Biology Reference
In-Depth Information
probably involved in reduction of As(V) to As(III). ArsB is a putative arse-
nite and antimonite exporter, while no function has been assigned to ArsH
(
Lopez-Maury et al., 2003
).
3.4.4. Allosteric regulation and autoregulation
Direct interaction of the metal ions in the regulatory sites of the SmtB/
ArsR repressors negatively regulates the specific operator/promoter-
binding affinity in vitro
.
Negative allosteric regulation of DNA binding is
a common characteristic of these family members (
Liu et al., 2004
).
The binding of two Zn
2+
ions to the α5 site in the dimer interface of
Synechococcus
SmtB probably induces an overall compaction of the repres-
sor from its conformation in the metal-free state. This conformational
change in SmtB could disrupt the geometry necessary for the interaction
of the regulator with the binding sites (
Kar, Adams, Lebowitz, Taylor, &
Hall, 1997
). In
Synechocystis
ZiaR, binding of the metal to both α3N and
α5 sites seems to be necessary, implying co-operativity between these sites
(
Thelwell et al., 1998
).
Anabaena
AztR lacks the dimer interface metal-
binding residues (α5 site), thus metal binding to the helix-turn-helix is the
proposed simple mechanism to induce DNA dissociation (
Liu et al., 2005
).
Oscillatoria
BxmR is dissociated from specific DNA in vitro upon the addi-
tion of both monovalent and divalent metal ions. Cu(I), Ag(I) and Cd(II)
inhibit BxmR-
bxa1
O/P DNA binding in an equally effective way, using
α3N as metal-binding sites. Concerning Zn(II), this metal is capable of
functioning through both metal sites although binding to the α5 site results
in a more effective negative allosteric regulation (
Liu et al., 2008
).
A mechanism of autoregulation has been described for cyanobacterial
SmtB-like proteins. Since the SmtB orthologues are encoded in the diver-
gently transcribed operons that are induced by themselves, their expression
is also controlled by metal availability. This autoregulatory mechanism allows
returning to the repression condition. However, in
Synechocystis
, the
arsR
gene is not autoregulated since it is expressed constitutively at low level. It is
worth noting that in the absence of ArsR, normal growth parameters in
Syn-
echocystis
are observed, in spite of the constitutive expression of ArsB which
results toxic for
E. coli
when it is overexpressed (
Lopez-Maury et al., 2003
).
3.5. The MerR Family of Proteins
MerR proteins act as dimeric transcriptional activators that may directly
interact with RNA polymerase to achieve a functional fit to DNA. A metal-
induced DNA-conformational change distorts the operator structure,
