The Xis2d protein of CTnDOT binds to the intergenic region between the mob and tra operons

• Xis2d is the first excisionase from an ICE that has been characterized as a transcriptional regulator.
• Xis2d specifically binds a region of dyad symmetry.
• Xis2d induces bends in the DNA.
• Xis2d and Exc interact to form higher order complexes.

CTnDOT is a 65 kbp integrative and conjugative element (ICE) that carries genes encoding both tetracycline and erythromycin resistances. The excision operon of this element encodes Xis2c, Xis2d, and Exc proteins involved in the excision of CTnDOT from host chromosomes. These proteins are also required in the complex transcriptional regulation of the divergently transcribed transfer (tra) and mobilization (mob) operons of CTnDOT. Transcription of the tra operon is positively regulated by Xis2c and Xis2d, whereas, transcription of the mob operon is positively regulated by Xis2d and Exc. Xis2d is the only protein that is involved in the excision reaction, as well as the transcriptional regulation of both the mob and tra operons. This paper helps establish how Xis2d binds the DNA in the mob and tra region. Unlike other excisionase proteins, Xis2d binds a region of dyad symmetry. The binding site is located in the intergenic region between the mob and tra promoters, and once bound Xis2d induces a bend in the DNA. Xis2d binding to this region could be the preliminary step for the activation of both operons. Then the other proteins, like Exc, can interact with Xis2d and form higher order complexes.