An expression system and process for the production of Diphtheria toxin polypeptides or mutated forms thereof, such as the toxoid CRM197 polypeptide, in genetically-modified E. coli with high yield is described. The system and process is based on the uncoupling of biomass growth from recombinant protein induction, i.e. using an inducer of protein production that cannot be used as a carbon source for growth by the bacteria. The use of specific components and conditions that improve protein yields are also described.