A simple structural feature is a major determinant of the identity of a transfer RNA.

Analysis of a series of mutants of an Escherichia coli alanine transfer RNA shows that substitution of a single G-U base pair in the acceptor helix eliminates aminoacylation with alanine in vivo and in vitro. Introduction of that base pair into the analogous position of a cysteine and a phenylalanine transfer RNA confers upon each the ability to be aminoacylated with alanine. Thus, as little as a single base pair can direct an amino acid to a specific transfer RNA.