Biology Reference
In-Depth Information
mutant (constructed by transformation of
bfrA
+
bfrB
mutant with
mrgA
disruption construct) cultured
in iron-suffi cient (10 µM) media was conducted. In addition, cells of wild-type and three disruption
strains (
mrgA
,
bfrA
+
bfrB
and
bfrA
+
bfrB
+
mrgA
) were grown on iron-suffi cient (10 µM Fe in an EDTA
amended medium) and iron-limited (0.3 µM) media, washed free of iron, held in iron-free medium
and exposed to different concentrations of H
2
O
2
in darkness for 20 hrs. Such cells were also subjected
to DNA microarray analysis. A number of genes [such as
isiAB
and iron transport components
futC
and
feoB
, the putative outer membrane transporter (
slr1406
) and the entire ABC-type iron transport
genes (
slr1316
to
slr1319
)] were up-regulated in wild-type cells treated with DFB. Inactivation of
mrgA
showed signifi cant changes in the pattern of expression of 255 genes about half of which were
up-regulated (polysaccharide metabolism and all hypothetical genes) and half of them were down-
regulated (HliA,
ssl2542
and HliB,
ssr2595
). By contrast, treatment of
mrgA
mutant cells with DFB
affected the expression of 914 genes which is 4.8 times higher than that observed in wild-type. Direct
comparison of DFB-treated wild-type cells and
mrgA
mutant cells yielded 779 differentially regulated
genes. The down-regulated genes in
mrgA
mutant treated with DFB pertained to photosynthesis,
respiration, ATP synthase, cytochrome oxidase, NADH dehydrogenase (associated with low affi nity
and high affi nity CO
2
uptake mechanisms, 18 of 20 genes), phycobiliproteins, ferredoxin-nitrite
reductase (
nir
,
slr0898
), nitrate/nitrite transporter subunit (
nrtD
,
sll1453
), glutamine synthetase (
glnA
,
slr1756
), the glutamate synthase large subunit (gltB, sll1502) and the entire nitrate/nitrite transporter
system (
nrt
,
sll1450
-
sll1453
). The up-regulated genes belonged to
isiA
(
sll0247
, an 8-fold increase in
transcript level; and all the genes in this operon), fi ve detoxifi cation genes (of which catalase,
sll1987
and SOD,
slr1516
increased by 2-fold) and Fur-like protein (only
slr 1738
perR
gene, not the other
two Fur-like proteins,
sll0517
and
sll1937
). Virtually all genes under PerR regulon including
isiA
and
futA2
(
slr0513
) were up-regulated with a signifi cant increase in the transcripts of
perR
gene (13.3
fold) in
mrgA
mutant treated with DFB than in wild-type. These results provide strong relationship
between iron defi ciency and oxidative stress. Iron-defi cient wild-type cells could withstand H
2
O
2
stress (up to 8 mM) 2-fold higher than iron-suffi cient cells. Mutant
mrgA
was much more sensitive to
H
2
O
2
but
bfrA
+
bfrB
double mutant withstood exposure of up to 4 mM H
2
O
2
and were not affected by
iron availability. The triple mutant was much more sensitive to H
2
O
2
when compared to
bfrA
+
bfrB
or
wild-type. These results thus emphasize that the combined action of the two iron storage complexes
coordinate accumulation and gradual release of iron for utilization minimizing the oxidative damage
from its interactions with ROS produced in abundance during photosynthesis.
I) Salinity and oxidative stress:
The expression of
isiA
and
isiB
genes of
Synechocystis
sp. strain
PCC 6803 (Vinnenmeir
et al
., 1998) and several genes of Prxs of
Synechocystis
and
S
.
elongatus
PCC 7942 (Stork
et al
., 2005) were highly induced in response to salt stress. Bagchi
et al
. (2007)
characterized a mutant of
S
.
elongatus
PCC 7942 that exhibited high tolerance to salinity as well as
high constitutive expression of
isiA
gene.
S
.
elongatus
PCC 7942 transformed with
katE
from
E
.
coli
showed overexpression of catalase with a concomitant resistance to salinity (Kaku
et al
., 2000). The
observations on
A
.
doliolum
and
M
.
aeruginosa
point out that high salinity caused oxidative damage
due to breakdown of ROS scavenging mechanism (Singh and Kshatriya, 2002) and induced the
release of H
2
O
2
(Ross
et al
., 2006), respectively. Finally it is concluded that any physiological condition
that decreases the balance of ATP to NADPH would result in ROS production and hence oxidative
stress (Latifi
et al
., 2009).
