Amblyomma americanum (Lone star tick) is an important disease vector in the 
United States. It transmits several human pathogens, including the agents of 
human monocytic ehrlichiosis, tularemia, and southern tick-associated rash 
illness. Blood-feeding insects (Class Insecta) depend on bacterial endosymbionts 
to provide vitamins and cofactors that are scarce in blood. It is unclear how 
this deficiency is compensated in ticks (Class Arachnida) that feed exclusively 
on mammalian blood. A bacterium related to Coxiella burnetii, the agent of human 
Q fever, has been observed previously within cells of A. americanum. Eliminating 
this bacterium (CLEAA, Coxiella-like endosymbiont of A. americanum) with 
antibiotics reduced tick fecundity, indicating that it is an essential 
endosymbiont. In an effort to determine its role within this symbiosis, we 
sequenced the CLEAA genome. While highly reduced (656,901 bp) compared with C. 
burnetii (1,995,281 bp), the CLEAA genome encodes most major vitamin and 
cofactor biosynthesis pathways, implicating CLEAA as a vitamin provisioning 
endosymbiont. In contrast, CLEAA lacks any recognizable virulence genes, 
indicating that it is not a pathogen despite its presence in tick salivary 
glands. As both C. burnetii and numerous "Coxiella-like bacteria" have been 
reported from several species of ticks, we determined the evolutionary 
relationship between the two bacteria. Phylogeny estimation revealed that CLEAA 
is a close relative of C. burnetii, but was not derived from it. Our results are 
important for strategies geared toward controlling A. americanum and the 
pathogens it vectors, and also contribute novel information regarding the 
metabolic interdependencies of ticks and their nutrient-provisioning 
endosymbionts.