Evidence for Horizontal Gene Transfer in Evolution of Elongation Factor Tu in Enterococci

The elongation factor Tu (EF-Tu) is a GTP binding protein playing a central role in protein synthesis. It mediates the recognition and transport of aminoacyl-tRNAs and their positioning to the A site of the ribosome (20). The highly conserved function and ubiquitous distribution render the elongation factor a valuable phylogenetic marker among eubacteria and even throughout the archaebacterial and eukaryotic kingdoms (3, 31). The tuf genes encoding EF-Tu are present in various copy numbers per bacterial genome. Most gram-negative bacteria contain two tuf genes (5, 15, 19, 39, 41, 43). As found in Escherichia coli, the two genes, while being almost identical in sequence, are located in different parts of the bacterial chromosome (15, 20, 41). However, recently completed maps of microbial genomes revealed that only one tuf gene is found in Helicobacter pylori as well as in some obligate parasitic bacteria, such as Borrelia burgdorferi, Rickettsia prowazekii, and Treponema pallidum, and in some cyanobacteria (16, 18, 24, 32, 41, 44). In most gram-positive bacteria studied so far, only one tuf gene was found (8, 14, 17, 22, 28–30, 32, 35, 39). However, Southern hybridization showed that there are two tuf genes in some clostridia (39) as well as in Streptomyces coelicolor and Streptomyces lividans (46, 47). Up to three tuf-like genes have been identified in Streptomyces ramocissimus (48). Although massive prokaryotic gene transfer is suggested to be one of the factors responsible for the evolution of bacterial genomes (12, 27, 42), the genes encoding components of the translation machinery are thought to be highly conserved and difficult to transfer horizontally due to the complexity of their interactions (23). However, a few recent studies demonstrated evidence that horizontal gene transfer has also occurred in the evolution of some genes coding for the translation apparatus, namely, 16S rRNA and some aminoacyl-tRNA synthetases (6, 27, 45, 48, 49). No further data suggest that such a mechanism is involved in the evolution of the elongation factors. Previous studies concluded that the two copies of tuf genes in the genomes of some bacteria resulted from an ancient event of gene duplication (10, 39). Moreover, a study of the tuf gene in R. prowazekii suggested that intrachromosomal recombination has taken place in the evolution of the genome of this organism (41). To date, little is known about the tuf genes of enterococcal species. In this study, we analyzed partial sequences of tuf genes in 17 enterococcal species, namely, Enterococcus avium, E. casseliflavus, E. cecorum, E. columbae, E. dispar, E. durans, E. faecalis, E. faecium, E. gallinarum, E. hirae, E. malodoratus, E. mundtii, E. pseudoavium, E. raffinosus, E. saccharolyticus, E. solitarius, and E. sulfureus. We report here the presence of two divergent copies of tuf genes in 11 of these enterococcal species. The six other species carried a single tuf gene. The evolutionary implications are discussed. (This study was presented in part at the 100th General Meeting of the American Society for Microbiology, Los Angeles, Calif., 21 to 25 May 2000.)