The schematic illustrates the approach used by DeBenedictis et al. to evolve triplet-decoding tRNAs (left) into tRNAs with expanded anticodon loops (red dots) to decode quadruplet codons consisting of four nucleotide bases (N; middle), and ultimately into efficient quadruplet codon decoders (right). To test the efficiency of four-base translation, DeBenedictis et al. created reporter genes (such as luciferase) with a single base insertion (also referred to as +1 frameshift). The ability of a particular tRNA variant to read that +1-frameshift mutation as a four-base codon can be measured as a function of how much full length and active reporter protein (e.g., luciferase) is made in cells. DeBenedictis et al. then recorded the translation efficiency of tRNAs with simple mutations to expand the anticodon loop (middle, red dots). Next, tRNAs were evolved through various mutations into more effective quadruplet-decoding tRNAs (qtRNAs, purple dots, right). The work represents an important step towards engineering a quadruplet genetic code with 256 codons.
Image credit: Tarana Siddika, Ilka U. Heinemann, Patrick O’Donoghue (CC BY 4.0).