SEQUENCING AND ANALYSIS OF THE GENE ENCODING THE ALPHA-TOXIN OF CLOSTRIDIUM-NOVYI PROVES ITS HOMOLOGY TO TOXIN-A AND TOXIN-B OF CLOSTRIDIUM-DIFFICILE

A library of total Clostridium novyi DNA was established and screened for the alpha-toxin gene (tcn alpha) by hybridization with oligonucleotides derived from a partial N-terminal sequence and by using specific antisera. Overlapping subgenic tcn alpha fragments were isolated and subsequently the total sequence of tcn alpha was determined. The 6534 nucleotide open reading frame encodes a polypeptide of M(r) 250 166 and pI 5.9. The N-terminal alpha-toxin (Tcn alpha) sequence MLITREQLMKIASIP determined by Edman degradation confirmed the identity of the reading frame and the assignment of the translation start point. The toxin is not modified posttranslationally at its N-terminus nor does it consist of different subunits. Overall the amino acid sequence shows 48% homology between the Tcn alpha and both toxins A (TcdA) and B (TcdB) of Clostridium difficile. The C-terminal 382 residues of Tcn alpha constitute a repetitive domain similar to those reported for TcdA and TcdB of C. difficile. The individual repeat motifs of these three toxins consist of oligopeptides some 19-52 amino acids in length, arranged in four to five different groups. Genetic, biochemical and pharmacological data thus confirm that the three toxins belong to one subgroup, designated large clostridial cytotoxins (LCT). Further definition of their structure and detailed molecular action should allow the LCTs to be used tools for the analysis of microfilament assembly and function.