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sll1621 and slr1738 . Thus the Fur homologue slr1738 of Synechocystis plays an important regulatory
role in the induction of antioxidant sll1621 in response to oxidative stress (Kobayashi et al ., 2004).
Two ORFs, sll1621 and slr1198 of Synechocystis sp. PCC 6803 encode AphC/TSA family protein
and antioxidant protein, respectively (Nakamura et al ., 1998). Hosoya-Matsuda et al . (2005) recognized
these proteins by Trx-affi nity chromatography and showed that these proteins are identical in
sequence to Type II Prx ( sll1621 ) and 1-Cys Prx. ( slr1198 ). Recombinant SLL1621 protein expressed in
E . coli when purifi ed not only interacted with Trx but also exhibited glutathione-dependent peroxidase
activity. Gene ( sll1621 ) disruptant mutant was not viable even under very weak light conditions
emphasizing the essential role of this Prx for photosynthetic growth. Although protein SLR1198 did
not show peroxidase activity, the slr1198 disruptant mutant exhibited poor growth. A bioinformatic
analysis of the genomes of Synechocystis sp. strain PCC 6803 and S . elongatus PCC 7942 for Prxs
and their transcript accumulation revealed that 1-Cys and 2-Cys Prxs of both organisms exhibited
very high similarities of 90% and 88% with identities of 83% and 73%, respectively. The two PrxQ
genes ( sll0221 and sll0242 ) of Synechocystis and one PrxQ gene (1668) of S . elongatus are atypical in
possessing only one conserved Cys residue where as the other three PrxQ genes (310,439 and 662)
of the latter organism possess two Cys residues in the conserved region that are spaced by only few
amino acids similar to the eukaryotic PrxQs (Stork et al ., 2005). The mRNA levels of the fi ve Prxs of
Synechocystis and six Prxs of S . elongatus under standard growth and induced oxidative stress (H 2 O 2
5 mM, MV 50 µM, light intensity 20, 200 and 800 µE m -2 s -1 for 48 hrs, iron limitation and NaCl 0.6
M) conditions exhibited differences in between the two organisms. With respect to light intensity,
the transcript levels of most of the Prx genes of Synechocystis (1-Cys Prx, 2-Cys Prx, Type II Prx and
PrxQ-B1) accumulated to a higher level at medium and high light intensities whereas those (1-Cys
Prx, 2-Cys Prx and PrxQ-A1) of S . elongatus were already up-regulated under low light intensity
itself. The transcripts of Type II Prx showed highest up-regulation in response to H 2 O 2 exposure
than MV-treated cells consistent with earlier studies (Kobayashi et al ., 2004; Hosoya-Matsuda et al .,
2005). By contrast, in S . elongatus the transcript levels of 2-Cys Prx were strongly up-regulated in
H 2 O 2 as well as after MV-treatment confi rming the earlier work of Perleman et al. (2003). In case of
NaCl stress, most of the Prxs in both the organisms were up-regulated at different times, i.e. at 24
or 48 hrs (Stork et al ., 2005). Gene alr2503 of Anabaena sp. strain PCC 7120 that encodes a putative
PrxQ homologue is located in the same gene cluster as pkn22 which ecncodes a Ser/Thr kinase. A
knock-out mutant of pkn22 (Mp22) was very sensitive to oxidative stress because of its failure to
express prxQ - A . The mutant tolerated oxidative stress only after the introduction and expression
of the prxQ - A gene in trans position. The properties of the recombinant PrxQ-A overexpressed in
E . coli revealed that it (i) it protects DNA from being degraded by ROS; (ii) reduces H 2 O 2 in presence
of dithiothrietol and (iii) shows Trx-dependent peroxidase activity. The recognition of Cys-47 as
the peroxidative residue and its replacement by Ser completely abolished the peroxidase activity.
Type II Prx of Anabaena sp. strain PCC 7120 is a hybrid protein with a Grx domain fused at the
C-terminal end that contains the conserved -CXXC- domain. This hybrid protein was expressed in
E . coli and shown to have the highest peroxidase activity toward H 2 O 2 using glutathione as electron
donor. The existence of this protein in both vegetative cells and heterocysts has been demonstrated
by immunoblot analysis (Hong et al ., 2008). Pérez-Pérez et al . (2009) conducted a comprehensive
analysis of Prxs of Synechocystis sp. PCC 6803 and demonstrated that fi ve Prxs (1-Cys Prx, 2-Cys
Prx, Type II Prx and PrxQ1 and PrxQ2) of this organism could use TrxA, TrxB and TrxQ as electron
donors. Glutathione or Grxs were ineffi cient as electron donors for the Prxs. Type II Prx and TrxQ
combination proved to be the best that showed highest catalytic effi ciency and this coincided
with the maximum levels of expression of the corresponding gene transcripts and proteins under
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