Progress toward Eukaryotic Semisynthetic Organisms: Translation of Unnatural Codons.

We have created a bacterial semisynthetic organism (SSO) that retains an unnatural base pair (UBP) in its DNA, transcribes it into mRNA and tRNA with cognate unnatural codons and anticodons, and after the tRNA is charged with a noncanonical amino acid synthesizes proteins containing the noncanonical amino acid. Here, we report the first progress toward the creation of eukaryotic SSOs. After demonstrating proof-of-concept with human HEK293 cells, we show that a variety of different unnatural codon-anticodon pairs can efficiently mediate the synthesis of unnatural proteins in CHO cells. Interestingly, we find that there are both similarities and significant differences between how the prokaryotic and eukaryotic ribosomes recognize the UBP, with the eukaryotic ribosome appearing more tolerant. The results represent the first progress toward eukaryotic SSOs and, in fact, suggest that such SSOs might be able to retain more unnatural information than their bacterial counterparts.