Outside of the mitochondria, tRNA^Lys _CUU is charged with lysine, its cognate amino acid. However, by changing the anticodon from C-U-U to C-A-U, the mutated tRNA is subsequently charged with methionine. To be able to track mutant tRNAs, an ³⁵S-radiolabelled methionine was used. This experiment demonstrated that aminoacylation was necessary for transport of tRNA into the mitochondria, although the identity of the amino acid that was charged onto the tRNA was less important.