= Elongator transfer RNA
An \b[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.<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.]{ref}
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.
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