STRESS RESPONSES - Cyanobacteria - page 700

growth rate. Of the four isoforms of Trx, the gene product of slr0623 , the homologue of m -type Trx
(i.e. TrxA) is the most abundant type and that accumulation of Trx isoforms is dependent on the
expression of the redox-related proteins. A comparative analysis of NTR-2-Cys Prx system from
plants and cyanobacteria has been presented and the gene sequences of NTR from Anabaena sp.
strain PCC 7120 could complement the knock-out mutants for NTR gene of Arabidopsis thaliana but
not those from Synechocystis sp. strain PCC 6803. So in this respect the Anabaena enzyme fulfi lls the
role of plant enzyme (Pascual et al ., 2011).
F) Glutathione (GSH): GSH is a tripeptide of γ-L-glutamyl-L-Cysteinyl-Glycine and it is a non-
protein thiol. It is widely distributed in some prokaryotes and in all eukaryotes with the exception
of those that lack chloroplast or mitochondria. Among the prokaryotes, it is present in cyanobacteria,
proteobacteria as well as in a few strains of gram-positive bacteria. The concentration of GSH ranges
from 0.1 to 10 mM in bacteria (Fahey et al ., 1978; Fahey and Sundquist, 1991; Newton et al ., 1996).
The thiol group of GSH is responsible for its antioxidant activity which forms a thiyl radical on
oxidation, and then reacts with a second oxidized glutathione to form disulphide bonded dimeric
form GSSG. The ratio of GSH:GSSG is carefully controlled in the cytoplasm and it mostly exists in its
reduced state. GSH functions directly as a free radical scavenger by reacting chemically with singlet
oxygen, superoxide and hydroxyl radicals. It plays a critical role in protection against oxidative
stress caused by peroxides such as H 2 O 2 or alkylhydroperoxides, and other environmental stresses
such as osmotic shock, acidity and toxicity due to certain chemicals (methylglyoxal, hypochlorous
acid and monochloramine). It also stabilizes membranes by removing acyl peroxides formed by
lipid peroxidation reactions. Glutathione peroxidase uses GSH to reduce H 2 O 2 producing GSSG.
The redox potential of GSH is suffi cient enough to reduce dehydroascorbate to ascorbic acid or to
reduce disulphide bonds of proteins.
The reduction of GSSG to GSH is catalysed by the enzyme glutathione reductase (GR) the product
of gor gene. Reducing equivalents from NADPH are used. Thus GR serves as the key link between
the two redox couples [GSH/GSSG and NAD(P)H/NAD(P)] in the cell. However, GSH and NAD(P)
H/NAD(P) do not exchange electrons directly. Multiple forms of GR exist in plants associated with
different subcellular compartments. In cyanobacteria, for example in Synechocystis no gene encoding
GR has been found. So how GSH could be regenerated from GSSG is not known (Li et al ., 2007).
Under oxidative stress, GSH serves as a protein reductant either directly or through the reduction of
Grxs. Glutathionylation is a process in which mixed disulphides are formed between GSH and the
cysteines of proteins. At any given time ~2% of the total glutathione content in E . coli cells is in the
form of protein disulphides (Miranda-Vizuete et al ., 1996). Protein glutathionylation seems to take
place by direct oxidation of GSH and protein thiol followed by the formation of mixed disulphide
bond (Thomas et al ., 1995; Hondrop and Matthews, 2004). This is a protective mechanism to prevent
cellular proteins from being inactivated by oxidative stress, because overoxidation of protein thiols
leads to the formation of sulfi nic acid or higher oxidation states. Such proteins cannot be reduced by
Trx or Grx and this leads to their irreversible inactivation. Glutathionylated proteins can be brought
back into an active state by Grxs that mediate the reduction of disulphides, once the organism is
brought back to normal environment (Ritz and Beckwith, 2001; Hondrop and Mathews, 2004). The
many faces of glutathione in bacteria have thus been exposed (Masip et al ., 2006).
NADPH-dependent glutathione peroxidases (GPxs; EC 1.11.1.9 and EC 1.11.1.12) catalyze the
reduction of H 2 O 2 and organic hydroperoxides to water and corresponding alcohols using GSH as
electron donor. Most of the cyanobacterial genomes (fully/partially) sequenced show the presence of
an ORF with homology to plant GPxs (all strains of P . marinus , 11 strains of Synechococcus , G . violaceus