STRESS RESPONSES - Cyanobacteria - page 685

strain PCC 7120 resulted in a strain (A17) that could not grow in the absence of combined nitrogen
and was more sensitive to photoinhibition caused by increased light intensity. The restoration of
wild-type character was possible by complementing the wild-type sodA gene in A17. The presence
of Mn-SOD in vegetative cells and heterocysts was confi rmed by immunoblotting. The inability
of A17 to grow in a nitrogen-free medium was explained on the basis that the ROS produced in
heterocysts inactivated nitrogenase in the absence of Mn-SOD. Thus the membrane associated Mn-
SOD protects the photosynthetic apparatus as well as nitrogenase from oxidative damage (Zhao et
al ., 2007). L . valderiana BDU20041, a marine isolate tolerant to acid black1, exhibited a 3-fold increase
in the activity of Mn-SOD. The cloned gene (550 bp) exhibited 63.8% homology with other bacterial
Mn-SODs and its deduced amino acid sequence revealed the active site residues to be His4, His58
and Asp141 along with highly conserved Mn-specifi c residues. The production of ROS in presence of
acid black1 was confi rmed by incubating the dye-treated cells with 2',7'-dichlorofl uorescein diacetate
(5 µM) in dark at 27 ± 2°C for one hour and fl uorescence measured in a spectrofl uorimeter confi rmed
the presence of ROS (Priya et al ., 2010).
Phylogenetic analysis of various Fe- and Mn-SODs (83 SOD sequences of which 2 bacterial, three
fungal, and six cyanobacterial Mn-SODs and eight cyanobacterial Fe-SODs included) revealed that:
(i) the Fe-SODs were normally well separated from the Mn-SODs; (ii) combialistic SODs that can
accept either Fe- or Mn- as co-factors are generally grouped with Fe-SODs; and (iii) the Fe-SODs of
Archaea and thermophilic bacteria form a clade within the Mn-SOD cluster. The identity and per
cent similarity of various cyanobacterial Mn-SODs (given in parentheses) are Anabaena sp. strain
PCC 7120 — N . punctiforme M1 (57/71), Anabaena sp. strain PCC 7120— N . punctiforme M2 (68/78),
Anabaena sp. strain PCC 7120— P . boryanum (56/70) and Anabaena sp. strain PCC 7120— E . coli (50/63)
and that of Anabaena sp. strain PCC 7120 and E . coli Fe-SOD is 44/59. Phylogenetic analysis of Fe-
SODs showed that the fi lamentous species formed a well separated cluster and the unicellular forms
were well segregated into another cluster (Regelsberger et al ., 2002, 2004). All the four metalloforms
of SODs formed distinct clusters in a radial Neighbour-Joining analysis of 105 cyanobacterial SOD
sequences (of which 38 are Fe-SODs, 36 Mn-SODs, 13 Cu,Zn-SODs and 18 Ni-SODs; Priya et al .,
2010; Fig. 19).
An obligate requirement of nickel is shown by marine picoplanktonic cyanobacteria for the
expression of sodN gene regardless of the presence of the type of nitrogen source in the medium.
In all the strains of Prochlorococcus and some of the Synechococcus sp. Ni-SOD is alone represented.
The presence of ORFs bearing signifi cant similarity to Ni-SOD pre-proteins has been demonstrated
in some of these cyanobacterial genomes sequenced (Dufresne et al ., 2003; Rocap et al ., 2003). The
presence of an ORF that encodes a peptidase ( sodX ) downstream of the sodN gene in P . marinus
strains (MED4, MIT9313, and SS120), Synechococcus sp. strain IMS101 and C . watsonii WH8501
has been detected (Eitinger, 2004). When oxygen-sentive E . coli mutant lacking the endogenous
Fe- and Mn-SOD was transformed with sodN and sodX from P . marinus MIT9313, in the transformant
co-expression of both the genes took place. The recombinant SOD of E . coli cells restored oxygen
tolerance. It means for the production of catalytically active Ni-SOD, the co-expression of sodX with
sodN is essential. A comparison of Synechococcus sp. strain CC9311 (a coastal isolate) and Synechococcus
sp. strain WH8102 (an isolate from open oceans) for Ni-requirement showed that the former strain
that possesses Cu,Zn-SOD besides Ni-SOD was able to withstand the Ni-defi ciency because of its
slow growth in presence of NH 4 + and in the absence of Ni whereas the latter strain that contained
only Ni-SOD could not grow in any nitrogenous medium devoid of Ni. It means Synechococcus strain
CC9311 had a greater tolerance for Ni-starvation and that the Cu,Zn-SOD could not completely
replace the Ni-SOD (Dupont et al ., 2008). Likewise, Ni-starvation inhibited growth of Synechococcus