The present invention is directed to a method to diversify the chemical
composition of proteins produced in vivo comprising the step of
disabling, particularly by mutagenesis, the editing function of one of
its aminoacyl tRNA synthetases. The present invention is also directed to
nucleic acid sequences encoding such mutated aminoacyl tRNA synthetases
having their editing site mutated and capable of mischarging its cognate
tRNA with a noncanonical amino acid. Also described herein is an improved
method for obtaining transformed cells capable of synthetizing in vivo
proteins comprising at least a noncanonical amino acid and their use for
the production of such proteins.