Biology Reference
In-Depth Information
sensitivity to UV-B radiation and the repair activity of the cells was impaired in visible light after
UV-B exposure but Deg-G mutants resembled the wild-type in their inactivation and reactivation
kinetics. Further, the degradation of UV-B damaged D1 and D2 proteins and their recovery during
the repair period was retarded in FtsH mutants as revealed by immunoblot assays (Cheregi
et al
.,
2007).
d) Ctp proteases
: This family of carboxy-terminal peptidases belongs to the C-terminal processing
peptidases. Tsp peptidase of
E
.
coli
is involved in the degradation of mistranslated proteins that
are targeted for degradation. Such proteins acquire
ssrA
sequence at their C-terminus which can be
specifi cally recognized by the Tsp peptidase (Jentsch, 1996; Keiler
et al
., 1996). This peptidase has a
characteristic PDZ domain at its C-terminus that is required for substrate recognition (Ponting, 1997;
Beebe
et al
., 2000).
Synechocystis
sp. strain PCC 6803 has three genes that encode Ctp-peptidases, Ctp
A, Ctp B and Ctp C that are homologous to Tsp peptidases of
E
.
coli
(Hara
et al
., 1991; Silver
et al
.,
1992). The cyanobacterial Ctp peptidases reside in the periplasm (Fulda
et al
., 2000; Zak
et al
., 2001).
The functional role of the
ctp
genes in
Synechocystis
has been examined in relation to processing of
the precursor form of D1 (pD1) with its C-terminal extension. In
Synechocystis
the carboxy-terminal
extension in pD1 is 16 amino acids long (Nixon
et al.
, 1992). For correct integration of D1 into
tetramanganese cluster essential for catalysis of water oxidation reaction, the C-terminal extension
has to be removed by the CtpA peptidase (Diner
et al
., 1988). Shestakov
et al.
(1994) identifi ed the role
of CtpA peptidase in the biogenesis of photosynthetic machinery by the analysis of
Synechocystis
sp.
strain PCC 6803 CtpA-defi cient mutant. This mutant was unable to grow under photoautotrophic
conditions and showed impaired C-terminal processing of pD1. By insertion of kanamycin resistance
gene in the sequence of
ctpA
of
Synechocystis
, a mutant was isolated that showed D1 protein larger
(by 2 kDa) than in the wild-type cells. Though the PSII complexes of the mutant exhibited functional
reaction centres of PSII complex, it was unable to accept electrons from water (Anbudurai
et al
.,
1994). The amino acid residues Ser313 and Lys338 have been found to be critical for
in vivo
catalytic
activity of CtpA as identifi ed by X-ray crystal structure analysis of CtpA from
Scenedesmus obliquus
(Liao
et al
., 2000). Detailed analysis of site specifi c mutations introduced at the conserved residues
of CtpA in
Synechocystis
revealed three additional amino acid residues (Asp253, Arg255 and Glu316)
important for the catalytic activity. These fi ndings are supported by the photoautotrophic growth
potential of the mutants along with their ability to process pD1 protein and the amount of pD1
versus mature D1 inside the cells (Inagaki
et al
., 2001a,b). CtpC gene was found to be essential for
cell viability as it was not possible to obtain complete gene-inactivated mutants (Ivleva
et al
., 2002).
Complete inactivation of CtpB did not lead to any visible phenotypic or physiological changes.
e) Spp family
:
This family is characterizd by the presence of two signal peptide peptidases
differing
in their molecular mass, with the shorter version representing the C-terminal domain of the longer
one.
Synechocystis
sp. strain PCC 6803
has two genes
SppA1
and
SppA2
and a homologue of the
former gene is found in
Arabidopsis
thaliana
(Lensch
et
al
., 2001).
iv) Photosynthesis and heat shock
:
Due to heat shock, photosynthetic apparatus appears to be affected
fi rst before the impairment of any other metabolic process. Chlorophyll
a
fl uorescence provides
an important intrinsic probe
for measuring the thermal stability of PSII-protein complexes. Heat
inactivation of photosynthesis can as well be measured in terms of O
2
evolution. A disturbance in
the integrity of thylakoid membranes causes damage to the integral proteins of the photosynthetic
electron transport. Nishiyama
et al
. (1993) showed that thermal adaptation of photosynthetic
