Expanding the genetic code of Escherichia coli

A unique transfer RNA (tRNA)/aminoacyl-tRNA synthetase pair has been genera ted that expands the number of genetically encoded amino acids in Escherich ia coli, When introduced into E, coli, this pair leads to the in vivo incor poration of the synthetic amino acid O-methyl-L-tyrosine into protein in re sponse to an amber nonsense codon, The fidelity of translation is greater t han 99%, as determined by analysis of dihydrofolate reductase containing th e unnatural amino acid. This approach should provide a general method for i ncreasing the genetic repertoire of Living cells to include a variety of am ino acids with novel structural, chemical, and physical properties not foun d in the common 20 amino acids.