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

The IStron Cd/St1 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/St1 is always integrated in ORB encoding bacterial protein. In case Cd/St1 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/St1 integration does not abolish protein function. Both elements seem to exist in molecular symbiosis and Cd/St1 could thus be a prototype of a novel class of genetic elements. Moreover. integration of the Cd/St1 into the genome could be advantageous for the bacterium, a motor function for evolution of bacterial proteins is discussed. In clinical practice Cd/St1 might well serve as a tool for epidemiological studies of C. difficile infections. (C) 2004 Elsevier Ltd. All rights reserved.