“…The outstanding level of primary sequence conservation within the methionine tRNAs, and as a consequence, the high conservation in secondary and tertiary structures of these tRNAs, make the tRNA Met family a unique case+ Interestingly, there is only one type of tRNA Met encoded in the mammalian mt DNAs, so that a single molecule has to serve both as initiator and elongator tRNA+ Thus, apart from being methionylated by methionyl-tRNA synthetase, this single tRNA must on one hand, be recognized by translation initiation factors and the mt methionyl-tRNA transformylase, and be able to read start codons of the mt mRNAs, and, on the other hand, the unformylated methionyl-tRNA must be specifically recognized by the elongation complex and be able to decode elongation codons+ This need for a relatively high number of different interactions undergone by mt tRNA Met , as compared to the other mt tRNAs, is likely the reason for their high degree of conservation+ Each interaction with a specific protein would require different conserved features+ The two conserved T50-T64 and Y51-N63 mismatches in the T-stem and a A5-R68 mismatch in the acceptor stem are features that have so far not been identified in any other tRNA, and are therefore prime candidates as signals for recognition (or against recognition) by either of the above mentioned proteins+ Among the different deviations from the universal genetic code found in mitochondria (Watanabe & Osawa, 1995) is the AUA codon, otherwise assigned to isoleucine, which has been taken over by methionine+ A unique posttranscriptional modification of C34 has been identified as 5-formylcytidine and is thought to be responsible for the unique decoding properties (Moriya et al+, 1994;Takemoto et al+, 1995)+ Because the mammalian initiation and elongation factors (Ma & Spremulli, 1995;Zhang & Spremulli, 1998) and the mt methionyl-tRNA transformylase (Takeuchi et al+, 1998) are known, the MetRS seems to be the only missing piece to solve these highly interesting mechanisms+…”