68-Codon Genetic Code Enables Simultaneous Incorporation of Four Distinct Non-Canonical Amino Acids

Published in Nature Chemistry in 2021, this paper demonstrated that four distinct non-canonical amino acids can be simultaneously incorporated into a single protein using a 68-codon genetic code with automated orthogonal mRNA design.

This work sits between the triply orthogonal system (three ncAAs, published in 2020) and the quintuply orthogonal system (five ncAAs, published in 2023) in the progression of multi-ncAA incorporation capability from the Chin group. It addressed a specific technical challenge: as the number of freed codons and orthogonal tRNA/synthetase pairs increases, so does the complexity of designing mRNA sequences that are correctly decoded by all systems simultaneously. The team developed automated approaches to mRNA design that account for codon context effects and ensure each freed codon is read by the intended tRNA without ambiguity.

Four simultaneous ncAA positions means four independent chemical modifications in a single molecule. In therapeutic terms, this could mean a conjugation handle for drug attachment, a protease-resistant backbone modification, a lipidation site for half-life extension, and a fluorescent probe for tracking, all genetically specified in one fermentation step.

The automated mRNA design aspect is as significant as the four-ncAA result itself. Manual design of multi-ncAA expression constructs becomes impractical as the number of orthogonal systems increases, because each combination of freed codons creates constraints on the surrounding sequence context. Automation makes the system scalable: adding a fifth or sixth ncAA does not require proportionally more design effort.

This paper represents an intermediate step in expanding the chemical complexity accessible through Constructive Bio's platform, building toward the current capability of up to three different ncAAs per molecule in manufacturing and the longer-term potential demonstrated by the quintuply orthogonal system.