Abstract
Protein synthesis involves two methionine-isoaccepting tRNAs, an initiator and an elongator. In eubacteria, mitochondria, and chloroplasts, the addition of a formyl group gives its full functional identity to initiator Met-tRNA(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 animal mitochondria, only one tRNA(Met) species has yet been described. It is admitted that this species can engage itself either in initiation or elongation of translation, depending on the presence or absence of a formyl group. In the present study, we searched for the identity elements of tRNA(Met) that govern its formylation by bovine mitochondrial transformylase. The main 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 for the protection against formylation of the mitochondrial tRNAs that share with tRNA(Met) an A(1)U(72) motif.
Highlights
In protein biosynthesis, methionine is universally used as the starting amino acid, and a particular initiator methionine tRNA ensures initiation of translation
Influence of the Nucleotidic Composition of the Acceptor Stem of tRNA on the Formylation Reaction Catalyzed by FMTmt— The sequence of tRNAMet from bovine mitochondria is shown in Fig. 1, together with that of E. coli tRNAfMet
As previously reported [11], FMTmt can formylate E. coli elongator Met-tRNAm Met in vitro. The efficiency of this reaction is only 500-fold lower than that measured with E. coli initiator Met-tRNAfMet
Summary
Methionine is universally used as the starting amino acid, and a particular initiator methionine tRNA ensures initiation of translation. The rule of the occurrence of two distinct methionine tRNAs breaks down in the mitochondria of many animals, such as mammals and insects. Single tRNAMet, believed to participate in both the initiation and the elongation of protein biosynthesis, has yet been evidenced [4, 5]. The current working hypothesis assumes that this Met-tRNAMet molecule can either bind elongation factor Tu and further participate in chain elongation or undergo formylation through the action of mitochondrial methionyltRNAMet transformylase (FMTmt) and be directed to the initiation machinery. Competition between elongation factor Tu and FMT for the binding of Met-tRNAMet would ensure a correct balance between the elongator and initiator functions of mitochondrial tRNAMet
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