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cyanobacterial ArsR orthologues have been published. The X-ray structure
of apo-SmtB revealed that the α3 helix contains this metal-binding motif;
however, in this regulator, the last cysteine residue is naturally substituted by
a glycine residue: ELCVGDL (
Cook et al., 1998
). Surprisingly, the remain-
ing cysteine residue in the motif, Cys
61
, is not essential in SmtB for Zn
2+
-
sensing in vivo (
Turner, Glands, Samson, & Robinson, 1996
).
Conversely, the α5 metal-binding site is composed of ligands derived
exclusively from the α5 helix (
Fig. 4.5
). Substitution of His
105
/His
106
in
SmtB or His
116
in ZiaR by arginine residues in the α5 helix resulted in a
loss of induction by zinc, suggesting that this metal site may be important
for Zn
2+
-sensing in vivo for both regulators (
Thelwell et al., 1998
;
Turner
et al., 1996
). The α5 metal-binding site probably resists distortion to accom-
modate larger metal ions, interacting preferably with smaller divalent ions
such as Zn(II), Co(II) and Ni(II) (
Pennella & Giedroc, 2005
).
Anabaena
AztR lacks the α5 metal-binding site, and thus, it utilizes the
α3N site to sense not only small essential Zn(II) ions but also larger toxic
Cd(II)/Pb(II) ions (
Liu et al., 2005
). The noteworthy structural plasticity of
the α3N site has also been shown by its ability to allow direct binding of
monovalent ions Cu(I)/Ag(I) in
Oscillatoria
BxmR (
Liu et al., 2008
). This
adaptation of the metal-binding site to different kinds of metals suggests an
Figure 4.5
Sequence alignment of cyanobacterial SmtB/ArsR family metalloregula-
tors generated using ClustalW2 (
http://www.ebi.ac.uk/Tools/msa/clustalw2/
). The pro-
posed 'metal-binding box' with the Cys-X-Cys motif is boxed and grey shaded. Residues
known or predicted to be metal ligands in the
α
3N site are denoted with an asterisk, and
those in the
α
5 site are boxed. The secondary structure assignment is based on the stud-
ies of SmtB by
Cook et al. (1998)
. For colour version of this figure, the reader is referred
to the online version of this topic.
