CODON REASSIGNMENT AND AMINO-ACID-COMPOSITION IN HEMICHORDATE MITOCHONDRIA

In the mitochondrial genome of the hemichordate Balanoglossus carnosus , the codon AAA, which is assigned to lysine in most metazoans but to asparagine in echinoderms, is absent, Furthermore, the lysine tRNA gen e carries an anticodon substitution that renders its gene product unab le to decode AAA codons, whereas the asparagine tRNA gene has not chan ged to encode a tRNA with the ability to recognize AAA codons, Thus, t he hemichordate mitochondrial genome can be regarded as an intermediat e in the process of reassignment of mitochondrial AAA codons, where mo st metazoans represent the ancestral situation and the echinoderms the derived situation, This lends support to the codon capture hypothesis , We also show that the reassignment of the AAA codon is associated wi th a reduction in the relative abundance of lysine residues in mitocho ndrial proteins.