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presence of an active SOD has been demonstrated by 63 Ni labelling studies and DGGE. Moreover,
Synechococcus sp. strain WH7803 showed a requirement of Ni for its acclimatization in low-iron and
high-light conditions of growth and depletion of Ni affected other metabolic pathways leading to
a low SOD activity (Qiu and Price, 2009).
B) Monofunctional catalases (EC 1.11.1.6): Cyanobacteria in general lack genes for the synthesis of
typical monofunctional catalases. The only complete gene sequence for a typical catalase is present
in N . punctiforme PCC 73102 with a proximal haem ligand Tyr and the conserved distal residues
His, Asn and Ser (Zámocký et al ., 2008). This enzyme contains haem b at the active site and uses
NADPH as a redox-active co-factor. Truncated C-terminal gene sequences are present for typical
catalases in S . elongatus PCC 7942 and Cyanothece sp. ATCC 51142. In Anabaena sp. strain PCC 7120
a fusion protein with a typical catalase-related domain was reported that bears closer resemblance
to the fusion protein of allene oxide synthase from Plexaura homomalla and related corals (Oldham
et al ., 2005). Some of the important properties of monofunctional catalases are that: (i) they can be
inhibited by 3-amino-1,2,4-triazole, an inhibitor of eukaryotic catalases; (ii) their activity is constant
over several pH units and (iii) they can not be reduced by dithionite. On the basis of their size, two
subgroups are recognized, the small subunit (55 to 69 kDa) enzymes associated with haem b and
the large subunit enzymes (75 to 84 kDa). The monofunctional catalase of N . punctiforme PCC 73102
belongs to the former group. The monofunctional catalases characterized in great detail are known
to exist as homotetramers (Obinger et al ., 1997; Jakopitsch et al ., 1999).
Another minor gene family of catalases is represented by Mn-catalases (non-haem or dimanganese
catalases or Mn-Cats). With the exception of G . violaceus PCC 7421, one ORF with similarity to Mn-
Cat has been reported in the heterocystous, diazotrophic species. There are two [ N . punctiforme PCC
73102 and Anabaena sp. strain PCC 7120] and three ( Cyanothec e sp. PCC 7424) paralogues for Mn-
Cats in certain of them. At least ~50% of the investigated species contain neither a typical catalase
nor bifunctional catalase-peroxidase or Mn-Cat. Studies on functional aspects of Mn-Cat genes
(structural analysis, transcription or expression) do not exist in literature.
C) Bifunctional catalase-peroxidases (KatGs; EC 1.11.1.7 ) : Initially ascorbate peroxidase (APx)
activity was reported from crude extracts of cyanobacteria (Miyake et al ., 1991). So it was thought
convenient to distinguish cyanobacteria into two groups, those possessing APx and those that do not.
But analysis of 44 fully or partially sequenced cyanobacterial genomes did not reveal the presence
of gene sequences for APx (Bernroitner et al ., 2009). KatGs are the only peroxidases that possess a
very high catalase activity besides a substantial peroxidase activity and do not possess any sequence
homology to the monofunctional catalases. But KatGs bear suffi cient homology to plant APxs and
yeast cytochrome c peroxidase (CcP) and that is why they are included in the Class I superfamily
of peroxidases to which plant, fungal, protist and bacterial haem peroxidases belong (Welinder,
1992; Passardi et al ., 2007). The KatG encoding genes are present in nearly 30% of the cyanobacterial
genomes with one katG gene in each of them. For example one KatG gene is found in G . violaceus
PCC 7421, S . elongatus PCC 6301, S . elongatus PCC 7942, eight strains of Synechococcus sp. (strains
WH7803, WH7805, CC9605, RS9916, RS9917, RCC307, WH5701, PCC7002), Synechocystis sp. strain
PCC 6803, A . marina MBIC11017 and Cyanothece sp. CCY0110. Of these, only the last mentioned
species is known to fi x nitrogen and the rest are unable to fi x nitrogen. In all P . marinus strains
(AS9601, MIT9211, MIT9215, MIT9301, MIT9303, MIT9312, MIT9515, NATL1A, NATL2A, subsp.
marinus strains CCMP1375, CCMP1986) some Synechococcus strains [BL107, CC9311, CC9902, JA-
2-3B'a(2-13) and JA-3-3Ab], M . aeruginosa NIES-843, T. elongatus BP-1,, C . watsonii WH8501, strains
of Cyanothece (ATCC 51142, PCC 7424, PCC 8801) Lyngbya sp. PCC 8106, T . erythraeum IMS101,
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