A chimeric ribozyme in Clostridium difficile combines features of group I introns and insertion elements

CdISt1, a DNA insertion of 1975 bp, was identified within tcdA-C34, the enterotoxin gene of the Clostridium difficile isolate C34. Located in the catalytic domain A1-C34, CdISt1 combines features of two genetic elements. Within the first 434 nt structures characteristic for group I introns were found; encoding the two transposase-like proteins tlpA and tlpB nucleotides 435-1975 represent the remainder of a IS605-like insertion element. We show that the entire CdISt1 is accurately spliced from tcdA-C34 primary transcripts and that purified TcdA-C34 toxin is of regular size and catalytic activity. A search for CdISt1-related sequences demonstrates that the element is widespread in toxinogenic and non-toxinogenic C. difficile strains, indicating the mobility of CdISt1. In strain C34, we characterize 10 CdISt1 variants; all are highly homologous to CdISt1 (> 93% identity), integrated in bacterial open reading frames (ORFs), show the typical composite structure of CdISt1 and are precisely spliced from their primary transcripts. CdISt1-like chimeric ribozymes appear to combine the invasiveness of an insertion element with the splicing ability of a group I intron, rendering transposition harmless for the interrupted gene.