Efficiencies of the incorporation of various nonnatural amino acids carrying aromatic side groups into streptavidin were examined. The aromatic amino acids were linked to a mixed dinucleotide, pdCpA, and the resulting aminoacyl pdCpAs were coupled with tRNAcccg(−CA) to afford chemically aminoacylated tRNAcccg's. Mutant streptavidin mRNA containing a CGGG 4 base codon at the Tyr83 site was prepared and added to an Escherichia coli in vitro translation system with the aminoacyl tRNAcccg. The expression of the full-length mutant streptavidins was confirmed by a Western blot analysis, and their biotin binding activity was examined by a dot blot analysis. The Western blot analysis indicated that the efficiencies of the incorporation were higher for aromatic groups with straight configurations than those with widely expanded or bend configurations. The incorporation efficiencies were also examined in a rabbit reticulocyte lysate. In the latter system, the efficiencies were markedly improved for nonnatural amino acids with large side groups such as pyrene and anthraquinone.

Here, we describe a novel reagentless fluorescent biosensor strategy based on the antigen-dependent removal of a quenching effect on a fluorophore attached to antibody domains. Using a cell-free translation-mediated position-specific protein labeling system, we found that an antibody single chain variable region (scFv) that had been fluorolabeled at the N-terminal region showed a significant antigen-dependent fluorescence enhancement. Investigation of the enhancement mechanism by mutagenesis of the carboxytetramethylrhodamine (TAMRA)-labeled anti-osteocalcin scFv showed that antigen-dependency was dependent on semiconserved tryptophan residues near the V(H)/V(L) interface. This suggested that the binding of the antigen led to the interruption of a quenching effect caused by the proximity of tryptophan residues to the linker-tagged fluorophore. Using TAMRA-scFv, many targets including peptides, proteins, and haptens including morphine-related drugs could be quantified. Similar or higher sensitivities to those observed in competitive ELISA were obtained, even in human plasma. Because of its versatility, this "quenchbody" is expected to have a range of applications, from in vitro diagnostics, to imaging of various targets in situ.

We designed and synthesized new, fluorescent, non-natural amino acids that emit fluorescence of wavelengths longer than 500 nm and are accepted by an Escherichia coli cell-free translation system. We synthesized p-aminophenylalanine derivatives linked with BODIPY fluorophores at the p-amino group and introduced them into streptavidin using the four-base codon CGGG in a cell-free translation system. Practically, the incorporation efficiency was high enough for BODIPYFL, BODIPY558 and BODIPY576. Next, we incorporated BODIPYFL-aminophenylalanine and BODIPY558-aminophenylalanine into different positions of calmodulin as a donor and acceptor pair for fluorescence resonance energy transfer (FRET) using two four-base codons. Fluorescence spectra and polarization measurements revealed that substantial FRET changes upon the binding of calmodulin-binding peptide occurred for the double-labeled calmodulins containing BODIPY558 at the N terminus and BODIPYFL at the Gly40, Phe99 and Leu112 positions. These results demonstrate the usefulness of FRET based on the position-specific double incorporation of fluorescent amino acids for analyzing conformational changes of proteins.

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