Asymmetric allosteric activation of the symmetric ArgR hexamer

Hexameric arginine repressor, ArgR, bound to L-arginine serves both as the master transcriptional repressor/activator at diverse regulons in a wide range of bacteria and as a required cofactor for resolution of ColE1 plasmid multimers. Multifunctional ArgR is thus unusual in possessing features of specific gene regulators, global regulators, and non-specific gene organizers; its closest functional analog is probably CAP, the cyclic AMP receptor/activator protein. Isothermal titration calorimetry, surface plasmon resonance, and proteolysis indicate that binding of a single L-arginine residue per ArgR hexamer triggers a global conformational change and resets the affinities of the remaining five sites, making them 100-fold weaker. The analysis suggests a novel thermodynamic signature for this mechanism of activation. (C) 2004 Elsevier Ltd. All rights reserved.