Production of diphtheria toxin CRM228 in B. subtilis.

The gene coding for a nontoxic diphtheria toxin (DT), tox228, was isolated from lysogenic Corynebacterium diphtheriae and cloned into pBR322. A mature form of the tox228 gene, lacking its signal sequence, was expressed in Bacillus subtilis using a B. amyloliquefaciens alpha-amylase secretion vector. To test the possibility of producing partially deleted DT molecules, which could be used for cell-directed toxin conjugates, a truncated form lacking 151 amino acids from the C-terminus of the DT was generated by oligonucleotide mutagenesis. Both the truncated and intact DT were efficiently secreted into the culture medium. During prolonged cultivation, the truncated form was less stable than the intact DT molecule.