Enlarging the amino acid set of Escherichia coli by infiltration of the valine coding pathway

Aminoacyl transfer RNA (tRNA) synthetases establish the rules of the geneti c code by catalyzing the aminoacylation of tRNAs. For some synthetases, acc uracy depends critically on an editing function at a site distinct from the aminoacylation site. Mutants of Escherichia coli that incorrectly charge t RNA(Val) with cysteine were selected after random mutagenesis of the whole chromosome. All mutations obtained were located in the editing site of valy l-tRNA synthetase. More than 20% of the valine in cellular proteins from su ch an editing mutant organism could be replaced with the noncanonical amino butyrate, sterically similar to cysteine, Thus, the editing function may ha ve played a central role in restricting the genetic code to 20 amino acids. Disabling this editing function offers a powerful approach for diversifyin g the chemical composition of proteins and for emulating evolutionary stage s of ambiguous translation.