STRESS RESPONSES - Cyanobacteria - page 678

SODs thus scavenge O 2 •- radicals from the cells in a reaction that involves the formation of H 2 O 2 .
It can be written as:
2 O 2 •- + 2 H + H 2 O 2 + O 2 (Reaction 1).
The rate of this reaction very much depends on the rate of diffusion of O 2 •- anion into the active
site of the enzyme and the availability of hydrogen ions (Getzoff et al ., 1992). Depending on the type
of prosthetic metal group present at the active site of the enzyme, SODs are classifi ed into iron (Fe-
SOD; sodB ), manganese (Mn-SOD; sodA ), copper,zinc (Cu,Zn-SOD; sodC ) and nickel (Ni-SOD; sodN )
containing enzymes. Among these, Fe-SOD is widely distributed in the algae, cyanobacteria and other
prokaryotes and the stroma of chloroplasts. Though Mn-SOD is widely distributed in all prokaryotes
and eukaryotes, its occurrence in cyanobacteria is restricted. After its fi rst discovery in Streptomyces sp.
with Ni as the catalytic metal (Youn et al. , 1996), Ni-SODs have been described subsequently in many
groups of organisms. In 1997, Fridovich wrote in his review that research in this fi eld has reached a
happy state of maturity. Truely enough, we now know the crystal structures of all the SODs and also
the lethal effects of the accumulated mutant proteins of the SODs (Miller, 2004).
The distribution of the different metal forms of SODs among cyanobacteria has been reported.
A comparison of 64 cyanobacterial SOD sequences (including 24 completely sequenced genomes)
revealed the occurrence of one sodN homologue mainly in P . marinus strains (AS9601, CCMP1986,
CCMP1375, MIT9301, MIT9303, MIT9211, MIT9312, MIT9313, MIT9515, NATL1A, NATL2A),
Synechococcus sp. (strains BL107, CC9605, CC9311, WH8102) C . watsonii WH8501 and T . erythraeum
IMS101. Cu,Zn-SODs have been noted in strains of Synechococcus (BL107, CC9605, CC9311, CC9902,
RS9916, WH7805) and Lyngbya sp. PCC 8106. The sodA gene sequences have been observed in
C. watsonii WH8501, Synechococcus sp. (WH5701, RS9917), T . elongatus BP-1, T . erythraeum IMS101,
P. boryanum UTEX485, Leptolyngbya valderiana BDU20041, N . punctiforme PCC 73102, and
A . variabilis ATCC 29413. Among these, P . boryanum has three genes and N . punctiforme PCC 73102 and
A . variabilis ATCC 29413 have two genes each while the others have one gene each encoding Mn-SOD.
By far the largest distribution is that of Fe-SODs that are present in Synechococcus sp. (strains RS9916,
RS9917, JA-3-3Ab, JA-2-3B'a, WH5701, WH7805; one gene each), S . elongatus PCC 6301 (two genes),
S . elongatus PCC 7942 (two genes), Synechocystis sp. strain PCC 6803, Cyanothece sp. CCY0110,
T . elongatus BP-1, G. violaceus PCC 7421, Lyngbya sp. PCC 8106, S . platensis , P . boryanum UTEX485 (one gene
each), Anabaena ( Nostoc ) sp. PCC 7120 (two genes), N . punctiforme PCC 73102, A . variabilis ATCC 29413,
N . linckia , N . commune , N . commune CHEN (one gene each; Regelsberger et al ., 2004; Priya et al .,
2007). This is in so far as the distribution of individual SODs in different cyanobacteria is concerned.
But a close scrutiny reveals that while it is true that Ni-SOD is the only form found in many of the
unicellular strains, a combination of Ni-SOD with either Cu,Zn-SOD ( Synechococcus sp. strains BL107,
CC9605, CC9311) or Mn-SOD ( C . watsonii WH8501 and T . erythraeum IMS101) is also known. The
occurrence of Cu,Zn-SOD is considerably rare but its occurrence in association with Fe-SOD is also
seen ( Synechococcus sp. strains RS9916, WH7805 and Lyngbya sp. PCC 8106). The presence of Fe- and
Mn-SOD forms is noted in unicellular ( Synechococcus sp. strains WH5701, RS9917, T . elongatus BP-1,
G . violaceus PCC 7421), fi lamentous non-heterocystous ( P . boryanum UTEX485) and heterocystous
forms [ Anabaena sp. strain PCC 7120, A . variabilis ATCC 29413 and N . punctiforme PCC 73102; Priya
et al ., 2007]. However, Dupont et al. (2008) reported that P. marinus SS120 and Synechococcus sp.
strain CC9902 also possess a sodN gene in each of them besides the sodC gene sequence in the latter.
According to them Synechococcus sp. RS9917 has Fe and Cu,Zn-SODs but according to Priya et al .
(2007) this strain possesses Fe- and Mn-SODs. Dupont et al . (2008) further showed the presence of
Fe-SOD in Synechococcus sp. RCC307 and Synechococcus sp. strain WH7803 but the latter contained
a gene encoding Cu,Zn-SOD as well.