Metalloregulatory proteins of the MerR family are transcriptional activators
that sense/control the concentration of various metal ions inside bacteria.1
The Cu+ efflux regulator CueR, similarly to other MerR proteins, possesses a
short multiple Cys-containing metal binding loop close to the C-terminus.
CueR has a high selectivity for Cu+, Ag+ and Au+, but exhibits no
transcriptional activity for the divalent ions Hg2+ and Zn2+.2 The two Cys-
residues of the metal binding loop were shown to settle M+ ions into a linear
coordination environment but other factors may also play a role in the
recognition of cognate metal ions.2 Nevertheless, it is an interesting question
whether the same sequence, when removed from the protein, shows a
flexibility to adopt different coordination environments and may efficiently
bind metal ions having preferences for larger coordination numbers.