Syn57: E. coli Engineered with the Most Compressed Genetic Code Ever Created, Freeing Seven Codons for ncAA Incorporation

Published in Science in 2025, this paper describes the creation of an E. coli strain with a 57-codon genetic code, the most radically recoded organism reported to date, pushing genome engineering beyond the three-codon compression of Syn61 to free a total of seven codons for reassignment.

Where Syn61 removed three codons from the genome, Syn57 removes seven. This is not simply a quantitative extension; it is a qualitative leap in engineering complexity. Each additional codon removal requires the systematic replacement of all instances of that codon across the genome, thousands of individual changes, while ensuring that the resulting organism remains viable, grows well, and can serve as a manufacturing host. The team achieved this by building on the continuous genome synthesis (CGS) methods developed in their earlier work, progressively replacing genome sections with synthetic DNA encoding the compressed codon usage.

Seven freed codons translates directly into seven independent channels for ncAA incorporation. Constructive Bio's current BioForge platform operates with up to three different ncAAs per molecule, already well beyond what SPPS can achieve. Syn57 opens the path toward greater molecular complexity. Seven orthogonal ncAA positions in a single peptide or protein would enable combinations of functional modifications that are not currently accessible by other manufacturing methods: multiple conjugation chemistries, backbone and side-chain modifications, stability enhancements, and targeting moieties, all genetically encoded in a single fermentation step.

Syn57 also further strengthens the phage-resistance properties first demonstrated in Syn61. With seven codons removed from the host genome, the translational barrier to viral replication is substantially reinforced, as phages relying on these codons are unable to propagate.

This work represents the current frontier of genome recoding and the next generation of Constructive Bio's platform technology. It demonstrates that the synthetic biology approach to genetic code expansion is not limited to incremental improvements: the architecture of the genetic code can be progressively rewritten to serve therapeutic manufacturing.