Serine/threonine kinases and E2-ubiquitin conjugating enzymes in Planctomycetes: unexpected findings

The regulation of signal transduction by phosphorylation and ubiquitination is essential to ensure the correct behavior of eukaryotic cells. We searched for protein families involved in such signaling in several eukaryotic species and in a limited set of prokaryotes, where two members of the Planctomycetes phylum were included as they exhibit eukaryote-like features (Gemmata obscuriglobus and Pirellula staleyi). We identified sequences homologous to eukaryotic serine/threonine kinases (STKs) and E2-ubiquitin conjugating enzymes in the two Planctomycetes species. To extend these analyses to the Planctomycetes/Verrucomicrobia/Chlamydia super-phylum, we performed comparative analyses using domains from kinases, phosphatases and GTPases that serve as signaling signatures, and we analyzed their distributions. We found substantial differences in kinome densities with regards to other prokaryote clades and among the groups in the Planctomycetes/Verrucomicrobia/Chlamydia super-phylum. In addition, we identified the presence of classic eukaryotic E2-conjugating ubiquitin proteins in prokaryotes, these having previously believed to exist only in eukaryotes. Our phylogenetic analyses of the STKs signature domains and E2-enzymes suggest the existence of horizontal gene transfer.