The invention relates to a mutated cell such as a bacteria or yeast in which the thyA gene coding thymidylate synthase includes a nonsense codon, preferably the amber codon, said nonsense codon replacing a codon coding an amino acid and inducing the interruption of thyA gene translation and the auxotropy of the cell for the thymidine. Advantageously, the endA gene coding the endonuclease 1 and/or the recA gene coding the recombinase is inactivated in said mutated cell. The invention also relates to an expression plasmid including a transgene and a sequence of a suppressing ARM structural gene containing an anticodon that can be paired with the nonsense codon of the thyA gene and is specific of an amino acid capable of restoring the translation of the mutated thyA gene and thereby obtaining a protein of the wild or mutated type having a thymidylate synthase activity. The invention also relates to a method for the multiplication of the expression plasmid.