= Rickettsia
{c}
<Rickettsia> is a genus of intracellular <bacteria>, many associated with <arthropods>. Some <species> cause disease in <vertebrate> <hosts>, but a genus name does not identify one universal <vector>, disease or cellular mechanism. Dependence on <host> <cells> makes retained and lost metabolic functions especially informative.
The <genome> of <Rickettsia prowazekii> contains respiratory and citric-acid-cycle machinery alongside extensive biosynthetic losses. Comparisons with mitochondrial <gene> sets helped investigate the bacterial ancestry of <mitochondria>. <The genome sequence of Rickettsia prowazekii and the origin of mitochondria by Siv G. E. Andersson and others>[1998, abstract and comparative gene inventories: respiratory-chain and tricarboxylic-acid-cycle genes]{ref} Shared features and a reconstructed relationship are evidence about evolutionary history; they do not make a living rickettsial <species> the unchanged ancestor of every modern <mitochondrion>.
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