methionyltRNA

Methionyl-tRNA, or methionyl-tRNA, is a transfer RNA (tRNA) molecule charged with the amino acid methionine. It serves as the carrier of methionine during protein synthesis and plays a central role in translation, both for initiating protein synthesis and for incorporating methionine at internal AUG codons during elongation.

Biogenesis and charging: Methionyl-tRNA is produced when methionyl-tRNA synthetase (MetRS) catalyzes the acylation of tRNA^Met with methionine, using ATP as an energy source. MetRS recognizes cytosolic, mitochondrial, or chloroplast tRNA^Met species in different organisms and adds methionine to the tRNA’s 3′-terminal adenosine. Some organisms possess distinct MetRS isoforms for different cellular compartments.

Initiation and elongation: In bacteria, the initiator tRNA is formylated after charging, producing formylmethionyl-tRNA (fMet-tRNA^fMet), which together with initiation factors starts translation at the start codon AUG. In contrast, eukaryotes and many organelles use a non-formylated methionyl-tRNA^Met (often referred to as tRNAi^Met for the initiator form) for initiation, recognized by initiation factors such as eIF2. During elongation, Met-tRNA^Met participates in ribosome decoding of AUG codons and is delivered to the ribosome by elongation factors (EF-Tu in bacteria or EF-1α in eukaryotes).

Localization and variation: Mitochondria and chloroplasts often encode their own tRNA^Met and MetRS or import these components from the cytosol. Post-translational processing may remove the initiator Met from some proteins, depending on the N-terminal sequence, though this is not universal.

Overall, methionyltRNA is essential for accurate initiation and for supplying methionine during protein synthesis across diverse domains of life.