Biology Reference
In-Depth Information
(Zheng
et al
., 1998; Storz and Zheng, 2000; Pomposiello and Demple, 2001; Choi
et al
., 2001). Another
well studied repressor is PerR that was fi rst discovered in
B
.
subtilis
. The PerR regulon includes
a number of genes such as
katA
(for
catalase),
ahpCF
(for alkyl hydroperoxide reductsae),
mrgA
,
hemAXCDBL
,
fur
and
perR
. PerR is a metalloprotein and its regulation involves both oxidation and
metal ions (Herbig and Helmann, 2001; Fuangthong
et al
., 2002; Mongkolsuk and Helmann, 2002;
Helmann
et al
., 2003). Three dimeric proteins Fur, Zur and PerR coordinate gene exression in
B
.
subtilis
in response to iron, zinc and H
2
O
2
stress, respectively.
An ORF (
slr1738
) encoding PerR similar to that of
B
.
subtilis
has been identifi ed in the genome of
Synechocystis
sp. strain PCC 6803 by employing a full genome cDNA microarray. Consistent with the
known properties of PerR, its function as a repressor has been determined by the construction of a
perR
knock-out strain. It was possible to identify PerR-specifi c sequences upstream the promoters of
the genes under its control. Another ORF (
sll1621
) that encodes a peroxiredoxin is regulated by PerR.
The most important genes under its control are
isiA
,
sigD
,
sigB
and
mrgA
(
slr1894
). MrgA belongs
to ferretin family of proteins that helps in the storage of iron and plays a key role in oxidative stress
response. MrgA deletion mutants of
Synechocystis
sp. strain PCC 6803 have been found to be highly
sensitive to peroxide.The down-regulated genes included those of chlorophyll biosynthesis pathway,
regulatory genes and those of histidine kinases. A study of peroxide-induced gene expression in
PerR and thioredoxin (TrxA1) mutants revealed that neither PerR nor TrxA1 is essential for peroxide
protective response (Li
et al
., 2004). Screening of a library of Hik mutants (44) of
Synechocystis
sp.
strain PCC 6803 (by RNA slot blot hybridization and DNA microarray) revealed the involvement
of four Hiks (Hik33, Hik34, Hik16 and Hik41) in H
2
O
2
signal perception and transduction pathway.
Of a total of 77 H
2
O
2
-inducible genes 26 were regulated by these four Hiks with an induction factor
of 4.0. Hik33 was responsible for the induction of 22 of the 26 H
2
O
2
-inducible genes because all
these genes were not expressed in Hik33 mutant cells in response to H
2
O
2
. The expression of only
two genes was affected in case of Hik34, Hik16 or Hik41 mutants. H
2
O
2
-inducible genes are
hspA,
the
gifA
and
gifB
(for the subunits of glutamine synthetase inactivation factors),
hliA
,
hliB
and
hliC
genes,
nblA1
and
nblA2
,
dnaJ
and
perR
. Only nine H
2
O
2
-inducible genes are known to be under the
regulation of PerR. Interestingly, PerR mutation abolished the induction of
nblA1
,
nblA2
and
ndh2
(which are under the control of Hik33) and six other genes (whose induction is not controlled by
any of the four Hiks) suggesting that H
2
O
2
signal reception requires the participation of both Hik33
and PerR. The induction of rest of the 45 H
2
O
2
-inducible genes was not affected either in wild-type
or the Hik or PerR mutants implying the existence of another unknown H
2
O
2
sensor for these genes
(Kanesaki
et al
., 2007). Miller
et al
. (2007) synthesized two new fl uorescent probes, Peroxy Green
(PG1) and Peroxy Crimson (PC1) that are helpful in visualizing endogenous H
2
O
2
produced in living
cells. This opens up a possibility for detailed studies on brain cell signalling in response to H
2
O
2
and
in understanding the diseases caused by a defi ciency of catalases in human beings.
D) Peroxiredoxins (Prxs; EC 1.11.1.15):
This family of enzymes was fi rst described under the term
'peroxidoxin' (Chae
et al
., 1994) but shortly afterwards the term 'peroxiredoxin' was suggested
(Chae
et al
., 1994) which has been widely accepted. The Prxs belong to members of the thioredoxin
(Trx)-fold superfamily which have a thioredoxin-like fold and interact with either thiol- or disulfi de-
containing substrates. Six subclasses have now been recognized in Trx-fold superamily viz.,
glutathione peroxidases, glutathione-S-transferases, Trxs, glutaredoxins (Grx), DsbA (catalyzes
disulfi de formation) and Prxs (Schröder and Ponting, 1998). The structure, function and biology
of Prxs has been investigated in detail (Stone and Yang, 2006; Veal
et al
.,2007; Winterbourn, 2008;
Fourquest
et
al
., 2008; Hall
et al
., 2009).
