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-dependent repressor proteins in Gram-positive bacteria. This study aimed to increase understanding of the general regulatory principles of cation binding to DtxR.

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 domain (which contains two metal-binding sites) and a third, very flexible carboxy-terminal domain. Each DNA-binding domain contains a helix-turn-helix motif and has a topology which is very similar to catabolite gene activator protein (CAP). Molecular modeling suggests that bound DNA adopts a bent conformation with helices alpha 3 of DtxR interacting with the major grooves. The two metal-binding sites lie approximately 10 A apart. Binding site 2 is positioned at a potential hinge region between the DNA-binding and interface domains. Residues 98-108 appear to be crucial for the functioning of the repressor; these provide four of the ligands of the two metal-binding sites and three residues at the other side of the helix which are at the heart of the dimer interface.

CONCLUSIONS:

The crystal structure of the DtxR holorepressor suggests that the divalent cation co-repressor controls motions of the 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.