THE GENOME SEQUENCE OF RICKETTSIA-PROWAZEKII AND THE ORIGIN OF MITOCHONDRIA

We describe here the complete genome sequence (1,111,523 base pairs) o f the obligate intracellular parasite Rickettsia prowarekii, the causa tive agent of epidemic typhus, This genome contains 834 protein-ooding genes. The functional profiles of these genes show similarities to th ose of mitochondrial genes: no genes required for anaerobic glycolysis are found in either R. prowazekii or mitochondrial genomes, but a com plete set of genes encoding components of the tricarboxyilc acid cycle and the respiratory-chain complex is found in R. prowarekii, In effec t, ATP production In Rickettsia Is the same as that in mitochondria. M any genes involved in the biosynthesis and regulation of biosynthesis of amino acids and nucleosides in free-living bacteria are absent fron t R. prowazekii and mitochondria, Such genes seem to have been replace d by homologues in the nuclear (host) genome, the R. prowarekii genome contains the highest proportion of non-coding DNA (24%) detected so f ar in a microbial genome, Such non-coding sequences may be degraded re mnants of 'neutralized' genes that await elimination from the genome, Phylogenetic analyses Indicate that R. prowazekii is more closely rela ted to mitochondria than is any other microbe studied so far.