An exceptional disposition of the elongation factor genes is observed in Rickettsia prowazekii, in which there is only one tuf gene, which is distant from the lone fus gene. In contrast, the closely related bacterium Agrobacterium tumefaciens has the normal bacterial arrangement of two tuf genes, of which one is tightly linked to the fus gene. Analysis of the flanking sequences of the single tuf gene in R. prowazekii shows that it is preceded by two of the four tRNA genes located in the 5' region of the Escherichia coli tufB gene and that it is followed by rpsJ as well as associated ribosomal protein genes, which in E. coli are located downstream of the tufA gene. The fus gene is located within the str operon and is followed by one tRNA gene as well as by the genes secE and nusG, which are located in the 3' region of tufB in E. coli. This atypical disposition of genes suggests that intrachromosomal recombination between duplicated tuf genes has contributed to the evolution of the unique genomic architecture of R. prowazekii.