Comparative genomics of endomicrobial symbionts associated with distinct protist species in the termite gut
1Tokyo Institute of Technology Graduate School of Bioscience, 2The University of Sydney School of Biological Science, 3RIKEN CSRS BMEP Biomass Research Platform Team, 4RIKEN BRC-JCM
Various termite gut protists harbor endosymbiotic bacteria belonging to the class Endomicrobia in the phylum Elusimicrobia (formerly TG1). According to phylogenetic analysis based on 16S rRNA, it is assumed that these endomicrobial intracellular symbionts have been acquired independently in each protist family. We previously analyzed the complete genome of Candidatus Endomicrobium trichonymphae phylotype Rs-D17 and reported its reduced genome (1.1 Mb) that contains many pseudogenes. In addition, we also predicted functional role of Rs-D17 in the symbiotic system (Hongoh et al., 2008, PNAS).
In the present study, we reconstructed the complete genome sequence of another Rs-D17 genomovar, IZ. Additionally, we newly obtained draft genomes of Ca. E. trichonymphae phylotype HsjEm3 and two Endomicrobium species, each intracellularly and specifically harbored by different protist species in the termite gut. The predicted size of these three newly obtained genomes are comparable with that of Rs-D17 genomovars and the basic functions as symbionts, i.e., fermenting monosaccharides derived from the hydrolysis of lignocellulose by the host protist, and synthesizing amino acids and cofactors, appeared to be similar.
However, considerable differences existed in the variety of pseudogenes. For example, genes encoding important proteins, such as replication initiation factor (DnaA), 30S ribosomal protein S1 (RspA) and ammonium transporter (AmtB) have been pseudogenized only in the two genomovars of Rs-D17 but not in the other Endomicrobium species. We will report detailed comparison among these genomes of the dominant endosymbiotic group in the termite gut.
keywords:Comparative Genomics,Endosymbiosis,Genome reduction,Pseudogene,Evolution