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AztR only retains the two α3N sites per dimer, so it senses not only two
Zn(II) ions but also two Cd(II) and Pb(II) ions per homodimer through
this site (
Liu et al., 2005
). AzuR is the other metalloregulator in
Anabaena
that belongs to the SmtB/ArsR family. This has been proposed to be a
zinc repressor more closely related to SmtB (
Liu et al., 2008
). Finally,
Syn-
echocystis
ArsR is induced in vivo by As(III) and Sb(III), but not by As(V)
(
Lopez-Maury et al., 2003
).
3.4.2. DNA-binding sites
The SmtB/ArsR transcriptional repressors often regulate expression of
divergently transcribed genes, arranged in operons. These metalloregulators
are specifically bound to their DNA operator/promoter (O/P)-binding
sites in the metal-free state.
Most of the O/P sequences in this family contain one imperfect 12-2-12
inverted repeat, generally overlapping or located near the transcriptional
start site of the genes (
Fig. 4.6
). For instance, only one pair of contact sites
is described for
Synechocystis
ZiaR or
Anabaena
AzrR within the
zia
and
azt
divergons, respectively (
Fig. 4.6
A) (
Liu et al., 2005
;
Thelwell et al., 1998
).
Nonetheless, in
Synechococcus
, the
smt
operon displays two of these repeats
contacted by SmtB at the conserved TGA sequence: S2/S1 lies at the
smtA
transcriptional start site while S4/S3 lies between the
smtA
and
smtB
−10
sequences. At low concentrations, recombinant SmtB binds in vitro as a
monomer to either S1 or S2; as the SmtB concentration increases, it binds
as a dimer at both sites. At high SmtB concentrations, the metal-free regula-
tor may form a homotetrameric complex. The formation of all the three
DNA-SmtB complexes is inhibited by Zn
2+
in vitro (
Erbe, Taylor, & Hall,
1995
). In
Oscillatoria
, there is one imperfect 12-2-12 inverted repeat in the
region between
bxmR
and
bmtA
, and another one found in the promoter
region of
bxa1
(
Fig. 4.6
B) (
Liu et al., 2004
). The
Synechocystis arsBHC
operon is repressed by the product of a physically separated gene, ArsR.
The ArsR-binding site within the
arsBHC
O/P region contains two 17-bp
direct repeats of the sequence ATCAAGTTTTTTTGATG, each one con-
sisting of two inverted repeats (
Lopez-Maury et al., 2003
).
Synechococcus
mutants with an interrupted
smt
divergon are fivefold sensi-
tive to Zn
2+
and show some reduction in tolerance to Cd
2+
(
Turner, Morby,
Whitton, Gupta, & Robinson, 1993
). The same behaviour is observed in
Synechocystis zia
divergon mutants and both operons seem to be inter-
changeable. Restoration of metal resistance is achieved by the introduction
of
zia
in the
Synechococcus smt
mutant (
Thelwell et al., 1998
).
