Mammalian mitochondrial methionyl-tRNA transformylase from bovine liver. Purification, characterization, and gene structure

The mammalian mitochondrial methionyl-tRNA transformylase (MTF(mt)) was partially purified 2,200-fold from bovine liver mitochondria using column chromatography. The polypeptide responsible for MTF(mt) activity was excised from a sodium dodecyl sulfate-polyacrylamide gel and the amino acid sequences of several peptides were determined. The cDNA encoding bovine MTF(mt) was obtained and its nucleotide sequence was determined. The deduced amino acid sequence of the mature form of MTF(mt) consists of 357 amino acid residues. This sequence is about 30% identical to the corresponding Escherichia coli and yeast mitochondrial MTFs. Kinetic parameters governing the formylation of various tRNAs were obtained. Bovine MTF(mt) formylates its homologous mitochondrial methionyl-tRNA and the E. coli initiator methionyl-tRNA (Met- tRNA(fMet)) with essentially equal efficiency. The E. coli elongator methionyl-tRNA (Met-tRNA(mMet)) was also formylated although with somewhat less favorable kinetics. These results suggest that the substrate specificity of MTF(mt) is not as rigid as that of the E. coli MTF which clearly discriminates between the bacterial initiator and elongator Met-tRNAs. These observations are discussed in terms of the presence of a single tRNA(Met) gene in mammalian mitochondria.