V. Braun et al., A chimeric ribozyme in Clostridium difficile combines features of group I introns and insertion elements, MOL MICROB, 36(6), 2000, pp. 1447-1459
CdISt1, a DNA insertion of 1975 bp, was identified within tcdA-C34, the ent
erotoxin gene of the Clostridium difficile isolate C34. Located in the cata
lytic domain A1-C34, CdISt1 combines features of two genetic elements. With
in the first 434 nt structures characteristic for group I introns were foun
d; encoding the two transposase-like proteins tlpA and tlpB nucleotides 435
-1975 represent the remainder of a IS605-like insertion element. We show th
at the entire CdISt1 is accurately spliced from tcdA-C34 primary transcript
s and that purified TcdA-C34 toxin is of regular size and catalytic activit
y. A search for CdISt1-related sequences demonstrates that the element is w
idespread in toxinogenic and non-toxinogenic C. difficile strains, indicati
ng the mobility of CdISt1. In strain C34, we characterize 10 CdISt1 variant
s; all are highly homologous to CdISt1 (> 93% identity), integrated in bact
erial open reading frames (ORFs), show the typical composite structure of C
dISt1 and are precisely spliced from their primary transcripts. CdISt1-like
chimeric ribozymes appear to combine the invasiveness of an insertion elem
ent with the splicing ability of a group I intron, rendering transposition
harmless for the interrupted gene.