A versatile platform for single- and multiple-unnatural amino acid mutagenesis in Escherichia coli

To site-specifically incorporate an unnatural amino acid (UAA) into target proteins in Escherichia coli, we use a suppressor plasmid that provides an engineered suppressor tRNA and an aminoacyl-tRNA synthetase (aaRS) specific for the UAA of interest. The continuous drive to further improve UAA incorporation efficiency in E. coli has resulted in several generations of suppressor plasmids. Here we describe a new, highly efficient suppressor plasmid, pUltra, harboring a single copy each of the tRNA and aaRS expression cassettes that exhibits higher suppression activity than its predecessors. This system is able to efficiently incorporate up to three UAAs within the same protein at levels up to 30% of the level of wild-type protein expression. Its unique origin of replication (CloDF13) and antibiotic resistance marker (spectinomycin) allow pUltra to be used in conjunction with the previously reported pEVOL suppressor plasmid, each encoding a distinct tRNA/aaRS pair, to simultaneously insert two different UAAs into the same protein. We demonstrate the utility of this system by efficiently incorporating two bio-orthogonal UAAs containing keto and azido side chains into ketosteroid isomerase and subsequently derivatizing these amino acid residues with two distinct fluorophores, capable of Förster resonance energy transfer interaction. Finally, because of its minimal composition, two different tRNA/aaRS pairs were encoded in pUltra, allowing the generation of a single plasmid capable of dual suppression. The high suppression efficiency and the ability to harbor multiple tRNA/aaRS pairs make pUltra a useful system for conducting single- and multiple-UAA mutagenesis in E. coli.