Biology Reference
In-Depth Information
evolutionary mechanism developed to confer increased resistance to other
toxic heavy metals on certain cyanobacteria.
The sequence of ArsR orthologues does not usually contain the
N-terminal region present in SmtB-like proteins. Thus,
E. coli
ArsR pos-
sesses three α3 cysteines in the α3 helix, which induce metal responsiveness
due to the lack of the α5 site in this regulator. Similarly,
Synechocystis
ArsR
does not possess ligands related to the α5 site either, so it presumably binds
metal ions through the α3 cysteine site (
Osman & Cavet, 2010
).
An overall comparison of the cyanobacterial SmtB/ArsR repressors
reveals distinct metal-binding properties that are summarized in
Table 4.5
.
Synechococcus
SmtB binds two zinc ions per dimer through the α5 site
(
VanZile et al., 2002b
).
Synechocystis
ZiaR requires the binding of four zinc
ions to both α3N and α5 sites to induce zinc metalloregulation in vivo
(
Thelwell et al., 1998
).
Oscillatoria
BxmR also possesses both sites, using
either α5 or α3N to bind zinc ions. However, this regulator affects more
selective and effective zinc regulation by binding Zn(II) to the α5 site, while
the α3N site is required for Cu(I), Ag(I) or Cd(II) sensing. BxmR binds
four Cu(I) ions per dimer in α3N site, but only two Zn(II) or Cd(II) ions
per dimer in either α5 or α3N sites (
Osman & Cavet, 2010
).
Anabaena
Table 4.5
Summary and comparison of metal-binding properties of cyanobacterial
SmtB/ArsR proteins
'Metal-binding
box'
Sensor
Metal
Metal site(s) Ligands
Synechococcus
SmtB
ELCVGDL
Zn(II)
α5
Asp104 His106
His117 Glu120
Synechocystis
ZiaR
ELCVCDL
Zn(II)
α5 and
α3N
α5: Asp114 His116
His127 Glu130
α3N: Cys71 and/or
Cys73 Cys His
Oscillatoria
BxmR
ELCVCDL
Zn(II)
α5 and
α3N
α5: Asp119 His121
His132 Glu135
Cu(I), Ag(I),
Cd(II)
α3N
α3N: Cys23 Cys31
Cys75 Cys77
Anabaena
AztR
ELCVCDL
Zn(II)
Cd(II)
Pb(II)
α3N
α3N: Cys21 Cys72
Cys74 His
Anabaena
AzuR
ELCVSDL
Zn(II)?
Co(II)?
α5?
?
Synechocystis
ArsR
EQCVCDL
As(III)
Sb(III)
α3
?
