tRNAMet

tRNAMet is a transfer RNA molecule that carries the amino acid methionine to the ribosome during protein synthesis. It is charged with methionine by methionyl-tRNA synthetase and participates in translation by delivering Met to growing polypeptides. In most organisms, methionine is incorporated at start codons (AUG) and can also appear at internal AUG codons during elongation.

In bacteria, there are two distinct methionine tRNAs. The initiator tRNA, tRNAfMet, is formylated to formylmethionine (fMet) and is specialized for initiation, interacting with initiation factors and the ribosome at the start site. A separate elongator tRNAMet functions during elongation. In contrast, archaea and eukaryotes generally use a single methionine tRNA (tRNAMet) for both initiation and elongation; initiation employs a dedicated initiator form (tRNAiMet), but it is not formylated in these domains.

Structurally, tRNAMet shares the common tRNA cloverleaf fold and contains the anticodon CAU, which base-pairs with the start codon AUG and internal AUG codons. Its identity is shaped by specific sequence and structural elements recognized by methionyl-tRNA synthetase and, in initiation, by the relevant initiation factors and ribosome. Post-transcriptional chemical modifications further refine its function and ribosome recognition.

Genomic encoding varies: nuclear-encoded MET genes produce cytosolic tRNAMet in eukaryotes, while mitochondria and, in plants, chloroplasts may harbor organellar versions encoded by their own genomes. tRNAMet thus plays a central role in translating methionine-containing proteins across all domains of life.