Cloned fragment A of diphtheria toxin is expressed and secreted into the periplasmic space of Escherichia coli K12.

An 831-base pair segment of the corynebacteriophage beta tox-45 genome encoding fragment A of diphtheria toxin was cloned into plasmid pUC8 in Escherichia coli K12. Strains containing recombinant plasmids expressed the adenosine diphosphate ribosyl transferase activity characteristic of fragment A; this activity could be inhibited by polyvalent antiserum to fragment A as well as by the appropriate monoclonal antibodies to diphtheria toxin. The transferase activity was secreted into the periplasmic space of E. coli. These findings have implications for the future construction of genetically engineered chimeric toxins.