The gene for Clostridium septicum alpha-toxin was cloned and expressed in Escherichia coli from C. septicum BX96. The toxin was determined to be 443 amino acids in length, with a 31-residue signal peptide that was removed from the toxin during secretion. No extended hydrophobic regions were observed in the mature toxin sequence. Expression of alpha-toxin in E. coli BL21 resulted in the production of AT pro , which was identical to native toxin from C. septicum with respect to activity and

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... tion. The proteolytic activation site for alpha-toxin was determined to be on the carboxy-terminal side of arginine 398, which lies within the sequence KKRRGKR-398SVD. Previous work showing similarities in activation and mechanism between alpha-toxin and Aeromonas hydrophila aerolysin was extended to the primary structures of both toxins. The DNA-derived primary sequence of alpha-toxin exhibited 27% identity and 72% similarity over a 387-residue region with the primary structure of the A. hydrophila aerolysin toxin, a level of similarity heretofore unobserved between toxins produced by a gram-positive organism and a gram-negative organism.