Clostridium spiroforme toxin genes are related to C-perfringens lota toxin genes but have a different genomic localization

Clostridium spiroforme toxin belongs to the iota toxin family and consist of an ADP-ribosyltransferase component (Sa) and a binding component (Sb) which is involved in the binding and internalization of the toxin within the cell. In this study, the respective genes, sas and sbs, were characterized. The genes are orientated in the same way as the C. perfringens iota toxin genes and the C. difficile ADP-ribosyltransferase (CDT) genes. The sas gene is located upstream of the sbs gene. The genes are transcribed in the same orientation, and are separated by a short (41 bp) non coding sequence. The encoded proteins Sa and Sb are similar (about 80% identity) to the iota toxin and CDT, and these toxins have the same domain organization. In contrast to the iota toxin genes which are present on large plasmids in C. perfringens E, C, spiroforme toxin and CDT genes are present on the chromosome in the C. spiroforme and C. difficile strains analyzed, although these strains also contain large plasmids. A variation of the iota toxin sequences (80% identity) was observed in 3 of 4 C. perfringens E strains, and one strain had only the enzymatic component gene.