STRESS RESPONSES - Cyanobacteria - page 694

Prxs are generally divided into fi ve subclasses based on their different subunit composition
and the position and number of conserved cysteine residues. In the fi rst subclass, 1-Cys prxs, one
conserved cysteine residue is present in the N-terminal part of the enzyme whereas in the second
subclass, 2-Cys Prxs, two cysteine residues are present with one each at the N- and C-terminal
parts. Prx-Q is a chloroplastic enzyme which belongs to the third subclass, and is similar to E . coli
bacterioferretin comigratory protein. Type II Prxs are atypical members that belong to the fourth
subclass and possess only one conserved cysteine residue. These members can use Trx as well as
Grx as electron donors and multiple isoforms of this enzyme are localized in many sub-cellular
compartments. The last subclass is represented by NADPH-dependent glutathione peroxidase
(GPx)-like proteins that carry out reduction of H 2 O 2 and organic hydroperoxides to water and
corresponding alcohols (Bernroitner et al ., 2009).
A complete survey of the 44 (fully or partially) sequenced genomes of cyanobacteria revealed
the existence of gene sequences of all the fi ve subclasses of Prxs mentioned above. Except the 12
strains of P . marinus and 60% of the Synechococcus strains, the rest of the genomes of cyanobacteria
investigated possess one ORF with similarity to 1-Cys Prx. However, A . marina MBIC11017
contains two paralogues of 1-Cys Prx. The highly conserved sequence at the N-terminal domain is
Val-Leu-Phe-Ser-His-Pro-X-Asp-Tyr-Thr-Pro-Val-Cys-Thr-Thr-Glu- that has one cysteine residue.
The catalytic role of this enzyme has not yet been revealed in cyanobacteria. The presence of two
conserved cysteine residues is a characteristic feature of 2-Cys Prxs. The N-terminal conserved
domain possesses a sequence -Phe-Phe-Tyr-Pro-Leu-Asp-Phe-Thr-Phe-Thr=Phe-Val-Cys-Pro-Thr-
Glu- and the C-terminal possesses a 'resolving' cysteine residue -Val-Cys-Pro-. These are dimeric
enzymes and the two subunits are linked via a disulphide bond in the oxidized form. The presence
of one ORF similar to 2-Cys Prx has been reported in all the genomes of cyanobacteria investigated.
Type II Prxs are atypical Prxs in possessing one conserved cysteine residue. They utilize thioredoxin
as electron donor to reduce peroxides and so are called as thioredoxin peroxidases. The presence
of multiple isoforms of Type II Prxs has been demonstrated in the sub-cellular compartments of
plants. A gene sequence of Type II Prxs is present in 15% of P . marinus and Synechococcus strains,
M . aeruginosa , Synechocystis sp. strain PCC 6803, A . marina MBIC11017, C . watsonii WH8501, Cyanothece
(strains ATCC 51142, CCY0110, PCC 7424 and PCC 8106), T . erythraeum IMS101, A . variabilis ATCC
29413, No . spumigena CCY9414, N . punctiforme PCC 73102 and Anabaena sp. strain PCC 7120.
The cyanobacterial Type II Prxs contain a highly conserved sequence in the N-terminal region
with one cysteine (-Leu-Pro-Gly-Ala-Phe-Thr-Pro-Thr-Cys-Ser-Ser-) residue whereas the second
cysteine residue is not conserved in all species. It is absent in all P. marinus strains, in M . aeruginosa ,
T . erythraeum and all Nostocales. Type II Prx sequence is absent in 85% of P . marinus and Synechococcus
strains, G . violaceus PCC 7421, S. elongatus PCC 6301, S . elongatus PCC 7942 and T . elongatus BP-1. The
most widely represented Prx in cyanobacteria belongs to the PrxQ group with multiple isoforms
in all cyanobacterial species investigated. Except 15% of P . marinus strains and 5% of Synechococcus
strains that posess one isoform of Prx, the rest of them possess two (85% of P . marinus strains, 45%
of Synechococcus strains, Synechocystis sp. strain PCC 6803, C . watsonii WH8501 and Cyanothece
ATCC 51142), three [ M . aeruginosa NIES-843, 45% of Synechococcus strains, T . elongatus BP-1, strains
of Cyanothece (CCY0110 and PCC 8801), Lyngbya sp. PCC 8106, T . erythraeum IMS101] and four [ A .
variabilis ATCC 29413, No . spumigena CCY 9414, N . punctiforme PCC 73102 and Anabaena sp. strain PCC
7120] isoforms of PrxQ. Gene sequences of GPx are present in all P . marinus (with two such sequences
in 85% and one in 15%) strains, G . violaceus PCC 7421, S . elongatus PCC 7942, Synechococcus (with two
in 5% and one in 80%) strains, Synechocystis sp. strain PCC 6803 (two ORFs), C . watsonii WH8501,
N . punctiforme PCC 73102 and Cyanothece PCC 8801. The presence of GPx sequence in N . punctiforme