STRESS RESPONSES - Cyanobacteria - page 698

different stress conditions. The eukaryotic 2-Cys Prxs have been shown be sensitive to overoxidation
due to the presence of structural Gly-Gly-Leu-Gly and Tyr-Phe motifs. The overoxidation of 2-Cys
Prx results in its inactivation leading to a transient increase in H 2 O 2 concentration. This results in
H 2 O 2 -mediated signalling and is known as fl oodgate hypothesis. Thus this adaptation allows 2-Cys
Prxs to act as fl oodgates, keeping resting levels of H 2 O 2 low while permitting higher levels during
signal transduction (Wood et al ., 2003). To know whether such an adaptation exists in cyanobacterial
2-Cys Prxs, Pascual et al . (2010) showed that several prokaryotic 2-Cys Prx sequences including
those of cyanobacteria contained Gly-Gly(Leu-/Val-/Ile)-Gly and Try-Phe motifs characteristic of
overoxidation sensitive ezymes. Further two cyanobacterial 2-Cys Prx enzymes (from Anabaena
sp. strain PCC 7120 and Synechocystis sp. strain PCC 6803) have been tested for their sensitivity to
overoxidation. The wild-type and mutant enzyme from Anabaena showed overoxidation at the Cys
residue. In response to high light and H 2 O 2 , the 2-Cys Prx from Anabaena was reversibly overoxidized
in vitro . These results emphasize that the two organisms adopt different strategies to cope up with
H 2 O 2 . Synechocystis has low content of less sensitive 2-Cys Prx and high catalase activity whereas
Anabaena possesses abundant, sensitive 2-Cys Prx but low catalase activity. In this respect the latter
comes closer to the chloroplast system.
E) Thioredoxins (Trxs): Trxs are small proteins (of molecular weight ~12,000) ubiquitously present
in all living organisms. The common active site sequence -Trp-Cys-Gly-Pro-Cys- is well conserved
in all Trxs. These proteins share a common structure known as Trx motif consisting of four α-helices
and fi ve β-sheets (Eklund et al ., 1984; Capitani et al ., 2000). In photosynthetic organisms a number of
processes are regulated by light and this can be due to light-driven changes in intracellular pH, ion
concentration or reduced co-factors. The levels of dithiol form of Trxs are enhanced by light-driven
enhancement in the levels of both NADPH and ferredoxin. Trxs in turn can function as reducing
agents in processes such as sulphur metabolism and deoxynucleotide synthesis. Trxs mediate
dithiol-disulphide exchange and thus regulate the activity of enzymes through the formation or the
reduction of disulphide bridge in the target enzymes. This leads to a modulation of enzyme activity
via a redox control. The known enzymes whose activity is controlled by Trxs belong to the Calvin
cycle. The regulation of chloroplast NADP-dependent malate dehydrogenase by Trx m was reported
(Scheibe, 1987) followed by the regulation of glyceraldehyde-3-phosphate dehydrogenase, fructose-
1,6-biphosphatase, sedoheptulose-1,7-biphosphatase and phosphoribulokinase by Trx f and these
are known as thiol enzymes in the chloroplasts (Marcus et al ., 1988; Porter et al ., 1988; Jacquot et al .,
1997). In prokaryotes, three phylogenetic groups of Trxs, m, x and y are known. Their corresponding
homologues occur in chloroplasts and algae (Meyer et al ., 2005) and in cyanobacteria the genome
of Synechocystis sp. strain PCC 6803 possesses one homologue from each of these families. These
are TrxA ( slr0623 ), TrxB ( slr1139 ) and TrxQ ( slr0233 ) belonging to m , x and y groups, respectively
(Navarro and Florencio, 1996; Florencio et al ., 2006; Pérez-Pérez et al ., 2006).
A Trx from Anabaena sp. 7119 (Gleason et al ., 1985) and A . nidulans ( S . elongatus PCC 7942; Muller
and Buchanan, 1989) that was similar to m -type Trx in amino acid sequence and functional properties
has been reported. Alam et al . (1989) identifi ed two thioredoxin genes in Anabaena sp. strain PCC
7120. One of these encodes an unusual Trx that activates spinach fructose-1,6-biphosphatase and
considered to be the counterpart of chloroplast Trx f (Gleason, 1990) but later designated as Trx 2
bearing no resemblance in amino acid sequence to known Trxs (Eklund et al ., 1991). Although the
gene that encodes Trx 2 has been cloned from Anabaena sp. strain PCC 7120 and expressed in E . coli , its
presence in the cells of Anabaena could not be demonstrated (Alam et al ., 1989). Western immunoblotting
revealed the presence of Trx 2 in very low levels and it can be reduced by glutathione. The amino