The identities of stop codon reassignments support ancestral tRNA stop codon decoding activity as a facilitator of gene duplication and evolution of novel function

Stop codon reassignments are widely distributed in prokaryotic, eukaryotic and organellar genomes, but are remarkably convergent in terms of the stop codons and amino acids reassigned. Strikingly, the identities of stop codon reassignments are closely matched to the properties of naturally occurring nonsense suppressor (NONS) tRNAs, suggesting that pre-existing nonsense suppression in an ancestral tRNA facilitated the occurrence of stop codon reassignments. Here this idea is expanded, by exploring the mechanism by which the gene duplication of tRNAs has occurred, leading to the reassignment of stop codons. Two types of stop codon reassignment are identified: those that necessitate a tRNA gene duplication, and those that do not because a single tRNA can recognize the reassigned stop codon and the canonical codon(s) for the cognate amino acid. Where tRNA gene duplication has occurred, this implies a multi-functional ancestral NONS tRNA, followed by adaptive mutation in the anticodon of one of the gene duplicates to become complementary to the stop codon, constituting a clear example of escape from adaptive conflict. The best exemplar is the UAA + UAG → gln reassignment, which has occurred 9 times independently in a diverse range of genomes, and appears to reflect the widespread occurrence of naturally occurring nonsense suppression of the UAA + UAG stop codons by glutamine tRNAs. Consideration of pre-existing tRNA functionality and the mechanism of gene duplication provide new insights into the process of stop codon reassignment.