Abstract
The essential feature of the genetic code is the strict one-to-one correspondence between codons and amino acids. The canonical code consists of three stop codons and 61 sense codons that encode 20% of the amino acid repertoire observed in nature. It was originally designated as immutable and universal due to its conservation in most organisms, but sequencing of genes from the human mitochondrial genomes revealed deviations in codon assignments. Since then, alternative codes have been reported in both nuclear and mitochondrial genomes and genetic code engineering has become an important research field. Here, we review the most recent concepts arising from the study of natural non-standard genetic codes with special emphasis on codon re-assignment strategies that are relevant to engineering genetic code in the laboratory. Recent tools for synthetic biology and current attempts to engineer new codes for incorporation of non-standard amino acids are also reviewed in this article.
Highlights
The genetic code maps 64 codons onto a set of 20 amino acids plus the translational stop signal [1]
Two non-canonical amino acids are naturally incorporated into the genetic code, namely selenocysteine, which is inserted at specific UGA sites in a wide range of prokaryotes and eukaryotes [9,10] and pyrrolysine in the archeon Methanosarcina barkeri at selected
Incorporation of non-canonical amino acids, in particular, the isostructural ncAAs which are recognized by the endogenous host cell machinery, has been possible by replacement of canonical amino acids using a supplementation-based incorporation method (SPI)
Summary
The genetic code maps 64 codons onto a set of 20 amino acids plus the translational stop signal [1]. The genetic code is almost universal, 34 alterations in nuclear and organellar genomes (Table 1) from bacterial to eukaryotic species have been discovered [3] The majority of these codon reassignments involve sense to nonsense codon changes (or vice versa) and occur in mitochondria. Euplotes crassus tolerates the incorporation of two amino acids (selenocysteine and cysteine) at the UGA codon and the dual use of this codon can occur within the same gene [22]. These examples highlight high genetic code flexibility, but how natural variation in codon-amino acid assignments emerges and is selected as well as the consequences of engineering the genetic code remain unclear
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