Biology Reference
In-Depth Information
(
Le Brun, Crow, Murphy, Mauk, & Moore, 2010
). The deposition of iron
is accompanied by deposition of phosphate. In cyanobacteria, three dif-
ferent types of ferritin family proteins were identified. Ferritins and bac-
terioferritins function as ferroxidases, oxidizing Fe(II) to Fe(III), while
generating hydrogen peroxide. This ferroxidase activity is carried out by
the di-iron centre. Fe(III) is stored as iron oxide in the cavity at the centre
of their 24-mer ultrastructure (
Le Brun et al., 2010
).
Bacterioferritins differ from ferritins in the presence of a haeme mol-
ecule, anchored at the interface between two adjacent subunits. The role of
this haeme remains poorly defined but it was suggested that it contributes to
iron reduction and release from the core of the complex (
Carrondo, 2003
).
DNA protection during starvation (DPS) family (DNA-binding proteins
from starved cells) proteins have a very similar function. However, the elec-
tron acceptor in the case of DPS proteins is hydrogen peroxide instead of
water.
The ferroxidase di-iron centre of DPS proteins is located between two
helices. In addition, the structure of DPS proteins is shorter by one α-helix,
as compared to ferritins and bacterioferritins. As a result, DPS multimeric
structures are 12-mer, whereas ferritins and bacterioferritins form larger
24-mer structures. From an evolutionary standpoint, DPS proteins represent
a more diverse group than the other ferritin families with members func-
tioning as iron storage proteins, DNA-binding proteins protecting against
oxidative stress, cold shock proteins, neutrophile activators or pili compo-
nents (
Zeth, Offermann, Essen, & Oesterhelt, 2004
). These activates are not
necessarily connected to the ferroxidase activity.
To date, functional analysis of the role of these proteins in cyanobac-
teria is largely limited to freshwater species. In
Synechocystis
PCC 6803,
it was found that
bfr
genes belong to a subfamily of bacterioferritin
genes, in which one gene codes for a protein with a conserved haeme
ligand and the other codes for a protein with conserved di-iron centre
ligands (
Bertani, Huang, Weir, & Kirschvink, 1997
). Inactivation mutants
lacking either of the two proteins exhibited a loss of ∼50% of the cel-
lular iron quota and the induction of the iron stress response pathway
even under iron-replete growth conditions (
Keren, Aurora, & Pakrasi,
2004
). The double insertion mutant did not display a more severe phe-
notype, indicating that both proteins are required for effective iron stor-
age. Isolated bacterioferritins from
Synechocystis
PCC 6803 contain 2300
atoms of iron and 1500 molecules of phosphate per complex (
Laulhere
& Briat, 1993
).
