Five Mutually Orthogonal tRNA Systems Enable Up to Five Distinct Non-Canonical Amino Acids in a Single Protein

This 2023 Nature Chemistry paper extended the Chin group's earlier work on orthogonal translation systems from three to five mutually orthogonal pyrrolysyl-tRNA synthetase/tRNA pairs. These are molecular machines that can operate simultaneously in a single cell without cross-reactivity, expanding the number of distinct non-canonical amino acids that can be incorporated into one molecule.

Each synthetase/tRNA pair functions like a dedicated adapter: it recognises one specific ncAA from the growth medium and loads it onto a specific tRNA that reads a specific codon. For multiple pairs to work in the same cell, each pair must ignore the others' ncAAs, tRNAs, and codons. The engineering challenge scales combinatorially: adding a fourth pair means it must be orthogonal to three others; a fifth must be orthogonal to four.

The team achieved this by systematically engineering PylRS/tRNA pairs from five distinct archaeal species, selecting variants with high activity for their target ncAAs and negligible activity for the others. The quintuply orthogonal system was validated by demonstrating simultaneous, independent ncAA incorporation at five distinct positions.

Constructive Bio's current manufacturing platform incorporates up to three different ncAAs per molecule, already beyond what solid-phase peptide synthesis can practically achieve. This work expands the theoretical ceiling for molecular complexity. Five orthogonal pairs mean five different chemical modifications can be independently specified within a single peptide or protein: conjugation handles, stability modifications, half-life extensions, targeting moieties, and functional groups for activity, each genetically encoded at a precise position.

This expanding toolkit of orthogonal pairs is foundational to the platform's long-term capability. As therapeutic programmes require increasingly complex molecular architectures, including multi-functional peptides, multi-payload conjugates, or molecules requiring combinations of backbone and side-chain modifications, the ability to encode more independent chemical functions in a single fermentation step becomes a compounding advantage.