The complete amino acid sequence of the Clostridium botulinum type D neurotoxin, deduced by nucleotide sequence analysis of the encoding phage d-16 phi genome.

The complete nucleotide sequence of Clostridium botulinum type D strain CB16 neurotoxin was determined and the deduced amino acid sequence is reported here for the first time. The structure and function of botulinum type D neurotoxin is discussed from a molecular biological viewpoint. DNA was extracted from toxin-converting phage d-16 phi of C. botulinum type D strain CB16, and a fragment (about 10 kbp) coding for the neurotoxin was cloned into Escherichia coli using lambda gt11. A 21-mer oligonucleotide which corresponds to Phe7 to Val13 of the partial amino acid sequence near the N-terminus of the type D neurotoxin was synthesized and used as a probe to identify the gene encoding type D neurotoxin. The nucleotide sequence contained a single open reading frame coding for 1,275 amino acids (molecular weight of 146,785) and the deduced amino acid sequence corresponded exactly to the partial amino acid sequences determined by direct microsequencing of the neurotoxin fragments. In the dichain molecule of the neurotoxin, Thr2 and Asn443 formed the N-termini of the light chain (M.W. 50,410) and heavy chain (M.W. 96,394) respectively, and these two chains were linked with a disulfide bond between Cys437 on the light chain and Cys450 on the heavy chain. The nucleotide sequence of the D-CB16 neurotoxin differed from that previously reported for type D neurotoxin by three nucleotides.