Biology Reference
In-Depth Information
tolerance (
Turner et al., 1993
). SmtA scavenges excess zinc from adventi-
tious sites sequestering it in a nontoxic form. However, overexpressing
smtA
mutants are viable, implying that SmtA does not remove zinc from
advantageous sites (
Turner, Robinson, & Gupta, 1995
). The release of zinc
from SmtA has not yet been elucidated. Interaction with another protein
or even SmtA degradation has been proposed as zinc release possibilities
(
Robinson, Whitehall, & Cavet, 2001
). In addition to zinc detoxification,
another function in zinc accumulation has been suggested for the metallo-
thionein SmtA. The DNA-primase gene,
dnaG
, is located adjacent to
smtA
in
Synechococcus
PCC 7942. It is not known whether SmtA can influence
the zinc content of the predicted zinc-finger present in this DNA-primase
(
Robinson et al., 2001
).
The
Synechocystis zia
operon is organized in a similar way to the
smt
region; however, ZiaR controls the transcription of a zinc-exporting ATPase
ZiaA. In the absence of zinc, the operon is repressed but when metal avail-
ability increases, the expression of both the regulator ZiaR and the ATPase
ZiaA triggers zinc efflux into the periplasm until levels are low enough to
arrest
ziaA
transcription. In addition to zinc hypersensibility, reduced zinc
export to the periplasm is observed in
ziaA
mutants (
Barnett et al., 2012
;
Thelwell et al., 1998
).
Thus, two alternative ways of zinc detoxification are represented in these
two cyanobacteria: metal sequestration in cytosol by a metallothionein or
metal export to the periplasm by an ATPase. In
Oscillatoria
, both systems
are present since BxmR metalloregulates not only the expression of the
metallothionein BmtA but also the transcription of the ATPase Bxa1. As
soon as Zn(II) or Cd(II) are sensed by BxmR, the transporter Bxa1 is firstly
induced in a rapid response to restore the intracellular metal homeostasis,
while BmtA is transcribed relatively slowly as a long-term defence against
metallotoxicity (
Liu et al., 2004
).
In
Anabaena
, the AztR regulator responds to zinc, cadmium and lead
allowing the transcription of the ATPase AztA, which transports divalent
ions from cytosol to periplasm (
Liu et al., 2005
). Surprisingly, the genome of
this cyanobacterium also encodes the metallothionein BmtA but an associ-
ated SmtB/ArsR regulator has not yet been found (
Blindauer, 2008
). Nota-
bly, a second SmtB orthologue AzuR has been described in
Anabaena
, but
it has not yet been well characterized either biochemically or functionally
(
Liu et al., 2008
).
In
Synechocystis
, ArsR senses As(III) and Sb(III) ions and regulates the
arsBHC
operon involved in arsenic and antimony resistance. The
arsC
gene
encodes a putative arsenate reductase, related to arsenate detoxification,
