Polyadenylation creates the discriminator nucleotide of chicken mitochondrial tRNA(Tyr).

In the chicken mitochondrial genome, the gene for tRNA(Tyr) overlaps by one nucleotide with the downstream tRNA(Cys) gene, which is located on the same strand. The overlapping nucleotide, a guanosine residue, thus encodes both the discriminator base of the tRNA(Tyr) and the 5'base of the tRNA(Cys). When cDNA clones of circularized forms of the tRNA(Tyr) are analyzed, the discriminator nucleotide is an adenosine residue rather than the genomically encoded guanosine. Thus, the tRNA(Tyr) is subjected to an RNA editing activity similar to that shown to exist in the mitochondria of two other animal species. Interestingly, some cDNA clones have several adenosine residues at their 3'-ends instead of the expected CCA-sequence. Furthermore, a review of sequence data from animal mitochondrial genomes suggests that only tRNAs whose discriminator bases are adenosines tend to have genes that overlap with downstream genes. Thus, polyadenylation seems to be a major component of the RNA editing machinery that affects overlapping genes in animal mitochondria.