acetogenesis from H-2 plus CO2 and nitrogen fixation by gut bacteria play crucial roles in the host termites' nutrition by contributing to the energy demand of termites and supplying nitrogen poor in their diet, respectively. Fractionation of these activities and the identification of key genes from the gut community of the wood-feeding termite Hodotermopsis sjoestedti revealed that substantial activities in the gut-nearly 60% of reductive acetogenesis and almost exclusively for nitrogen fixation-were uniquely attributed to the endosymbiotic bacteria of the cellulolytic protist in the genus Eucomonympha. The rod-shaped endosymbionts were surprisingly identified as a spirochete species in the genus Treponema, which usually exhibits a characteristic spiral morphology. The endosymbionts likely use H-2 produced by the protist for these dual functions. Although H-2 is known to inhibit nitrogen fixation in some bacteria, it seemed to rather stimulate this important mutualistic process. In addition, the single-cell genome analyses revealed the endosymbiont's potentials of the utilization of sugars for its energy requirement, and of the biosynthesis of valuable nutrients such as amino acids from the fixed nitrogen. These metabolic interactions are suitable for the dual functions of the endosymbiont and reconcile its substantial contributions in the gut.
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