STRESS RESPONSES - Cyanobacteria - page 702

to Ser of the active site; Cys-Pro-Phe-Cys to Cys-Pro-Phe-Ser) immobilized on to an affi nity column,
cytosolic proteins were passed through the column. Among the 42 proteins so identifi ed 26 of
them were novel to the cyanobacterium. These proteins are supposed to be involved in 12 cellular
processes including oxidative stress response, Calvin's cycle, protein synthesis etc. Most importantly
the interaction of Grx2061with Type II Prx, catalase-peroxidase, Trx-reductase and Grx1562 has been
studied in detail. Loss of catalase activity (to nearly 30%) in presence of Grx suggested that catalase
was Grx-regulated. Type II Prx acivity confi rmed that it could also be regulated by Grx2061. So it
means that Grx-dependent activation of Prx and deactivation of catalase can occur in presence of
Grx2061. Another interesting feature is the interaction of Trx-reductase with Grx2061 that is reported
for the fi rst time from a photosynthetic organism. The interaction of Grx2061 with Grx1562 indicated
that oxidized Grx2061 could keep redox equilibrium and can be reduced by Trx reductase. It means
that Grx2061 can accept electrons from either glutathione or Trx-reductase. So the redox reactions
pass through NADPH-TR-GSH-Grx and protein.
The progress made in redox biology with the help of proteomics using redox-affinity
chromatography, two-dimensional electrophoresis and mass spectrometry has greatly improved
our understanding of the intricate network of physiological processes at work in cyanobacteria
experiencing oxidative stress (Nishiyama and Hisabori, 2009). Trxs and Grxs thus constitute unifying
elements in redox biology. Both proteins act by changing the structure and activity of a broad spectrum
of proteins, typically by modifying redox status. The range of target proteins in case of Trx and Grx
is bound to increase as new types of orthologues of Trx and Grx are yet to be discovered through
out the animal and plant kingdoms (Meyer et al ., 2009).
Oxidative stress in relation to iron limitation, high salinity, nitrogen metabolism and UV-
irradiation has also received attention (Latifi et al ., 2009). A direct relationship between iron limitation
and oxidative stress has been established by recent investigations. These will be presented along
with observations that connect salinity and oxidative stress.
H) Iron defi ciency and oxidative stress: The global transcriptional regulator, Fur has been identifi ed
to couple iron homeostasis and defence against ROS. Fur functions as a positive regulator of genes
encoding iron storage proteins and SODs. This has led to the suggestion that lack of iron regulation
might cause oxidative stress. Secondly, through Fenton reaction the reduced iron form can be used
to convert less reactive H 2 O 2 to more reactive oxygen species leading to oxidative damage. The
induction of isiA and isiB genes has been noted under iron defi ciency and also several other stresses
including oxidative stress (Huvaux et al ., 2005; Kouril et al ., 2005). In response to iron limitation, some
of the cyanobacterial species ( S . elongatus PCC 7942, Synechocystis sp. strain PCC 6803 and Anabaena
sp. strain PCC 7120) show the expression of isiA gene (Burnap et al ., 1993; Leonhardt and Straus,
1994; Xu et al ., 2003). The induction of isiA has also been observed under oxidative stress conditions
(Jeanjean et al ., 2003; Yousef et al ., 2003; Li et al ., 2004; Singh et al ., 2004). Xu et al . (2003) reported that
a putative Ser/Thr kinase gene pkn22 ( alr2502 ) in Anabaena sp. strain PCC 7120 is induced by both
iron starvation and oxidative stress and regulates the expression of isiA . A pkn22 insertion mutant
(Mp22 obtained by the insertion of a spectinomycin/streptomycin resistance cassette) was unable to
grow to the extent as that of wild-type under iron-limiting conditions and the gowth was completely
arrested in iron-depleted conditions. The expression of genes isiA , isiB , psbC and pkn22 was studied
in presence of iron and under iron-defi cient conditions both in wild-type and the Mp22 mutant.
Additionally, MV-treated cells were also subjected to measure the transcripts for pkn22 . In the wild-
type, the trancripts for pkn22 , isiA , isiB and psbC were noted in iron-limited conditions and in MV-
stressed cells showed transcripts for pkn22 . However, in case of Mp22 mutant the expression of isiA