Common ancestries, activities and structural determinants of a modular (bi-)polarization control system encoded in free-living and obligate intracellular α-proteobacteria, including the rickettsial pathogens, are described.

Diverse sophisticated phylogenetic analyses update the phylogeny of the Alphaproteobacteria and show that the parasitic Holosporales is a derived group within the Rhodospirillales order which comprises primarily free-living alphaproteobacteria.