Abstract The genetic code is a highly conserved feature of life. However, some “alternative” genetic codes use reassigned stop codons to code for amino acids. Here, we survey stop codon recoding across bacteriophages (phages) in human and animal gut microbiomes. We find that stop codon recoding has evolved in diverse clades of phages predicted to infect hosts that use the standard code. We provide evidence for an evolutionary path towards recoding involving reduction in the frequency of TGA and TAG stop codons due to low GC content, followed by acquisition of suppressor tRNAs and the emergence of recoded stop codons in structural and lysis genes. In analyses of two distinct lineages of recoded virulent phages, we find that lysis-related genes are uniquely biased towards use of recoded stop codons. This convergence supports the inference that stop codon recoding is a strategy to regulate the expression of late stage genes and control lysis timing. Interestingly, we identified prophages with recoded stop codons integrated into genomes of bacteria that use standard code, and hypothesize that recoding may control the lytic-lysogenic switch. Alternative coding has evolved many times, often in closely related lineages, indicating that genetic code is plastic in bacteriophages and adaptive recoding can occur over very short evolutionary timescales.

Stop codon recoding is widespread in diverse phage lineages and has the potential to regulate translation of late stage and lytic genes