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help in scavenging superoxide radicals and hydrogen peroxide, respectively have been identifi ed
(Nakamura et al ., 2002). Due to the high concentrations of metal ions in the coastal environment,
Synechococcus sp. strain CC9311 makes greater use of copper for purposes of making Cu-Zn SOD
not seen in marine cyanobacteria. A multicopper oxidase characteristically present in this strain
helps in the oxidation or detoxifi cation of higher levels of reduced copper (Palenik et al ., 2006). It
is suggested that oxidative stress in M . aeruginosa PCC 7806 leads to programmed cell death (PCD)
akin to the capsase-like activity of eukaryotic cells that mediates PCD. Of the fi ve genes identifi ed
in M . aeruginosa PCC 7806, three of them ( Mic0980 , Mic3930 and Mic4051 ) showed similarity in
sequences to those present in M . aeruginosa NIES-843. But due to lack of capsase-like activity in these
gene products their role in processes other than PCD is indicated. The gene products of the other
two genes Mic1068 and Mic5406 showed some resemblance to human capsase proteins (Frangeul
et al ., 2008).
6) Regulatory functions : The genes allocated for regulatory functions in nine cyanobacteria have been
listed in Table 10. Among these, genes encoding two-component regulatory systems, transcription
factors and circadian clock proteins have been identifi ed in different genomes of cyanobacteria.
These are presented in the following sub-sections.
a) Two-component regulatory systems : The signal sensing and transmitting component consists of a
sensor histidine kinase (or transmitter) which autophosphorylates an invariant histidine residue
in an ATP-dependent mechanism in response to an environmental signal. The phosphorylated
transmitter transfers the phosphate to an invariant aspartate residue in a cognate receiver protein
called response regulator (or receiver). The response regulators often have an input domain that
defi nes a DNA-binding motif through which the protein regulates transcription, although some have
an output domain. There also exists a class of complex signal transducers that contain both transmitter
and receiver domain. Genes for two-component systems identifi ed in some cyanobacterial genomes
are listed (Table 10). Although genes for biosynthesis of cAMP and cGMP have been identifi ed in a
number of cyanobacteria (Katayama and Ohmori, 1997; Kasahara and Ohmori, 1999; Terauchi and
Ohmori, 1999; Ochoa de Alda and Houmard, 2000; Ochoa de Alda et al ., 2000), for the fi rst time in
Synechocystis sp.strain PCC 6803 two putative cyclic nucleotide phosphodiesterases (the enzymes
that are involved in the degradation of cyclic nucleotides cAMP and cGMP) encoded by genes sll1624
and slr2100 have been identifi ed. The presence of fi ve proteins (probably cyclic nucleotide receptors)
Table 10: Distribution of genes related to regulatory functions in the genomes of some cyanobacteria.
Two-component systems
No. Name
Sensory
Response
Hybrid type
Ser/Thr protein
kinases
regulators
kinases
1.
Anabaena sp. strain PCC 7120
71
71
53
-
2.
G . violaceus PCC 7421
27
37
12
-
3.
M. aeruginosa NIES-843
22
23
-
-
4.
N . punctiforme PCC 73102
255
55
5.
Synechocystis sp. strain PCC 6803
26
38
17
-
6.
Synechococcus sp. WH8102
5
9
-
-
7.
Synechococcus sp. CC9311
11
17
-
-
8.
S . elongatus PCC 6301
13
21
3
-
9.
T . elongatus BP-1
17
27
6
11
(1) Kaneko et al . (2001); (2) Nakamura et al . (2003); (3) Kaneko et al . (2008); (4) Meeks et al . (2001); (5) Kaneko et al . (1996); (6)
& (7) Palenik et al . (2003); (8) Sugita et al . (2006) and (9) Nakamura et al . (2002).
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