Expression of immunogenically reactive diphtheria toxin fusion proteins under the control of the pR promoter of bacteriophage lambda.

The tox228 gene encoding the non-toxic, immunologically cross-reactive CRM228 mutant diphtheria toxin (DT) has been cloned downstream of the PR promoter and the cro translational initiation region of bacteriophage lambda carried by plasmid pCQV2 (Queen, 1983). Efficient transcription but no appreciable amount of a translational product corresponding to complete DT could be detected in Escherichia coli hosts. Deletion of 320 bp from the C-terminal region of the B-fragment of DT, and fusion of the truncated tox228 gene to lacZ yielded several hybrid beta-galactosidases (beta Gal) in an E. coli lon- strain in addition to beta Gal. The various DT fragments fused to beta Gal were immunologically reactive and were identified with antibodies specifically directed against the A- or the B-fragment of DT. Antibodies raised against the DT-beta Gal fusion proteins in guinea pigs cross-reacted with wild-type DT and its B-fragment and protected Vero cells in tissue culture against the lethal action of DT. Immunized guinea pigs survived upon injection of a five-fold lethal dose of wild-type DT.