3-DIMENSIONAL STRUCTURE OF THE DIPHTHERIA-TOXIN REPRESSOR IN COMPLEX WITH DIVALENT-CATION CO-REPRESSORS

Background: When Corynebacterium diphtheriae encounters an environment with a low concentration of iron ions, it initiates the synthesis of several virulence factors, including diphtheria toxin. The diphtheria toxin repressor (DtxR) plays a key role in this iron-dependent, global regulatory system and is the prototype for a new family of iron-depen dent repressor proteins in Gram-positive bacteria. This study aimed to increase understanding of the general regulatory principles of cation binding to DcxR. Results: The crystal structure of dimeric DtxR holo- repressor in complex with different transition metals shows that each subunit comprises an amino-terminal DNA-binding domain, an interface d omain (which contains two metal-binding sites) and a third, very flexi ble carboxy-terminal domain. Each DNA-binding domain contains a helix- turn-helix motif and has a topology which is very similar to catabolit e gene activator protein (CAP). Molecular modeling suggests that bound DNA adopts a bent conformation with helices alpha 3 of DtxR interacti ng with the major grooves. The two metal-binding sites lie similar to 10 Angstrom apart. Binding site 2 is positioned at a potential hinge r egion between the DNA-binding and interface domains. Residues 98-108 a ppear to be crucial for the functioning of the repressor; these provid e four of the ligands of the two metal-binding sites and three residue s at the other side of the helix which are at the heart of the dimer i nterface. Conclusions: The crystal structure of the DtxR holo-represso r suggests that the divalent cation co-repressor controls motions of t he DNA-binding domain. In this way the metal co-repressor governs the distance between operator recognition elements in the two subunits and , consequently, DNA recognition.