Initiator transfer RNA

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] 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.