The compatibility of D-amino acids with peptide elongation during translation has been examined in several studies. However, some of the studies have reported that D-amino acids are incompatible with translation, whereas others have reported that D-amino acids are incorporated into polypeptides. Here, we have reevaluated the incorporation of a series of D-amino acids into the nascent chain of short peptides with a reprogrammed genetic code by using the flexible in vitro translation (FIT) system. The FIT system enables the compatibility of each D-amino acid with elongation to be assessed quantitatively in the absence of potential competitors. The incorporation efficiencies were determined by Tricine-SDS-PAGE and the full-length peptide was detected by MALDI-TOF-MS. The D-amino acids were categorized into three groups based on their incorporation efficiencies relative to the corresponding L-amino acid. The D-isomers in group I showed efficiencies of 40% or higher (Ala, Ser, Cys, Met, Thr, His, Phe, and Tyr), and those in group II showed efficiencies of 10-40% (Asn, Gln, Val, and Leu). The D-amino acids in group III produced truncated peptides or no detectable full-length peptides (Arg, Lys, Asp, Glu, Ile, Trp, and Pro). When group I D-amino acids were used consecutively or were alternated with L-amino acids, this completely inhibited their elongation. However, when two or three L-amino acids were inserted between the D-amino acids, the double-incorporation efficiency was restored. Our results quantitatively reveal the compatibility of D-amino acids with peptide elongation and raise new questions about the mechanism of D-amino acid selection and incorporation by the ribosome.

The compatibility of β-amino acids with ribosomal translation was studied for decades, but it has been still unclear whether the ribosome can accept various β-amino acids, and whether the ribosome can introduce multiple β-amino acids in a peptide. In the present study, by using the Escherichia coli reconstituted cell-free translation system with a reprogramed genetic code, we screened β-amino acids that give high single incorporation efficiency and used them to synthesize peptides containing multiple β-amino acids. The experiments of single β-amino acid incorporation into a peptide revealed that 13 β-amino acids are compatible with ribosomal translation. Six of the tested β-amino acids (βhGly, l-βhAla, l-βhGln, l-βhPhg, l-βhMet, and d-βhPhg) showed high incorporation efficiencies, and seven (l-βhLeu, l-βhIle, l-βhAsn, l-βhPhe, l-βhLys, d-βhAla, and d-βhLeu) showed moderate incorporation efficiencies; whereas no full-length peptide was produced using other β-amino acids (l-βhPro, l-βhTrp, and l-βhGlu). Subsequent double-incorporation experiments using β-amino acids with high single incorporation efficiency revealed that elongation of peptides with successive β-amino acids is prohibited. Efficiency of the double-incorporation of the β-amino acids was restored by the insertion of Tyr or Ile between the two β-amino acids. On the basis of these experiments, we also designed mRNA sequences of peptides, and demonstrated the ribosomal synthesis of peptides containing different types of β-amino acids at multiple positions.

To combat SARS-CoV-2 and any unknown emerging pathogens in the future, the development of a rapid and effective method to generate high-affinity antibodies or antibody-like proteins is of critical importance. We here report a high-speed in vitro selection of multiple high-affinity antibody-like proteins against various targets including the SARS-CoV-2 spike protein. The sequences of monobodies against the SARS-CoV-2 spike protein were successfully procured within only four days. Furthermore, the obtained monobody efficiently captured SARS-CoV-2 particles from the nasal swab samples of patients and exhibited a high neutralizing activity against SARS-CoV-2 infection (IC50 = 0.5 nM). The high-speed in vitro selection of antibody-like proteins would be useful for the rapid development of a detection method and a neutralizing protein against a virus responsible for an ongoing, and possibly a future, pandemic.

We report the in vitro selection of thioether-macrocyclized peptides against vascular endothelial growth factor receptor 2 (VEGFR2) from multiple, highly diverse peptide libraries constructed utilizing genetic code reprogramming. The macrocyclic peptide libraries consisted of combinations of four types of amino acid linkers for cyclization and two types of elongator amino acid compositions, including four backbone-modified non-proteinogenic amino acids. Affinity selection from these libraries, using our recently developed TRAP (Transcription-translation coupled with Association of Puromycin-linker) display, yielded multiple anti-VEGFR2 macrocyclic peptide leads. Further antagonizing activity-based screening of the chemically synthesized lead peptides identified a potent macrocyclic peptide that inhibited VEGF-induced VEGFR2 autophosphorylation, proliferation, and angiogenesis of living vascular endothelial cells. The TRAP display-based selection from multiple, highly diverse peptide libraries followed by activity-based screening of selected peptides is a powerful strategy for discovering biologically active peptides targeted to various biomolecules.

Preventing the escape of hazardous genes from genetically modified organisms (GMOs) into the environment is one of the most important issues in biotechnology research. Various strategies were developed to create "genetic firewalls" that prevent the leakage of GMOs; however, they were not specially designed to prevent the escape of genes. To address this issue, we developed amino acid (AA)-swapped genetic codes orthogonal to the standard genetic code, namely SL (Ser and Leu were swapped) and SLA genetic codes (Ser, Leu, and Ala were swapped). From mRNAs encoded by the AA-swapped genetic codes, functional proteins were only synthesized in translation systems featuring the corresponding genetic codes. These results clearly demonstrated the orthogonality of the AA-swapped genetic codes against the standard genetic code and their potential to function as "genetic firewalls for genes". Furthermore, we propose "a codon-bypass strategy" to develop a GMO with an AA-swapped genetic code.