= Initiator transfer RNA
An <initiator transfer RNA> is a specialized <transfer RNA> used to establish the start of <translation (biology)> 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.<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]{ref} 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.
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