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oxidation whereas point mutation in the conserved motif of thioredoxin domain did not affect the
NADPH oxidase activity (Sueoka et al. , 2009).
The genome of Synechocystis sp. strain PCC 6803 revealed the presence of fi ve genes encoding
Prxs, two genes of PrxQ ( sll0221 and slr0242 ) and one gene each of 2-Cys prx ( sll0755 ), 1-Cys Prx
( slr1198 ) and Type II Prx ( sll1621 ) (Kaneko et al ., 1996; Kobayashi et al ., 2004; Hosoya-Matsuda et al. ,
2005 ). In case of S . elongatus PCC 7942 six putative genes of Prxs have been detected, i.e. one each
of 1-Cys Prx (gene 915), 2-Cys Prx (gene 782) and four of PrxQ (genes 310, 439, 662 and 1668). The
2-Cys Prx gene ( sll0755 ) of Synechocystis sp. strain PCC 6803 has been cloned and this gene was
overexpressed in E . coli . This enzyme was able to reduce H 2 O 2 and tertiary butyl hydroperoxide with
thioredoxin from E . coli as electron donor (Yamamoto et al ., 1999). The purifi ed protein (22.5 kDa)
possesses two cysteine residues Cys50 and Cys172 required for the catalytic cycle. These are present in
highly conserved motifs -FTFVCPTEI- (Phe-Thr-Phe-Val-Cys-Pro-Thr-Glu-Ile, the so-called F-motif)
and EVCP (glu-Val-Cys-Pro). During the reaction cycle an intersubunit disulphide bond is formed
between Cys50 of one subunit and Cys172 of the other. Signifi cant conformational changes seem to be
necessary during the catalytic cycle (Hirotsu et al ., 1999; Schröder et al ., 2000). So this novel antioxidant
enzyme has been designated as thioredoxin peroxidase (TPx). Synechocystis cells were transformed
with pY7Blue-T plasmid having TPx gene interrupted with spectinomycin/streptomycin resistance
marker to generate tpx - cells. The tpx - mutant cells lost the peroxidase activity coupled with the
electron fl ow in PSII. These results indicate that the TPx activity is coupled to photosynthetic electron
transport system. Investigations on the in vivo role of 2-Cys Prx in Synechocystis sp. strain PCC 6803
and its homologue in S . elongatus PCC 7942 and the corresponding gene disruption mutants showed
that 2-Cys Prx is important in protecting the cells against peroxide stress (Klughammer et al ., 1998;
Yamamoto et al ., 1999). Moreover, studies on the 2-Cys Prx disruptant mutants of Synechocystis sp.
strain PCC 6803 revealed that this Prx is important in photosynthetic adaptation as in the mutants
the quantum yield decreased with a simultaneous saturation of electron transport rates at lower
light intensity. These results thus suggest that the 2-Cys Prx plays an important protective role in
photosynthesis (Klughammer et al ., 1998; Yamamoto et al ., 1999).
Perelman et al . (2003) identifi ed an ORF downstream of nblR gene sequence (that encodes a
response regulator NblR essential for cell survival under high light and nutrient starvation) of
S . elongatus PCC 7942 that is homologous to thioredoxin-peroxidase-like (TplA). To understand the
H 2 O 2 detoxifi cation mechanisms in this organism, they generated single and double mutants for katG
and tplA genes. The double mutant surprisingly tolerated H 2 O 2 stress and its ability to detoxify H 2 O 2
compared very well with the wild-type and TplA mutant cells. This suggested that KatG activity
was essential for this process and tplA gene was dispensable but when the cells experience oxidative
stress under high light conditions TplA appears to be essential for growth. Cells lacking TplA could
not compete with wild-type. These results emphasize different physiological roles for the enzymes
depending on whether oxidative stress is caused by high light or H 2 O 2 .
A DNA microarray analysis of Synechocystis sp. strain PCC 6803 cells exposed to oxidative stress
(MV for 15 min) showed signifi cant induction of genes ( sll1621 , slr1738 , slr0074 , slr0075 and slr05890 )
commonly under conditions of both normal and high light. When Fur-type transcriptional regulator
encoded by slr1738 (located near to sll1621 gene) has been deleted, the deletion mutant under non-
stressed conditions showed greatest depression in the expression of sll1621 . The gene product of
sll1621 , a Type II Prx, has been shown to be essential for aerobic photoautotrophic growth both under
high light (in liquid and solid media) and low light (solid media) conditions. The purifi ed His-tagged
recombinant protein SLR1738 produced in E . coli exhibited binding to the intergenic region between
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