Tunable translational control using site-specific unnatural amino acid incorporation in Escherichia coli

Translation of target gene transcripts in Escherichia coli harboring UAG amber stop codons can be switched on by the amber-codon-specific incorporation of an exogenously supplied unnatural amino acid, 3-iodo-(L)-tyrosine. Here, we report that this translational switch can control the translational efficiency at any intermediate magnitude by adjustment of the 3-iodo-(L)-tyrosine concentration in the medium, as a tunable translational controller. The translational efficiency of a target gene reached maximum levels with 10(−5) M 3-iodo-(L)-tyrosine, and intermediate levels were observed with suboptimal concentrations (approximately spanning a 2-log(10) concentration range, 10(−7)–10(−5) M). Such intermediate-level expression was also confirmed in individual bacteria.