Termites harbor a symbiotic gut microbial community that is responsible for their ability to thrive on recalcitrant plant matter. The community comprises diverse microorganisms, most of which are as yet uncultivable; the detailed symbiotic mechanism remains unclear. Here, we present the first complete genome sequence of a termite gut symbiont-an uncultured bacterium named Rs-D17 belonging to the candidate phylum Termite Group 1 (TG1). TG1 is a dominant group in termite guts, found as intracellular symbionts of various cellulolytic protists, without any physiological information. To acquire the complete genome sequence, we collected Rs-D17 cells from only a single host protist cell to minimize their genomic variation and performed isothermal whole-genome amplification. This strategy enabled us to reconstruct a circular chromosome (1,125,857 bp) encoding 761 putative protein-coding genes. The genome additionally contains 121 pseudogenes assigned to categories, such as cell wall biosynthesis, regulators, transporters, and defense mechanisms. Despite its apparent reductive evolution, the ability to synthesize 15 amino acids and various cofactors is retained, some of these genes having been duplicated. Considering that diverse termite-gut protists harbor TG1 bacteria, we suggest that this bacterial group plays a key role in the gut symbiotic system by stably supplying essential nitrogenous compounds deficient in lignocelluloses to their host protists and the termites. Our results provide a breakthrough to clarify the functions of and the interactions among the individual members of this multilayered symbiotic complex.gut bacteria ͉ insect ͉ phi29 ͉ symbiosis T he termite gut harbors 10 6 -10 8 microorganisms comprising Ͼ300 species of protists, bacteria, and archaea (1-5). These are mostly unique to termites and are essential, as a highly structured symbiotic community, for the host to survive on recalcitrant plant matter (1, 6-9). Although this symbiosis has long been attracting researchers for both basic and applied interests, the complexity and formidable unculturability of the gut microbiota have hampered clarification of the symbiotic mechanism. Among the as-yet-uncultivable gut symbionts, bacteria belonging to the candidate phylum Termite Group 1 (TG1) are common and often predominate in the gut microbial communities (8, 10, 11). Recently, these TG1 bacteria have been identified as intracellular symbionts of diverse cellulolytic gut protists (10, 12, 13). Because of its predominance, commonness, and specific localization confined to the gut protist cells, we assumed that the TG1 bacteria play a key role in the termite gut symbiotic system. However, TG1 is one of the Ϸ40 candidate phyla without isolated representatives (Fig. 1a) (5, 14), and no physiological information have been obtained thus far.In the present study, we aimed to acquire a complete genome sequence of the TG1 bacteria to clarify their functions, which will provide a breakthrough to disentangle the complicated symbiotic web. We targeted phylotype...

Termites harbor diverse symbiotic gut microorganisms, the majority of which are as yet uncultivable and their interrelationships unclear. Here, we present the complete genome sequence of the uncultured Bacteroidales endosymbiont of the cellulolytic protist Pseudotrichonympha grassii, which accounts for 70% of the bacterial cells in the gut of the termite Coptotermes formosanus. Functional annotation of the chromosome (1,114,206 base pairs) unveiled its ability to fix dinitrogen and recycle putative host nitrogen wastes for biosynthesis of diverse amino acids and cofactors, and import glucose and xylose as energy and carbon sources. Thus, nitrogen fixation and cellulolysis are coupled within the protist's cells. This highly evolved symbiotic system probably underlies the ability of the worldwide pest termites Coptotermes to use wood as their sole food.