Characterization of the C3 gene of Clostridium botulinum types C and D and its expression in Escherichia coli.

Clostridium botulinum type C and D strains produce exoenzyme C3, which ADP-ribosylates the Rho protein, a 21-kDa regulatory GTP-binding protein. In a previous work, we demonstrated that the C3 gene is encoded by bacteriophages C and D of C. botulinum by using DNA-DNA hybridizations with oligonucleotides deduced from the C3 protein N-terminal sequence. The C3 coding gene was cloned and sequenced, but its upstream DNA region could not be studied because of its instability in Escherichia coli. In this work, the upstream DNA region of the C3 gene was directly amplified by the polymerase chain reaction and sequenced. The C3 gene encodes a polypeptide of 251 amino acids (27,823 Da) consisting of a 40-amino-acid signal peptide and a mature protein of 211 amino acids (23,546 Da). The C3 mature protein was expressed in E. coli under the control of the trc promoter. The recombinant polypeptide obtained was recognized by C3 antibodies and ADP-ribosylated the Rho protein. The C3 gene nucleotide sequence is identical on C and D phage DNAs. At the amino acid sequence level, no similarity was found among C3, other ADP-ribosylating toxins, or tetanus or botulinal A, C1, and D neurotoxins.