Conformational and thermodynamic hallmarks of DNA operator site specificity in the copper sensitive operon repressor from Streptomyces lividans

Metal ion homeostasis in bacteria relies on metalloregulatory proteins to upregulate metal resistance genes and enable the organism to preclude metal toxicity. The copper sensitive operon repressor (CsoR) family is widely distributed in bacteria and controls the expression of copper efflux systems. CsoR operator sites consist of G-tract containing pseudopalindromes of which the mechanism of operator binding is poorly understood. Here, we use a structurally characterized CsoR from Streptomyces lividans (CsoR(Sl)) together with three specific operator targets to reveal the salient features pertaining to the mechanism of DNA binding. We reveal that CsoR(Sl) binds to its operator site through a 2-fold axis of symmetry centred on a conserved 5'-TAC/GTA-3' inverted repeat. Operator recognition is stringently dependent not only on electropositive residues but also on a conserved polar glutamine residue. Thermodynamic and circular dichroic signatures of the CsoR(Sl)-DNA interaction suggest selectivity towards the A-DNA-like topology of the G-tracts at the operator site. Such properties are enhanced on protein binding thus enabling the symmetrical binding of two CsoR(Sl) tetramers. Finally, differential binding modes may exist in operator sites having more than one 5'-TAC/GTA-3' inverted repeat with implications in vivo for a mechanism of modular control.