STRESS RESPONSES - Cyanobacteria - page 699

acid sequences of Trxs T1 and T2 from Anabaena sp. strain PCC 7120 showed the former protein to be
similar to spinach chloroplast Trx m and the latter to a Grx. In this respect, cyanobacteria resemble
plant chloroplasts in having two dissimilar Trxs- one that is related to more common protein found
in prokaryotes and the other a Grx (Gleason, 1992).
The earliest studies on Trx gene disruption mutants of S . elongatus PCC 7942 (then described
as A . nidulans R2) corresponding to genes slr0623 and sll1980 of Synechocystis sp. PCC 6803 that
encode TrxA and a Trx-like protein respectively revealed that while the former is essential for
photosynthetic growth (Muller and Buchanan, 1989), the latter was required for correct assembly
of photosynthetic apparatus (Collier and Grossman, 1995). The fi rst indication that Trx can serve
as a source of reductant for Prxs came from the katG deletion mutant of Synechocystis that exhibited
a light-dependent thiol-specifi c peroxidase activity (Tichy and Vermaas, 1999) already described
earlier. Studies on Trx-deletion mutants of Synechocystis have shown that while TrxA is essential for
the survival of the organism (as deletion mutants could not be obtained), deletion of TrxQ caused
hypersensitivity to H 2 O 2 suggesting that TrxQ plays a role in peroxide tolerance (Nishiyama et al .,
2001; Pérez-Pérez et al ., 2009).
In order to identify Trx-linked processes in cyanobacteria, Lindahl and Florencio (2003) screened
cytosolic and peripheral membrane proteins by using an immobilized mutant form of Trx, TrxA
(where Cys35 was exchanged with Ser by site-directed mutagenesis) as a bait. The formation of a
covalent bond between the target proteins and TrxA was confi rmed by non-reducing/reducing two-
dimensional SDS/PAGE. Of the identifi ed cytosolic (18) and membrane-associated (8) proteins, 20
of them are new candidate substrates for Trx. Phosphoglucomutase is an important enzyme whose
activity was inhibited under oxidizing and activated under reducing (in presence of DTT) conditions
in vitro . Light-induced redox regulation by Trx seems to be extensive as processes of assimilation
and storage of carbon, sulphur and nitrogen, all enzymes of glycogen synthesis pathway (including
the rate-limiting enzyme ADP-glucose pyrophosphorylase), cyanophycin synthesis, ammonia
assimilation (GOGAT), carbon concentrating mechanism protein (CcmM) come under its infl uence.
This shows that reduced Trx has a profound effect on all the anabolic processes and depending
on the growth conditions the identities and relative abundance of Trx target proteins are likely to
change. Cyanobacterial redox-mediated signalling and the regulation of transcription of key genes
involved in acclimation response to a changing environment has remained obscure for a long time.
In this direction Nakamura and Hihara (2006) fi rst demonstrated that PedR, a LuxR-type regulator,
gets activated and inactivated under low and high light conditions, respectively with a concomitant
conformational change. Horiuchi et al . (2010) for the fi rst time showed that PedR interacts with Trx
to transmit the redox status of photosynthetic electron transport chain. This has been confi rmed
by the in vitro formation of intramolecular disulphide bond between Cys80 residue of the PedR
homodimer and its reduction by TrxM1 and TrxX. Mutants of Synechocystis that lack ferredoxin-Trx
reductase or NADPH-Trx reductase showed minimal conformational change in PedR or minimal
transcriptional regulation of key genes under high light. So a transient inactivation of PedR results
due to its reduction by Trx under high light. Trx-affi nity chromatography is proving to be a useful
method for understanding the Trx-network (Hisabori et al ., 2005).
The availability of reduced Trx very much depends on the reducing equivalents supplied by
either NADPH or reduced ferredoxin. In presence of two Trx reductases, NADPH-Trx reductase
(NTR) and ferredoxin-Trx reductase (FTR) the in vitro reduction of Trx has been demonstrated.
Hishiya et al . (2008) identifi ed the genes for these two Trx reductases and also four typical isoforms
of Trx. Their studies on NTR and FTR disruptant mutants suggested that the NTR-Trx pathway is
important for antioxidant system whereas FTR-Trx pathway may be benefi cial in the control of cell