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#initiator tRNA

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Translation in Prokaryotes and Eukaryotes

Initiator tRNA in eukaryotes carries

Eukaryotic cytosolic initiator tRNA carries unformylated methionine, distinguishing cytosolic machinery from bacterial and mitochondrial systems. Met-tRNAiMet is delivered by ternary complex eIF2 GTP Met-tRNAi to 40S P site within 43S pre-initiation complex during scanning initiation. Methionine remains first amino acid but often removed co-translationally by methionine aminopeptidases depending on second residue. Lack of formylation reflects distinct eIF2 recognition and insensitivity to initiator specific antibiotics, yet preserves N to C synthesis polarity and requirement for AUG start codon fidelity guided by Kozak context.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 7, Eukaryotic initiator Met-tRNAiMet without formylation

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Translation in Prokaryotes and Eukaryotes

Initiator tRNA in prokaryotes carries which amino acid?

In prokaryotes, initiator tRNA is first aminoacylated with methionine then formylated at alpha-amino group by methionyl-tRNA formyltransferase using formyl-tetrahydrofolate as donor, generating N-formyl-methionine. Formylation blocks free amino terminus ensuring it can only form peptide bond as donor at N-terminus, establishing directionality, and enhances binding to IF2 and P site with higher initiation efficiency. After nascent chain emergence, peptide deformylase removes formyl group and methionine aminopeptidase may excise methionine, revealing mature N-terminus for many cytosolic proteins. This refined regulation supports accurate ribosomal assembly, quality control and translational fidelity under diverse physiological conditions and growth states.

Ref: Watson Molecular Biology of the Gene, 7th ed., Chapter 15, Prokaryotic initiator fMet-tRNA and formylation by FMT

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