The complete genome sequence of Rickettsia prowazekii, the causative agent of epidemic typhus, has been determined. The genome is 1,111,523 base pairs long and contains 834 protein-coding genes. The functional profiles of these genes show similarities to those of mitochondrial genes, including a complete set of genes for the tricarboxylic acid cycle and respiratory-chain complexes. ATP production in Rickettsia is similar to that in mitochondria. Many genes involved in amino acid and nucleotide biosynthesis are absent in R. prowazekii and mitochondria, suggesting they have been replaced by homologues in the nuclear genome. The R. prowazekii genome has a high proportion of non-coding DNA (24%), which may be remnants of neutralized genes. Phylogenetic analyses indicate that R. prowazekii is more closely related to mitochondria than any other microbe studied. Rickettsia are α-proteobacteria that multiply in eukaryotic cells. R. prowazekii is the agent of epidemic, louse-borne typhus. The genome of R. prowazekii is small, containing only 1,111,523 base pairs. Its phylogenetic placement and other characteristics suggest it is a descendant of bacteria with larger genomes. The genome sequence indicates that the three features of R. prowazekii—its small size, intracellular lifestyle, and genetic simplification—may be related. These features are likely due to reductive evolution, where genetic information is lost as the organism becomes dependent on its host. The R. prowazekii genome contains a high proportion of non-coding DNA, which may be degraded remnants of genes that are no longer functional. The genome also shows signs of ongoing gene loss, with some genes becoming pseudogenes. The presence of non-coding DNA and pseudogenes suggests that R. prowazekii has undergone significant genetic simplification. The genome sequence supports the hypothesis that mitochondria may have originated from α-proteobacteria, with R. prowazekii being a close relative of the ancestor to mitochondria. The genome of R. prowazekii shares many features with mitochondria, including the presence of genes involved in ATP production and transport. The genome also shows similarities to the mitochondrial genome of Reclinomonas americana, suggesting a common evolutionary history. The genome of R. prowazekii has a reduced set of genes involved in DNA replication, repair, and recombination. It contains genes involved in homologous recombination, such as recA, recF, recJ, recN, and recR. The genome also contains genes involved in transcription and translation, including those for RNA polymerase and tRNA synthesis. The genome has a limited capacity for mismatch repair, with only mutL andThe complete genome sequence of Rickettsia prowazekii, the causative agent of epidemic typhus, has been determined. The genome is 1,111,523 base pairs long and contains 834 protein-coding genes. The functional profiles of these genes show similarities to those of mitochondrial genes, including a complete set of genes for the tricarboxylic acid cycle and respiratory-chain complexes. ATP production in Rickettsia is similar to that in mitochondria. Many genes involved in amino acid and nucleotide biosynthesis are absent in R. prowazekii and mitochondria, suggesting they have been replaced by homologues in the nuclear genome. The R. prowazekii genome has a high proportion of non-coding DNA (24%), which may be remnants of neutralized genes. Phylogenetic analyses indicate that R. prowazekii is more closely related to mitochondria than any other microbe studied. Rickettsia are α-proteobacteria that multiply in eukaryotic cells. R. prowazekii is the agent of epidemic, louse-borne typhus. The genome of R. prowazekii is small, containing only 1,111,523 base pairs. Its phylogenetic placement and other characteristics suggest it is a descendant of bacteria with larger genomes. The genome sequence indicates that the three features of R. prowazekii—its small size, intracellular lifestyle, and genetic simplification—may be related. These features are likely due to reductive evolution, where genetic information is lost as the organism becomes dependent on its host. The R. prowazekii genome contains a high proportion of non-coding DNA, which may be degraded remnants of genes that are no longer functional. The genome also shows signs of ongoing gene loss, with some genes becoming pseudogenes. The presence of non-coding DNA and pseudogenes suggests that R. prowazekii has undergone significant genetic simplification. The genome sequence supports the hypothesis that mitochondria may have originated from α-proteobacteria, with R. prowazekii being a close relative of the ancestor to mitochondria. The genome of R. prowazekii shares many features with mitochondria, including the presence of genes involved in ATP production and transport. The genome also shows similarities to the mitochondrial genome of Reclinomonas americana, suggesting a common evolutionary history. The genome of R. prowazekii has a reduced set of genes involved in DNA replication, repair, and recombination. It contains genes involved in homologous recombination, such as recA, recF, recJ, recN, and recR. The genome also contains genes involved in transcription and translation, including those for RNA polymerase and tRNA synthesis. The genome has a limited capacity for mismatch repair, with only mutL and