STRESS RESPONSES - Cyanobacteria - page 703

was completely abolished one hour after iron removal but there was no effect on the expression of
either isiB or psbC . In the chromosome of Anabaena sp. strain PCC 7120, alr2502 is located immediately
upstream of a gene ( alr2503 ) encoding a Prx. In the wild-type strain while alr2503 is expressed under
iron-limiting conditions, in case of Mp22 the expression of alr2503 ia also abolished suggesting that
pkn22 regulates both isiA and alr2503 induced by iron limitation. When Anabaena sp. strain PCC 7120
was subjected to iron-defi cient conditions, the cells experienced a 100-fold increase in the amount
of ROS compared to non-starved cells. Uncoupling of oxidative stress from iron defi ciency by the
use of a ROS quenching molecule tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl, a low
molecular weight membrane permeable antioxidant) resulted in the non-induction of isiA gene (that
encodes CP43' protein) even under iron defi ciency (Latifi et al. , 2005). Further, iron depletion also
caused the induction of a PrxQ-A and a mutant defi cient in the synthesis of this peroxidase was not
viable under iron-defi cent conditions.
Two ferretin-type storage complexes function in Synechocystis sp. strain PCC 6803 for the storage
of iron inside the cells. These are bacterioferretin and MrgA [a member of the DPS for DNA-binding
proteins from starved cells) proteins. Bacterioferretins (encoded by bfr genes) function as ferroxidases
(BFR ferroxidases) that oxidize Fe 2+ to Fe 3+ while generating H 2 O 2 and Fe 3+ is stored as iron oxide
in the cavity at the center of their 24-mer ultrastructure (Lewin et al ., 2005). Two bacterioferretin
proteins one with a conserved haem ligand and the other with a conserved di-iron center are encoded
by two bfr genes of Synechocystis . Mutant cells defi cient in the production of either of these two
proteins experienced a loss of nearly 50% of the cellular quota of iron even while growing under
iron-suffi cient conditions (Keren et al ., 2004). DPS proteins differ from other members of ferretin
proteins in the absence of fi fth C-terminal helix (Zeth et al ., 2004; Lewin et al ., 2005). Besides their
main function of storage of iron, DPS proteins also act as DNA-binding proteins protecting DNA
against oxidative stress, cold shock proteins, neutrophile activators or pili components (Zeth et al .,
2004). The DPS proteins form large (~150 kD) hexameric complexes that bind chromosomal DNA with
little sequence specifi city. As BFR ferroxidases generate H 2 O 2 , DPS proteins exhibit catalase activity
donating the electron from Fe 2+ to H 2 O 2 (Li et al ., 2004). Site-directed mutants of dpsA of S . elongatus
PCC 7942 exhibited very high sensitivity to photooxidative stress induced by high light and MV
treatment (Dwivedi et al ., 1997). Inactivation mutants of mrgA of Synechocystis sp. strain PCC 6803
grew much slower than wild-type when transferred from iron replete to iron-depleted conditions.
Moreover, when grown in iron-suffi cient conditions the internal quota of iron of mrgA mutants very
much resembled those of wild-type cells unlike the BFR mutants (Shcolnick et al ., 2007). Since in
Synechocystis sp. strain PCC 6803, mrgA gene is a part of PerR regulon that is up-regulated during
peroxide stress, an mrgA disrupted mutant was tested for its peroxide and light stresses. This mutant
was found to be highly sensitive to very low peroxide levels but showed the up-regulation of a gene
cluster ( sll1722 - sll1726 ) responsible for the synthesis of exopolysaccharide substance (EPS). Mutants
of wild-type and mrgA mutant disrupted in EPS cluster were found highly sensitive to oxidative
stress. In the absence of EPS the cells became more sensitive to peroxide and light. So the cells of
Synechocystis sp. strain PCC 6803 are liable to be damaged more readily to external oxidative and
light stresses in the absence of mrgA (Foster et al ., 2009). A comprehensive study by Shcolnick et al .
(2009) revealed a direct relationship between iron homeostasis and oxidative stress in Synechocystis
sp. strain PCC 6803. A comparative DNA microarray analysis of wild-type, wild-type cells treated
with DFB (deferoxamine B; an iron chelating agent), mrgA and perR mutants (Li et al ., 2004), mrgA
mutant treated with DFB, bfrA + bfrB double mutant (Keren et al ., 2004) and bfrA + bfrB + mrgA triple