Tying Up a Loose End: On the Role of the C‐Terminal CCHHRAG Fragment of the Metalloregulator CueR

The transcriptional regulator CueR is activated by the binding of Cu(I), Ag(I), or Au(I) to two cysteinates in a near‐linear fashion. The C‐terminal CCHHRAG sequence in Escherichia coli CueR present potential additional metal binding ligands and here we explore the effect of deleting this fragment on the binding of Ag(I) to CueR. CD spectroscopic and ESI‐MS data indicate that the high Ag(I)‐binding affinity of WT‐CueR is significantly reduced in Δ7C‐CueR.([111) Ag PAC spectroscopy demonstrates that the WT‐CueR metal site structure (AgS(2)) is conserved, but less populated in the truncated variant. Thus, the function of the C‐terminal fragment may be to stabilize the two‐coordinate metal site for cognate monovalent metal ions. In a broader perspective this is an example of residues beyond the second coordination sphere affecting metal site physicochemical properties while leaving the structure unperturbed.