Recognition of tRNAs by methionyl-tRNA transformylase from mammalian mitochondria

Protein synthesis involves two methionine-isoaccepting tRNAs, an initiator and an elongator, In eubacteria, mitochondria, and chloroplasts, the additi on of a formyl group gives its full functional identity to initiator Met-tR NA(Met). In Escherichia coli, it has been shown that the specific action of methionyl-tRNA transformylase on Met-tRNA(f)(Met) mainly involves a set of nucleotides in the acceptor stem, particularly a C(1)A(72) mismatch. In an imal mitochondria, only one tRNA(Met) species has yet been described. It is admitted that this species can engage itself either in initiation or elong ation of translation, depending on the presence or absence of a formyl grou p. In the present study, we searched for the identity elements of tRNA(Met) that govern its formylation by bovine mitochondrial transformylase, The ma in conclusion is that the mitochondrial formylase preferentially recognizes the methionyl moiety of its tRNA substrate. Moreover, the relatively small importance of the tRNA acceptor stem in the recognition process accounts f or the protection against formylation of the mitochondrial tRNAs that share with tRNA(Met) an A(1)U(72) motif.