Biochemical and phylogenetic analyses of methionyl-tRNA synthetase isolated from a pathogenic microorganism, Mycobacterium tuberculosis

Mycobacterium tuberculosis methionyl-tRNA synthetase (MetRS) has been cloned and characterized. The protein contains class I signature sequences but lacks the Zn²⁺ binding motif and the C-terminal dimerization appendix that are found in MetRSs from several organisms including E. coli MetRS. Consistent with these features, the enzyme behaved as a monomer in a gel filtration chromatography and did not contain the bound Zn²⁺. Nonetheless, it was active to the tRNA(Met) of E. coli as determined by in vivo genetic complementation and in vitro reaction. Phylogenetic analysis separated the M. tuberculosis and E. coli MetRSs into prokaryote and eukaryote-archaea group, respectively. This result is consistent with the taxonomic locations of the organism but is an interesting contrast to the case of its paralogous protein, isoleucyl-tRNA synthetase, and suggests that the two enzymes evolved in separate idiosyncratic pathways.