Phylogenetic distribution of the euryarchaeal archaellum regulator EarA and complementation of a mutant with heterologous homologues.

Archaella are the swimming organelles in the Archaea. Recently, the first archaellum regulator in the Euryarchaeota, EarA, was identified in , one of the model organisms used for archaellum studies. EarA binds to 6 bp consensus sequences upstream of the promoter to activate the transcription of the operon, which encodes most of the proteins required for archaella synthesis. In this study, synteny analysis showed that homologues are widely distributed in the phylum of Euryarchaeota, with the notable exception of extreme halophiles. We classified Euryarchaeota species containing homologues into five classes based on the genomic location of the genes relative to and chemotaxis operons. EarA homologues from , and successfully complemented the function of EarA in a mutant, demonstrated by the restoration of FlaB2 expression in Western blot analysis and the appearance of archaella on the cell surface in complemented cells. Furthermore, the 6 bp consensus sequence was also found in the promoter region in these methanogens, indicating that the EarA homologues ly use a similar mechanism to activate transcription of the operons in their own hosts. Attempts to demonstrate complementation of the function of EarA in a mutant by the EarA homologue of were unsuccessful, despite the presence of a copy of the 6 bp consensus EarA-binding sequence upstream of the promoter in the genome.