Clostridium difficile toxin A carries a C-terminal repetitive structure homologous to the carbohydrate binding region of streptococcal glycosyltransferases.

A detailed analysis of the 8130-bp open reading frame (ORF) of gene toxA and of an upstream ORF designated utxA, indicates the presence of a transcription terminator stem-loop for toxA, promoter sequences, and Shine-Dalgarno boxes for toxA and utxA. No transcription terminator between toxA and utxA is suggested by the sequence. ToxA contains two domains, one-third (C-terminal) with a repetitive structure and the residual two-thirds with no repetitions. The 2499-bp sequence encoding the repetitive structure is composed of nine groups of different short repetitive oligodeoxyribonucleotides (SRONs). A combination of these SRONs codes for five groups of combined repetitive oligopeptides (CROPs). Seven 50-amino acid (aa) CROPs and 23 CROPs of 21 aa in length are noticed. The CROPs are generally highly conserved, but four exhibit variability and possibly represent 'hot spots' of the repetitive structure. The reactivity of the C-terminal repeat with monoclonal antibody 1337C8 indicates that this part contains the carbohydrate-binding domain of ToxA. In this region homology exists between the ToxA repeats and the glucosyltransferases of Streptococci. We propose that binding of ToxA to cells occurs via the C-terminal repeat domain, with the N-terminal domain being responsible for toxic function.