Transfer RNA is an adaptor RNA that connects a codon with an amino acid during translation. Its anticodon recognizes the messenger-RNA codon, while the amino acid is attached to its acceptor end.
Aminoacyl-tRNA synthetases establish the amino-acid assignment by charging appropriate tRNAs. Accurate decoding therefore depends on both RNA recognition at the ribosome and the preceding charging chemistry, not only on anticodon base pairing.
An elongator transfer RNA is a transfer RNA used to supply an amino acid during extension of a growing protein chain. Charged as an aminoacyl-tRNA, it is delivered to a ribosomal A site, where its anticodon participates in recognition of a messenger RNA codon. The growing chain is transferred from the preceding peptidyl-tRNA to that incoming amino acid, forming a peptide bond. Subsequent ribosome movement advances the carriers. An initiator transfer RNA instead establishes the initial chain at a ribosomal P site.
This distinction concerns molecular function rather than merely which amino acid is attached. In bacteria, an elongator carrier for methionine and an initiating carrier can deliver the same underlying amino-acid species through different recognition pathways. Conversely, a transfer RNA with a matching anticodon still requires appropriate charging by an aminoacyl-tRNA synthetase; reading the intended codon does not independently verify the attached amino acid.
A revealing 1991 experiment altered the acceptor stem and anticodon of Escherichia coli initiator tRNA. Suppression of an internal amber stop codon tested elongator function. One mutant also initiated at an upstream UAG and suppressed a downstream UAG in the same reading frame. Thus initiation and elongation activities can coexist in an engineered carrier: the usual division of labor is enforced by recognition features and interactions, rather than being an unavoidable consequence of RNA chemistry.[ref]
Mutants of initiator tRNA that function both as initiators and elongators by Umesh Varshney and others. Abstract and pp. 18018–18024: acceptor-stem variants, amber suppression and dual-function mutant.
For a simple inventory, building a five-residue nascent chain requires one initiating delivery followed by four elongating deliveries and four peptide bonds. These are delivery events, not five permanently consumed RNA molecules: carriers can be released, recharged and reused. The count also says nothing about how many distinct tRNA sequences participate, since that depends on the codons and recognition system.
An initiator transfer RNA is a specialized transfer RNA used to establish the start of translation at a ribosomal P site. It delivers the first amino acid and participates in the initiation machinery, whereas an elongator transfer RNA supplies a subsequent residue during chain extension. Initiator identity depends on features of the RNA and its interactions, not only on the amino acid it carries.
In the canonical Escherichia coli system, the initiating carrier is charged with methionine and then carries formylmethionine. A 1987 experiment altered conserved G–C base pairs in its anticodon stem to those found in the elongator methionine tRNA. Researchers purified the variants, charged and formylated them, then compared initiation-factor binding, AUG-dependent P-site binding and protein synthesis.[ref]
Escherichia coli formylmethionine tRNA: mutations in GGGCCC sequence conserved in anticodon stem of initiator tRNAs affect initiation of protein synthesis and conformation of anticodon loop by B. L. Seong and U. L. RajBhandary. Abstract, engineered stem substitutions and separate functional assays
Testing these steps separately distinguishes a defect in preparing the carrier from a defect in its later use by the ribosome.For an original bookkeeping example, reaching a four-residue nascent chain requires one initiating amino-acid delivery and three subsequent elongation deliveries, forming three peptide bonds. Initiation is not four repeated uses of the initiating pathway. The tRNAs are molecular carriers that can be released and reused; the count describes delivery events, not permanent consumption of four RNA molecules.
The bacterial formylation example should not be turned into a definition of every initiator tRNA. The defining role is initiation, while the recognition machinery and chemical state of the delivered amino acid depend on the biological system.
A peptidyl-tRNA is a transfer RNA carrying the growing peptide chain through a covalent linkage at its acceptor end. During peptide-bond formation, that chain is transferred to the amino acid on another tRNA.
The ribosome then advances the tRNAs and messenger RNA through successive positions. Distinguishing peptidyl-tRNA from an aminoacyl-tRNA makes clear which carrier holds the existing chain and which supplies the next residue.
The three-nucleotide region of a transfer RNA that recognizes a corresponding codon through base pairing during biological translation.
A transfer RNA carrying an amino acid through an activated ester linkage. Aminoacyl-tRNA synthetase supplies the charging reaction; the linkage helps drive peptide-bond formation.
A transfer RNA charged with methionine. The initiator species has a distinct role in start-site decoding during biological translation.
