The "metallo-specific" response of proteins: a perspective based on the Escherichia coli transcriptional regulator NikR.

Transition metal ions are required by all cells but an excess of metal ions beyond physiological requirements has toxic consequences. Optimal cellular concentrations of transition metals are commonly maintained by metal-responsive transcription factors that regulate genes encoding the proteins responsible for transport, sequestration and/or use of the metals. These metalloregulators must discriminate between the bioavailable metals to properly effect metal homeostasis, but how this metal selectivity is achieved is poorly understood. This perspective examines the metal-selective response of the Escherichia coli Ni(II)-responsive metalloregulator NikR. Biochemical and structural studies of E. coli NikR reveal that the mechanism of metal-selective regulation is more complex than that defined by simple metal-binding thermodynamics. Here we examine the metal-dependent allosteric changes on NikR structure that affect DNA binding and discuss the correspondence with other metalloregulators. Given what we have learned of how metal selectivity is achieved by E. coli NikR, we propose a complete scheme for the regulatory function of NikR in E. coli nickel homeostasis.