Elongation factor Tu (EF-Tu) plays an important role in protein biosynthesis and is susceptible to antibiotics in prokaryotes like Escherichia coli. In order to understand the primary structure of EF-Tu in the intracellular pathogenic bacterium Mycobacterium leprae, the gene (tuf gene) coding for this protein was cloned and sequenced. The gene contains a coding region of 1,188 bp with GUG as start codon. The deduced amino acid sequence has 396 amino acids with a molecular weight of 43.6 kDa. Putative GTP-binding sites are located at amino acid positions 19-24, 83-87, and 138-141. Comparison of M. leprae EF-Tu amino acid sequence with those of M. tuberculosis, Micrococcus luteus, E. coli, and Salmonella typhimurium reveals 74-95% homology. Mitochondrial EF-Tu of Saccharomyces cerevisiae (62%) and chloroplast EF-Tu of Arabidopsis thalina (65.6%) also show strong homology with that of M. leprae. In contrast, the EF-Tu of the archaebacterium Halobacterium marismoruti exhibits relatively less homology (36.7%). Southern hybridization of M. leprae tuf gene with genomic DNA of slow growing and fast growing mycobacteria and related species like Corynebacterium fascians and Nocardia asteroides suggests that the gene is highly conserved in these organisms.