methioninetRNAiMet

Methionine tRNA iMet, commonly abbreviated as tRNAiMet, is a specialized initiator transfer RNA that delivers the first methionine to the ribosome during translation initiation in eukaryotes and archaea. It is distinct from the elongator methionyl tRNA (tRNAMet) used for internal coding and elongation of polypeptides.

Function and roles

tRNAiMet recognizes the start codon, typically AUG, and, in conjunction with initiation factors, occupies the P site of the 40S (or small) ribosomal subunit to establish the reading frame. This initiator tRNA must be selectively accepted by initiation factors and prevented from entering elongation until the start site is established. In bacteria, initiation uses a different system based on formylated methionine (fMet) tRNA, while eukaryotes and many archaea rely on tRNAiMet for initiation.

Biogenesis and charging

tRNAiMet is aminoacylated by methionyl-tRNA synthetase, which attaches methionine to its 3' terminus. The enzyme may recognize tRNAiMet alongside tRNAMet, depending on the organism. The initiator tRNA often bears unique structural or identity elements that promote recognition by initiation factors and proper placement on the ribosome. Post-transcriptional modifications contribute to its stability and functional identity.

Genetic encoding and distribution

In eukaryotes, tRNAiMet genes are encoded in the nuclear genome and the tRNA is processed and exported to the cytosol. In archaea, tRNAiMet is similarly used for initiation, with archaeal initiation factors partnering with the tRNA. Mitochondrial genomes may encode a dedicated initiator tRNA or rely on imported counterparts, depending on species.

Significance

tRNAiMet is essential for accurate start codon recognition and proper translation initiation, ensuring that methionine is the first residue of newly synthesized proteins in organisms that employ this initiation mechanism.