Mapping the Rickettsia prowazekii genome

Abstract: Rickettsia prowazekii is a member of alpha proteobacteria and the etiologic agent of epidemictyphus. Besides its pathogenicity as an obligate intracellular parasite it has been considered as amodel organism in a variety of studies.We have established a collection of overlapping clones, covering nearly the complete R.prowazekii genome. The clones were isolated from three genomic libraries in cosmid, lambdaphage and lambda-Zap vectors and arranged along the entire genome, using a bottom-upmethod. The ordered genomic libraries have served an important function in the sequencing ofthe entire genome and will be useful for further functional studies of the R. prowazekii genes.The sequencing and analysis of the entire R. prowazekii genome have provided evidencefor an exclusive obligate intracellular parasite with massive genomic reduction and a generalgenomic rearrangement in addition to an unusually high fraction (24%) of non-coding regions.There is evidence to suggest that the non-coding regions may be remnants of genes which areeliminated from the genome in an on-going process of genomic reduction. The genomicrearrangements are likely to have occurred through intra-chromosomal recombination events,mediated by repeated sequences in the ancestral genome which might have been deletedsubsequently. The rearrangement of the genome was evidenced in our earlier work whichdetected that the single copy of the 16S rRNA gene is located distantly from the linked 23S-5SrRNA genes in the R. prowazekii genome.Phylogenetic analysis based on a large number of R. prowazekii genes indicates that R.prowazekii is more closely related to mitochondria than any other microbial genome. Theobtained results are in accordance with the previously obtained data which was based onphylogentic analysis of rRNA genes. The partial similarities in gene composition between R.prowazekii and mitochondrial genomes is also additional evidence for this relationship.A comparative analysis between the complete genomes of the R. prowazekii and theChlamydia trachomatis, a distantly related obligate intracellular parasite, uncovered a total of325 genes with significant homology between the two genomes. A similar pattern of reductionin the corresponding functional categories is observed between these two genomes. Duringadaptation to their eukaryotic nutrient-rich environment, the two parasitic bacteria haveundergone a massive genomic loss and became more dependent on a host. In compensation,they have developed systems such as transporters that can exploit host cell provided nutrients.The comparative analysis has provided additional evidence for a convergent evolution duringadaptation to an intracellular life style in two distantly related bacteria.