STRESS RESPONSES - Cyanobacteria - page 684

An interruption in the electron fl ow between the two (4Fe-4S) centers of PSI and cytochrome f was
detected by measuring fl ash-induced absorption transient at 705 nm (Herbert et al ., 1992). The sodB -
mutant of S . elongatus PCC 7942 showed similar photosynthetic pigments and catalase activity as the
wild-type but the growth rate of the wild-type was faster with higher cyclic photosynthetic electron
transport. Further, the sodB - mutant was more sensitive to oxidative stress induced by MV but not
to the oxidative stress caused by norfl urazon (an inhibitor that blocks the synthesis of phytoene
desaturase and so the synthesis of β-carotene and other carotenoids; Thomas et al ., 1998). Based on
the extensive studies made on P . boryanum (Asada et al ., 1975; Okada et al ., 1979; Steinitz et al ., 1979),
Campbell and Laudenbach (1995) cloned the SOD genes in P . boryanum UTEX485 by employing a
23 base oligonucleotide primer (containing the sequence 5'-GATGTCTGGGAACACGCTTACTA-3')
designed by the reverse translation of conserved region of amino acids at the C-terminal region of
SOD. Of the four genes identifi ed, sodB governs the synthesis of Fe-SOD enzyme whereas sodA1 ,
sodA2 and sodA3 encoded Mn-SOD enzymes. In MV-resistant mutants sodB and sodA1 genes were
constitutively expressed. Under conditions of oxidative stress induced by MV, iron and nitrogen
limitation, wild-type cells showed the accumulation of transcripts for sodA2 . The MV-resistant mutant
of P . boryanum showed high levels of sodA3 transcipt accumulation. The genome of Anabaena sp.
strain PCC 7120 consists of sodB and sodA genes encoding Fe- and Mn-SODs, respectively. The sodA
gene exhibited a high homology to the sodA2 gene from P . boryanum described earlier (Campbell
and Laudenbach, 1995). The sodB and sodA genes of Anabaena sp. strain PCC 7120 were differentially
expressed and differed in their localization patterns. Fe-SOD is present in the soluble fractions of
vegetative cells and heterocysts whereas Mn-SOD is localized in the thylakoid membranes attached
towards the luminal side by a motif at the N-terminus of the mature protein. The pre-protein of Mn-
SOD also possesses a leader peptide which is shown to be essential for its activity as confi rmed by its
expression in E. coli cells. Though signifi cant amounts of Fe-SOD have been detected in the isolated
heterocysts, they did not contain any Mn-SOD. These observations emphasize the differential role
of SODs in relation to the type of cells in which they are localized (Li et al ., 2002). Such a localization
of Fe- and Mn-SODs has also been confi rmed by the studies on Anabaena sp. strain PCC 7120.
Alongside, studies on kinetics of enzyme activities of Fe- and Mn-SODs led to the determination of
their catalytic rate constants (Regelsberger et al ., 2002, 2004). In order to fi nd out the importance of
N-terminal region and linker region of the Mn-SOD, two recombinant proteins were produced in
E . coli BL21 (DE3) cells transformed with an expression vector containing Mn-Sod gene constructs
which lacked either the N-terminal hydrophobic region (amino acids 2-29) or both the hydrophobic
and the linker region (amino acids 2-61). The recombinant Mn-SODs produced by the transformants
of E . coli were found to be homodimeric in organization and exhibited catalytic activity assayed by
the conventional ferricytochrome c and by stopped fl ow spectroscopy. These results emphasize that
the linker region is not involved in oligomerization and that the soluble catalytic portion is fully
active (Regelsberger et al ., 2002). They confi rmed the localization of Fe-SOD in the soluble fractions
of vegetative cells and heterocysts of Anabaena sp. strain PCC 7120 with the help of activity, staining,
inhibition patterns and immunogold labelling. E . coli BL21 (DE3) cells transformed with an expression
vector, pET-28a carrying the cloned sodB gene from Anabaena, produced the recombinant enzyme.
The purifi ed enzyme (having a molecular mass of 42 kDa) was shown to exist in a homodimeric
state (Regelsberger et al ., 2004). The Fe-SOD gene ( slr1516 ) of Synechocystis sp. strain PCC 6803 has
been cloned and its overexpression in E . coli BL21 (DE3) cells led to the production of recombinant
protein at signifi cantly higher levels. The recombinant protein was purifi ed by gel fi ltration and
ion-exchange chromatography to near homogeneity. The transformant E . coli cells tolerated the
oxidative stress induced by MV (Bhattacharya et al ., 2004). The sodA gene inactivation in Anabaena sp.