Emergent rules for codon choice elucidated by editing rare arginine codons in Escherichia coli

The degeneracy of the genetic code allows nucleic acids to encode amino acid identity as well as non-coding information for gene regulation and genome maintenance. The rare arginine codons AGA and AGG (AGR) present a case study in codon choice, with AGRs encoding important transcriptional and translational properties distinct from the other synonymous alternatives (CGN). We created a strain of Escherichia coli with all 123 instances of AGR codons removed from all essential genes. We readily replaced 110 AGR codons with the synonymous CGU, but the remaining thirteen \"recalcitrant\" AGRs required diversification to identify viable alternatives. Successful replacement codons tended to conserve