Assembly of a complete genome sequence for Gemmata obscuriglobus reveals a novel prokaryotic rRNA operon gene architecture

Schlesneria sphaerica sp. nov., a neutrophilic, xylan-degrading planctomycete from a mountain wetland, and emended description of the genus Schlesneria

Aerobic chemoorganotrophic planctomycetes of the genus Schlesneria colonize a wide spectrum of freshwater ecosystems. The only described species of this genus, S. paludicola, is represented by ellipsoid-shaped, moderately acidophilic bacteria isolated from acidic peat bogs. Here, we characterize a novel neutrophilic member of this genus from a mountain wetland, strain T3-172T. This isolate possesses unpigmented, non-motile, spherical cells, which grow on media containing some mono- and polysaccharides in the pH range of 6.0-7.5 and at temperatures between 15 and 37°C. Good growth is observed on xylan, one of the major plant cell wall biopolymers. The genome of strain T3-172T is 7.16 Mb in size and contains two unlinked sets of rRNA genes, 93 tRNA genes, and about 5500 potential protein-coding genes. Among the latter, the GH5 family protein is identified as the most reliable candidate for the cleavage of xylan. The 16S rRNA gene sequence of strain T3-172T displays 97.8% similarity to that of S. paludicola MPL7T, and the average nucleotide identity between the genomes of these planctomycetes is 71.6%. We, therefore, propose a novel species of the genus Schlesneria, S. sphaerica sp. nov., with strain T3-172T (=KCTC 102306T = VKM B-3856T) as the type strain.

Paralogization and New Protein Architectures in Planctomycetes Bacteria with Complex Cell Structures

Bacteria of the phylum Planctomycetes have a unique cell plan with an elaborate intracellular membrane system, thereby resembling eukaryotic cells. The origin and evolution of these remarkable features is debated. To study the evolutionary genomics of bacteria with complex cell architectures, we have resequenced the 9.2-Mb genome of the model organism Gemmata obscuriglobus and sequenced the 10-Mb genome of G. massiliana Soil9, the 7.9-Mb genome of CJuql4, and the 6.7-Mb genome of Tuwongella immobilis, all of which belong to the family Gemmataceae. A gene flux analysis of the Planctomycetes revealed a massive emergence of novel protein families at multiple nodes within the Gemmataceae. The expanded protein families have unique multidomain architectures composed of domains that are characteristic of prokaryotes, such as the sigma factor domain of extracytoplasmic sigma factors, and domains that have proliferated in eukaryotes, such as the WD40, leucine-rich repeat, tetratricopeptide repeat and Ser/Thr kinase domains. Proteins with identifiable domains in the Gemmataceae have longer lengths and linkers than proteins in most other bacteria, and the analyses suggest that these traits were ancestrally present in the Planctomycetales. A broad comparison of protein length distribution profiles revealed an overlap between the longest proteins in prokaryotes and the shortest proteins in eukaryotes. We conclude that the many similarities between proteins in the Planctomycetales and the eukaryotes are due to convergent evolution and that there is no strict boundary between prokaryotes and eukaryotes with regard to features such as gene paralogy, protein length, and protein domain composition patterns.