tRNA – the Adapter Molecule

• Francis Crick postulated that an adapter molecule must exist, because amino acids have no structural speciality to directly read the genetic code on mRNA.
• The tRNA molecule acts as a vehicle that picks up amino acids scattered in the cytoplasm and reads specific codes of mRNA molecules - hence it is called the adapter molecule.
• tRNA was earlier called sRNA (soluble RNA); its adapter role was established after the genetic code was understood.

• Robert Holley proposed the two-dimensional cloverleaf model of tRNA secondary structure.
• He determined the first complete nucleotide sequence of alanine tRNA (1965), for which he was awarded the Nobel Prize.
• The secondary structure of tRNA based on his model looks like a cloverleaf.
• In its actual (3D) structure, tRNA is a compact molecule that looks like an inverted L.

Holley’s two-dimensional clover leaf model of transfer RNA

The cloverleaf model shows three main arms (with loops at their ends) and an additional variable loop:

• Four regions of double-stranded RNA are formed during the folding process.
• Modified bases are especially common in tRNA.

• The process of attaching an amino acid to tRNA is called aminoacylation or charging.
• The resultant product is a charged tRNA (aminoacyl-tRNA).
• This reaction is catalysed by the enzyme aminoacyl-tRNA synthetase.
• Aminoacylation is an endothermic reaction (requires energy).
• When two such charged tRNAs are brought together, peptide bond formation is energetically favoured.

• The tRNA charged with an amino acid serves as the adapter molecule to decode information on mRNA.
• This is achieved through the interaction of the anticodon on tRNA with the codon on mRNA (complementary base pairing).
• Wobbling between the anticodon and codon allows some tRNA molecules to read more than one codon.

t-RNA - the adapter molecule

• Each amino acid has at least one specific tRNA.
• There is a specific initiator tRNA used only for the initiation of translation.
• No tRNAs exist for stop codons.

• tRNA is the adapter molecule (Crick) that picks up amino acids and matches them to the correct mRNA codon.
• Robert Holley proposed the cloverleaf model (2D); in 3D, tRNA looks like an inverted L.
• It has three arms - DHU (binds aminoacyl-tRNA synthetase), anticodon (pairs with codon), TΨC (binds ribosome) - plus a 3′ CCA end for amino acid attachment.
• The amino acid is attached by aminoacyl-tRNA synthetase in a process called aminoacylation (charging), which needs energy.
• Each amino acid has a specific tRNA, with a special initiator tRNA for translation start; no tRNAs exist for stop codons.