Orthogonal Replication System Enables Accelerated Directed Evolution of Proteins in Synthetic E. coli

Published in Science in 2024, this paper established a synthetic orthogonal replication system in E. coli: a parallel DNA replication machinery that operates independently of the cell's native genome replication, enabling targeted accelerated evolution of specific genes without destabilising the host organism.

The problem it addresses is speed. Engineering new aminoacyl-tRNA synthetases that recognise novel non-canonical amino acids typically involves laboratory-based directed evolution: building libraries of synthetase variants, screening for desired activity, and iterating. This process works, but each cycle takes days to weeks of bench work. An orthogonal replication system allows the cell itself to continuously diversify and evolve target genes in vivo, on its own replication timeline, while the rest of the genome remains stable and unaffected.

The team built a synthetic replication system using components that do not interact with E. coli's native DNA replication. Genes of interest are placed on a DNA element that replicates using this orthogonal machinery, which can be tuned to introduce mutations at elevated rates. Because only the orthogonal replicon mutates, not the host genome, the cell remains healthy while the target gene undergoes rapid diversification. Variants with improved function can be selected directly in the living cell.

For Constructive Bio, this technology accelerates the development of new orthogonal tRNA/synthetase pairs, the molecular components that determine which ncAAs can be incorporated, how efficiently, and with what specificity. Each new therapeutic programme may require synthetase variants optimised for a specific ncAA or combination of ncAAs. Orthogonal replication compresses the engineering timeline from months of iterative library screening to continuous in-cell evolution, enabling the platform to expand its ncAA repertoire faster and respond more rapidly to the chemical requirements of new drug candidates.