title: The Coxiella burnetii cryptic plasmid is enriched in genes encoding type IV secretion system substrates.
abstract: The intracellular bacterial pathogen Coxiella burnetii directs biogenesis of a phagolysosome-like parasitophorous vacuole (PV), in which it replicates. The organism encodes a Dot/Icm type IV secretion system (T4SS) predicted to deliver to the host cytosol effector proteins that mediate PV formation and other cellular events. All C. burnetii isolates carry a large, autonomously replicating plasmid or have chromosomally integrated plasmid-like sequences (IPS), suggesting that plasmid and IPS genes are critical for infection. Bioinformatic analyses revealed two candidate Dot/Icm substrates with eukaryotic-like motifs uniquely encoded by the QpH1 plasmid from the Nine Mile reference isolate. CpeC, containing an F-box domain, and CpeD, possessing kinesin-related and coiled-coil regions, were secreted by the closely related Legionella pneumophila Dot/Icm T4SS. An additional QpH1-specific gene, cpeE, situated in a predicted operon with cpeD, also encoded a secreted effector. Further screening revealed that three hypothetical proteins (CpeA, CpeB, and CpeF) encoded by all C. burnetii plasmids and IPS are Dot/Icm substrates. By use of new genetic tools, secretion of plasmid effectors by C. burnetii during host cell infection was confirmed using β-lactamase and adenylate cyclase translocation assays, and a C-terminal secretion signal was identified. When ectopically expressed in HeLa cells, plasmid effectors trafficked to different subcellular sites, including autophagosomes (CpeB), ubiquitin-rich compartments (CpeC), and the endoplasmic reticulum (CpeD). Collectively, these results suggest that C. burnetii plasmid-encoded T4SS substrates play important roles in subversion of host cell functions, providing a plausible explanation for the absolute maintenance of plasmid genes by this pathogen.
authors.affiliation: Department of Microbiology and Immunology, University of Arkansas for Medical Sciences, 4301 W. Markham Street, Little Rock, AR 72205, USA. dvoth@uams.edu
authors.forename: Daniel E
authors.initials: DE
authors.lastname: Voth
authors.forename: Paul A
authors.initials: PA
authors.lastname: Beare
authors.forename: Dale
authors.initials: D
authors.lastname: Howe
authors.forename: Uma M
authors.initials: UM
authors.lastname: Sharma
authors.forename: Georgios
authors.initials: G
authors.lastname: Samoilis
authors.forename: Diane C
authors.initials: DC
authors.lastname: Cockrell
authors.forename: Anders
authors.initials: A
authors.lastname: Omsland
authors.forename: Robert A
authors.initials: RA
authors.lastname: Heinzen
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D058947:Bacterial Secretion Systems / Q000235:genetics / Q000502:physiology*; D001483:Base Sequence; D002460:Cell Line; D016997:Coxiella burnetii / Q000235:genetics / Q000378:metabolism*; D004269:DNA, Bacterial / Q000235:genetics; D015964:Gene Expression Regulation, Bacterial / Q000502:physiology*; D006367:HeLa Cells; D006801:Humans; D010957:Plasmids / Q000235:genetics*; D021381:Protein Transport
chemical_list: D001426:Bacterial Proteins; D058947:Bacterial Secretion Systems; D004269:DNA, Bacterial