title: A Rickettsiales symbiont of amoebae with ancient features.
abstract: The Rickettsiae comprise intracellular bacterial symbionts and pathogens infecting diverse eukaryotes. Here, we provide a detailed characterization of 'Candidatus Jidaibacter acanthamoeba', a rickettsial symbiont of Acanthamoeba. The bacterium establishes the infection in its amoeba host within 2 h where it replicates within vacuoles. Higher bacterial loads and accelerated spread of infection at elevated temperatures were observed. The infection had a negative impact on host growth rate, although no increased levels of host cell lysis were seen. Phylogenomic analysis identified this bacterium as member of the Midichloriaceae. Its 2.4 Mb genome represents the largest among Rickettsiales and is characterized by a moderate degree of pseudogenization and a high coding density. We found an unusually large number of genes encoding proteins with eukaryotic-like domains such as ankyrins, leucine-rich repeats and tetratricopeptide repeats, which likely function in host interaction. There are a total of three divergent, independently acquired type IV secretion systems, and 35 flagellar genes representing the most complete set found in an obligate intracellular Alphaproteobacterium. The deeply branching phylogenetic position of 'Candidatus Jidaibacter acanthamoeba' together with its ancient features place it closely to the rickettsial ancestor and helps to better understand the transition from a free-living to an intracellular lifestyle.
authors.affiliation: Department of Microbiology and Ecosystem Science, University of Vienna, Althanstraße 14, Vienna, Austria.
authors.forename: Frederik
authors.initials: F
authors.lastname: Schulz
authors.affiliation: Department of Cell and Molecular Biology, Science for Life Laboratory, Uppsala University, Husargatan 3, Uppsala, Sweden.
authors.forename: Joran
authors.initials: J
authors.lastname: Martijn
authors.affiliation: Department of Microbiology and Ecosystem Science, University of Vienna, Althanstraße 14, Vienna, Austria.
authors.forename: Florian
authors.initials: F
authors.lastname: Wascher
authors.affiliation: Department of Microbiology and Ecosystem Science, University of Vienna, Althanstraße 14, Vienna, Austria.
authors.forename: Ilias
authors.initials: I
authors.lastname: Lagkouvardos
authors.affiliation: Department of Biology, University of Ljubljana, Večna pot 111, Ljubljana, Slovenia.
authors.forename: Rok
authors.initials: R
authors.lastname: Kostanjšek
authors.affiliation: Department of Cell and Molecular Biology, Science for Life Laboratory, Uppsala University, Husargatan 3, Uppsala, Sweden.
authors.forename: Thijs J G
authors.initials: TJ
authors.lastname: Ettema
authors.affiliation: Department of Microbiology and Ecosystem Science, University of Vienna, Althanstraße 14, Vienna, Austria.
authors.forename: Matthias
authors.initials: M
authors.lastname: Horn
mesh_terms: D000048:Acanthamoeba / Q000382:microbiology* / Q000502:physiology; D020561:Alphaproteobacteria / Q000145:classification / Q000235:genetics / Q000302:isolation & purification* / Q000502:physiology; D016680:Genome, Bacterial; D010802:Phylogeny; D013559:Symbiosis