A wide variety of non-canonical amino acids (ncAAs) can be incorporated into proteins through the coordinated action of a stop codon suppressing tRNA and aminoacyl-tRNA synthetase. However, methods to discover and characterize suppressor tRNAs are generally lacking. In this work, we show that cell-free systems can express functional suppressor tRNAs using endogenous machinery and characterize their activity. This method is compatible with widely used orthogonal tRNAs, such as the Methanocaldococus jannaschii tyrosyl tRNA, the Methanosarcina barkeri pyrrolysyl tRNA, the Methanomethylophilus Alvus pyrrolysyl tRNA, and an engineered Int pyrroysyl tRNA. Modifying the workflow to evaluate TAA suppression revealed that the M. jannaschii and M. alvus are highly functional TAA suppressors in cell-free systems. Finally, we show that we can express two distinct tRNAs simultaneously, enabling the incorporation of multiple, distinct ncAAs. In total, our work shows that cell-free systems are useful platforms to express and characterize tRNAs.
Non-canonical amino acids (ncAAs) can be incorporated into peptides and proteins to create new properties and functions. Site-specific ncAA incorporation is typically enabled by orthogonal translation systems comprising a stop codon suppressing tRNA (typically UAG), an aminoacyl-tRNA synthetase, and an ncAA of interest. Unfortunately, methods to discover and characterize suppressor tRNAs are limited because of laborious and time-consuming workflows in living cells. In this work, we develop anEscherichia coli crude extract-based cell-free gene expression system to rapidly express and characterize functional suppressor tRNAs. Our approach co-expresses orthogonal tRNAs using endogenous machinery alongside a stop-codon containing superfolder green fluorescent protein (sfGFP) reporter, which can be used as a simple read-out for suppression. As a model, we evaluate the UAG and UAA suppressing activity of several orthogonal tRNAs. Then, we demonstrate that co-transcription of two mutually orthogonal tRNAs can direct the incorporation of two unique ncAAs within a single modified sfGFP. Finally, we show that the cell-free workflow can be used to discover putative UAG-suppressor tRNAs found in metagenomic data, which are nonspecifically recognized by endogenous aminoacyl-tRNA synthetases. We anticipate that our cell-free system will accelerate the development of orthogonal translation systems for synthetic biology.
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