The primary structure of Clostridium septicum alpha-toxin exhibits similarity with that of Aeromonas hydrophila aerolysin

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Abstract
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 ATpro, which was identical to native toxin from C. septicum with respect to activity and activation. 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.