The IStron CdISt1 of Clostridium difficile: molecular symbiosis of a group I intron and an insertion element.

Abstract The IStron Cd ISt 1 was first discovered as an insertion into the tcdA gene of the clinical isolate C34. It combines structural and functional properties of a group I intron at its 5′-end with those of an insertion element at its 3′-end. Up to date four different types could be found, mainly differing in their IS-element portions. Contrasting classical group I introns, Cd ISt 1 is always integrated in ORFs encoding bacterial protein. In case Cd ISt 1 had only the IS-element function such insertion would inactivate the protein encoded by the host gene. It is only due to the self-splicing activity of the group I intron parts that Cd ISt 1 integration does not abolish protein function. Both elements seem to exist in molecular symbiosis and Cd ISt 1 could thus be a prototype of a novel class of genetic elements. Moreover, integration of the Cd ISt 1 into the genome could be advantageous for the bacterium, a motor function for evolution of bacterial proteins is discussed. In clinical practice Cd ISt 1 might well serve as a tool for epidemiological studies of C. difficile infections.