Design, synthesis, and testing toward a 57-codon genome

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Science  19 Aug 2016:
Vol. 353, Issue 6301, pp. 819-822
DOI: 10.1126/science.aaf3639

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Recoding and repurposing genetic codons

By recoding bacterial genomes, it is possible to create organisms that can potentially synthesize products not commonly found in nature. By systematic replacement of seven codons with synonymous alternatives for all protein-coding genes, Ostrov et al. recoded the Escherichia coli genome. The number of codons in the E. coli genetic code was reduced from 64 to 57 by removing instances of the UAG stop codon and excising two arginine codons, two leucine codons, and two serine codons. Over 90% functionality was successfully retained. In 10 cases, reconstructed bacteria were not viable, but these few failures offered interesting insights into genome-design challenges and what is needed for a viable genome.

Science, this issue p. 819


Recoding—the repurposing of genetic codons—is a powerful strategy for enhancing genomes with functions not commonly found in nature. Here, we report computational design, synthesis, and progress toward assembly of a 3.97-megabase, 57-codon Escherichia coli genome in which all 62,214 instances of seven codons were replaced with synonymous alternatives across all protein-coding genes. We have validated 63% of recoded genes by individually testing 55 segments of 50 kilobases each. We observed that 91% of tested essential genes retained functionality with limited fitness effect. We demonstrate identification and correction of lethal design exceptions, only 13 of which were found in 2229 genes. This work underscores the feasibility of rewriting genomes and establishes a framework for large-scale design, assembly, troubleshooting, and phenotypic analysis of synthetic organisms.

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